From charioteer7 at gmail.com  Sun Mar  1 02:58:50 2015
From: charioteer7 at gmail.com (Geoffrey Bays)
Date: Sat, 28 Feb 2015 21:58:50 -0500
Subject: [Haskell-beginners] Using findIndex and then splitAt in
	Data.List
In-Reply-To: <54F1E431.7000704@orlitzky.com>
References: <CAD8ukx7J9usf=TwVnBisMiL0cgXu1+6gyc8R1HYbxo=CeTTXww@mail.gmail.com>
 <54F1CBEE.5050806@orlitzky.com>
 <CAD8ukx7CeHtOgKFHT9sgo7C2PzJdmbmJJ5NqhwiE6vd6_UB3Hg@mail.gmail.com>
 <54F1E431.7000704@orlitzky.com>
Message-ID: <CAD8ukx5HQxZWNNzbtPMk6x0TNjBDxz-O+owVBfX-VJvgZ=WDAg@mail.gmail.com>

Michael:
Yes, a map with a function that leaves everything I do not want to change
alone, that would indeed be the smooth, obvious FP way to do it. I will
change my code to work this way. Thanks.
Geoffrey
On Feb 28, 2015 10:52 AM, "Michael Orlitzky" <michael at orlitzky.com> wrote:

> On 02/28/2015 10:02 AM, Geoffrey Bays wrote:
> > Michael:
> > Thanks, split on would work nicely.
> >
> > I probably should have backed up in my reasoning process to say that my
> > real task is to replace a data item in a list with another altered data
> > item in the same position, or rather in FP, create a new list with the
> > altered item in the same place. And I need a predicate to find the name
> of
> > the item I want to replace.
> > Any thoughts?
> >
>
>
> You can define a function to "fix" the thing that you want to alter, and
> then map that function over the list. Just make sure that the function
> only alters the thing that you want to alter. For example, suppose the
> other numbers are afraid of 7, so you want to change 7 into 8 in some list:
>
>   -- Turn 7 into 8; leave everything else alone.
>   fix_seven :: Int -> Int
>   fix_seven 7 = 8
>   fix_seven x = x
>
>   numbers :: [Int]
>   numbers = [1..10]
>
>   ghci> print $ map fix_seven numbers
>   [1,2,3,4,5,6,8,8,9,10]
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From jakep at arqux.com  Sun Mar  1 04:40:49 2015
From: jakep at arqux.com (DJ)
Date: Sat, 28 Feb 2015 23:40:49 -0500
Subject: [Haskell-beginners] trying to install libraries for Haskell School
	of Expression
Message-ID: <54F29851.9040202@arqux.com>

Trying to install the package soegtk.

I am running linux mint 17 with the latest (as of Feb. 28) haskell 
platform and cabal-install.

I did install gtk2hs-buildtools and put the .cabal/bin in my path.

Cannot install soegtk:

Failed to install glib-0.12.5.4
cabal: Error: some packages failed to install:
cairo-0.12.5.3 failed during the configure step. The exception was:
ExitFailure 1
gio-0.12.5.3 depends on glib-0.12.5.4 which failed to install.
glib-0.12.5.4 failed during the configure step. The exception was:
ExitFailure 1
gtk-0.12.5.7 depends on glib-0.12.5.4 which failed to install.
pango-0.12.5.3 depends on glib-0.12.5.4 which failed to install.
soegtk-0.12.1 depends on glib-0.12.5.4 which failed to install.

Confession: I abandoned Haskell two years ago because of frustration 
with cabal and hackage. I decided to get back to the language today, and 
to start with Haskell School of Expression. I immediately run into the 
problem that the first thing I try to install with cabal does not work.

Thanks for any help. Please tell me things are not just as bad now as 
they were when I left ;-)

- DJP -


From sumit.sahrawat.apm13 at iitbhu.ac.in  Sun Mar  1 05:52:38 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Sun, 1 Mar 2015 11:22:38 +0530
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <54F29851.9040202@arqux.com>
References: <54F29851.9040202@arqux.com>
Message-ID: <CAJbEW8Novpm+51OpM+Fm08PRMGFqjCThLfJtKX2xTiA_CexT1w@mail.gmail.com>

We have sandboxes now, which (I think) weren't there 2 yrs ago.
Try this:

$ mkdir hse                                          # Directory for Haskell
school of expression
$ cd hse
$ cabal sandbox init                                 # Create a new sandbox
$ cabal install gtk2hs-buildtools                    # Install
gtk2hs-buildtools inside sandbox
$ export PATH=".cabal-sandbox/bin:$PATH"             # Add the binaries
installed in (current directory's) sandbox to $PATH
$ cabal install gtk                                  # Install gtk (and
anything else you need)

The above works in my case. Also, if you have a lot of packages installed
in "~/.cabal", then I recommend that you remove them and install them in
sandboxes where they are actually required.
Manually, you'd do it as follows:

$ mv ~/.cabal/packages ~/temporary_package_storage   # Long name to avoid
clash, save downloaded package sources
$ rm -rf ~/.ghc ~/.cabal                             # Remove all packages,
and ghc's knowledge of them
$ mkdir ~/.cabal                                     # Make a new ~/.cabal
directory
$ mv ~/temporary_package_storage ~/.cabal/packages   # Reinsert the saved
downloaded package sources

For more info, look here:
https://www.haskell.org/cabal/users-guide/installing-packages.html#developing-with-sandboxes
Also, after some time it becomes natural to deal with cabal.

For example, I have that export line in my .zshrc. Thus the sandbox
residing in current directory is always in my path.
Also, a shell function that you may want to use.

cabal-reset () {
    if [[ ! -d ~/.cabal-packages ]] ; then           # If backup does not
exist
        mv ~/.cabal/packages ~/.cabal-packages       # Make a backup
    fi
    rm -rf ~/.ghc ~/.cabal                           # Nuke everything
    mkdir ~/.cabal                                   # Create new ~/.cabal
    ln -s ~/.cabal-packages ~/.cabal/packages        # Symlink backup to
~/.cabal/packages
}

Hope this helps.

On 1 March 2015 at 10:10, DJ <jakep at arqux.com> wrote:

> Trying to install the package soegtk.
>
> I am running linux mint 17 with the latest (as of Feb. 28) haskell
> platform and cabal-install.
>
> I did install gtk2hs-buildtools and put the .cabal/bin in my path.
>
> Cannot install soegtk:
>
> Failed to install glib-0.12.5.4
> cabal: Error: some packages failed to install:
> cairo-0.12.5.3 failed during the configure step. The exception was:
> ExitFailure 1
> gio-0.12.5.3 depends on glib-0.12.5.4 which failed to install.
> glib-0.12.5.4 failed during the configure step. The exception was:
> ExitFailure 1
> gtk-0.12.5.7 depends on glib-0.12.5.4 which failed to install.
> pango-0.12.5.3 depends on glib-0.12.5.4 which failed to install.
> soegtk-0.12.1 depends on glib-0.12.5.4 which failed to install.
>
> Confession: I abandoned Haskell two years ago because of frustration with
> cabal and hackage. I decided to get back to the language today, and to
> start with Haskell School of Expression. I immediately run into the problem
> that the first thing I try to install with cabal does not work.
>
> Thanks for any help. Please tell me things are not just as bad now as they
> were when I left ;-)
>
> - DJP -
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>



-- 
Regards

Sumit Sahrawat
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From allbery.b at gmail.com  Sun Mar  1 07:29:54 2015
From: allbery.b at gmail.com (Brandon Allbery)
Date: Sun, 1 Mar 2015 02:29:54 -0500
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <54F29851.9040202@arqux.com>
References: <54F29851.9040202@arqux.com>
Message-ID: <CAKFCL4Urz9Tr-snqwNeXA_iGZg5Z1-xpa6XijtNSwRJjeWWsGA@mail.gmail.com>

On Sat, Feb 28, 2015 at 11:40 PM, DJ <jakep at arqux.com> wrote:

> I am running linux mint 17 with the latest (as of Feb. 28) haskell
> platform and cabal-install.
> I did install gtk2hs-buildtools and put the .cabal/bin in my path.
>

Do you have the devel libraries needed by gtk installed? cabal cannot
install them for you, and the runtime libraries are not sufficient. You'll
want libcairo2-dev for this, and others for other gtk components (notably
libgtk-3-dev but there will be others as well).

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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From magnus at therning.org  Sun Mar  1 08:30:35 2015
From: magnus at therning.org (Magnus Therning)
Date: Sun, 1 Mar 2015 09:30:35 +0100
Subject: [Haskell-beginners] trees on Haskell : Do I understand it right
	?
In-Reply-To: <54EF2732.90005@home.nl>
References: <54EF1DCF.40906@home.nl>
 <CAAbahfQO5o+Db_qy_B+vb3E6WWb1bdeQBmpAPa6E-5TwTjerjg@mail.gmail.com>
 <54EF2732.90005@home.nl>
Message-ID: <20150301083035.GA3078@tatooine>

Roelof, please consider changing your mail client to send *both* HTML
and plain-text emails!  With your current setting you send *only* HTML
sometimes and it results in a slightly broken list archive, see [1]
for an example of the result.

To boot, you also make it more difficult for those of us who use
plain-text mail clients to benefit from the diskussions you
participate in.

/M

[1]: https://mail.haskell.org/pipermail/beginners/2015-February/014808.html

On Thu, Feb 26, 2015 at 03:01:22PM +0100, Roelof Wobben wrote:
> <html>
>   <head>
>     <meta content="text/html; charset=windows-1252"
>       http-equiv="Content-Type">
>   </head>
>   <body bgcolor="#FFFFFF" text="#000000">
>     <div class="moz-cite-prefix">Oke, <br>
>       <br>
>       So a leaf is a node which has no "branch"<br>
>       <br>
>       I have made a exercise where I have to made the? logMessages. <br>
>       Now I have to turn them into a tree <br>
>       <br>
>       Where does the second entry goes then ?<br>
>       <br>
>       Roelof<br>
>       <br>
>       <br>
>       Konstantine Rybnikov schreef op 26-2-2015 om 14:56:<br>
>     </div>
>     <blockquote
> cite="mid:CAAbahfQO5o+Db_qy_B+vb3E6WWb1bdeQBmpAPa6E-5TwTjerjg at mail.gmail.com"
>       type="cite">
>       <div dir="ltr">
>         <div>
>           <div>
>             <div>
>               <div>
>                 <div>
>                   <div>
>                     <div>Hi Roelof,<br>
>                       <br>
>                     </div>
>                     I think you misunderstood it.<br>
>                     <br>
>                   </div>
>                   <div>There are two things here: types and values
>                     (value-constructors). They exist in different world,
>                     not touching each other.<br>
>                     <br>
>                   </div>
>                   <div>In Haskell, you define a type as:<br>
>                     <br>
>                   </div>
>                   <div>data &lt;Type_Name&gt; =
>                     &lt;ValueConstructor_Name&gt; &lt;Type_Name&gt;
>                     &lt;Type_Name&gt; &lt;Type_Name&gt;<br>
>                     <br>
>                   </div>
>                   <div>You can create values as:<br>
>                     <br>
>                   </div>
>                   <div>let varName = &lt;ValueConstructor_Name&gt;
>                     &lt;Value&gt; &lt;Value&gt; &lt;Value&gt;<br>
>                     <br>
>                   </div>
>                   <div>You need to put &lt;Value&gt; of some type, not
>                     type name itself in place of those &lt;Value&gt;s.<br>
>                     <br>
>                   </div>
>                   <div>So, with datatype you provided, you have two
>                     data-constructors:<br>
>                     <br>
>                   </div>
>                   <div>Leaf<br>
>                     <br>
>                   </div>
>                   <div>and<br>
>                     <br>
>                   </div>
>                   <div>Node &lt;val&gt; &lt;val&gt; &lt;val&gt;<br>
>                     <br>
>                   </div>
>                   <div>You can create a leaf:<br>
>                     <br>
>                   </div>
>                   <div>let leaf = Leav<br>
>                     <br>
>                   </div>
>                   <div>or a node:<br>
>                     <br>
>                   </div>
>                   <div>let node = Node Leaf "msg" Leaf<br>
>                     <br>
>                   </div>
>                   <div>You can see that Node is a data-constructor that
>                     takes 3 values, not type-names as it's parameters.<br>
>                     <br>
>                   </div>
>                   <div>Hope this helps.<br>
>                   </div>
>                 </div>
>               </div>
>             </div>
>           </div>
>         </div>
>       </div>
>       <div class="gmail_extra"><br>
>         <div class="gmail_quote">On Thu, Feb 26, 2015 at 3:21 PM, Roelof
>           Wobben <span dir="ltr">&lt;<a moz-do-not-send="true"
>               href="mailto:r.wobben at home.nl" target="_blank">r.wobben at home.nl</a>&gt;</span>
>           wrote:<br>
>           <blockquote class="gmail_quote" style="margin:0 0 0
>             .8ex;border-left:1px #ccc solid;padding-left:1ex">Hello,<br>
>             <br>
>             Suppose we have this definition of a tree :<br>
>             <br>
>             data MessageTree = Leaf<br>
>             ? ? ? ? ? ? ? ? ?| Node MessageTree LogMessage MessageTree<br>
>             ? deriving (Show, Eq)<br>
>             <br>
>             let Message? = LogMessage "E 1 this is a test error"<br>
>             let Message = LogMessage "e 2 this is the second test error
>             "<br>
>             <br>
>             As I understand it right I can make the first entry like
>             this : first_entry = Node Messagetree? Message Messagetree<br>
>             <br>
>             And the second one like this second_entry = Node Message
>             Messagetree Message2 Messagetree ??<br>
>             <br>
>             Roelof<br>
>             <br>
>             _______________________________________________<br>
>             Beginners mailing list<br>
>             <a moz-do-not-send="true"
>               href="mailto:Beginners at haskell.org" target="_blank">Beginners at haskell.org</a><br>
>             <a moz-do-not-send="true"
>               href="http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners"
>               target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners</a><br>
>           </blockquote>
>         </div>
>         <br>
>       </div>
>       <br>
>       <fieldset class="mimeAttachmentHeader"></fieldset>
>       <br>
>       <pre wrap="">_______________________________________________
> Beginners mailing list
> <a class="moz-txt-link-abbreviated" href="mailto:Beginners at haskell.org">Beginners at haskell.org</a>
> <a class="moz-txt-link-freetext" href="http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners">http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners</a>
> </pre>
>     </blockquote>
>     <br>
>   </body>
> </html>

> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners


-- 
Magnus Therning                      OpenPGP: 0xAB4DFBA4 
email: magnus at therning.org   jabber: magnus at therning.org
twitter: magthe               http://therning.org/magnus

Heuristic is an algorithm in a clown suit. It?s less predictable, it?s more
fun, and it comes without a 30-day, money-back guarantee.
     -- Steve McConnell, Code Complete 
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From joel.neely at gmail.com  Sun Mar  1 13:45:24 2015
From: joel.neely at gmail.com (Joel Neely)
Date: Sun, 1 Mar 2015 07:45:24 -0600
Subject: [Haskell-beginners] trees on Haskell : Do I understand it right
	?
In-Reply-To: <54EF66A5.5050107@home.nl>
References: <54EF1DCF.40906@home.nl>
 <CAAbahfQO5o+Db_qy_B+vb3E6WWb1bdeQBmpAPa6E-5TwTjerjg@mail.gmail.com>
 <54EF2732.90005@home.nl>
 <CAAbahfRW8-XtPgGs1Md5-t4EjqKapsDrd0b4TnqdU0tQzU2NNg@mail.gmail.com>
 <54EF307A.40204@home.nl> <54EF66A5.5050107@home.nl>
Message-ID: <CAEEzXAi=TN_+wmYqFqhN7puJVV0gc-nxt3G3KCU6oGw_FqtBkw@mail.gmail.com>

Roelof,

Perhaps it would help for you to draw diagrams for the cases you're working.
If a leaf has nothing below it, and a node always has exactly three things
below it (the left child, the message, and the right child),
what would your picture look like for 1, 2, and 3 nodes?
And how would you write out constructor expressions that match those shapes?

Hope that helps,
-jn-


On Thu, Feb 26, 2015 at 12:32 PM, Roelof Wobben <r.wobben at home.nl> wrote:

>  Oke,
>
> I send it a second time but now in plain text.
>
> So for 3 it will be like this :
>
> Node = Node  (Node Leaf "msg1" Leaf) (Node Leaf "msg2") (Node  "msg3"
> Leaf)  ???
>
>
> Roelof
>
>
>
>
>
> Konstantine Rybnikov schreef op 26-2-2015 om 15:08:
>
>  In my second example you can see a minimal node with a message:
>
> node = Node Leaf "msg" Leaf
>
>  Instead of either left or right Leaf you can put another value of type
> MessageTree, for example:
>
>  node = Node Leaf "msg1" (Node Leaf "msg2" Leaf)
>
> On Thu, Feb 26, 2015 at 4:01 PM, Roelof Wobben <r.wobben at home.nl> wrote:
>
>>  Oke,
>>
>> So a leaf is a node which has no "branch"
>>
>> I have made a exercise where I have to made the  logMessages.
>> Now I have to turn them into a tree
>>
>> Where does the second entry goes then ?
>>
>> Roelof
>>
>>
>> Konstantine Rybnikov schreef op 26-2-2015 om 14:56:
>>
>>     Hi Roelof,
>>
>>  I think you misunderstood it.
>>
>>  There are two things here: types and values (value-constructors). They
>> exist in different world, not touching each other.
>>
>>  In Haskell, you define a type as:
>>
>>  data <Type_Name> = <ValueConstructor_Name> <Type_Name> <Type_Name>
>> <Type_Name>
>>
>>  You can create values as:
>>
>>  let varName = <ValueConstructor_Name> <Value> <Value> <Value>
>>
>>  You need to put <Value> of some type, not type name itself in place of
>> those <Value>s.
>>
>>  So, with datatype you provided, you have two data-constructors:
>>
>>  Leaf
>>
>>  and
>>
>>  Node <val> <val> <val>
>>
>>  You can create a leaf:
>>
>>  let leaf = Leav
>>
>>  or a node:
>>
>>  let node = Node Leaf "msg" Leaf
>>
>>  You can see that Node is a data-constructor that takes 3 values, not
>> type-names as it's parameters.
>>
>>  Hope this helps.
>>
>> On Thu, Feb 26, 2015 at 3:21 PM, Roelof Wobben <r.wobben at home.nl> wrote:
>>
>>> Hello,
>>>
>>> Suppose we have this definition of a tree :
>>>
>>> data MessageTree = Leaf
>>>                  | Node MessageTree LogMessage MessageTree
>>>   deriving (Show, Eq)
>>>
>>> let Message  = LogMessage "E 1 this is a test error"
>>> let Message = LogMessage "e 2 this is the second test error "
>>>
>>> As I understand it right I can make the first entry like this :
>>> first_entry = Node Messagetree  Message Messagetree
>>>
>>> And the second one like this second_entry = Node Message Messagetree
>>> Message2 Messagetree ??
>>>
>>> Roelof
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>
>>
>>
>> _______________________________________________
>> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
>
> _______________________________________________
> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>


-- 
Beauty of style and harmony and grace and good rhythm depend on simplicity.
- Plato
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From objitsu at gmail.com  Sun Mar  1 14:56:44 2015
From: objitsu at gmail.com (emacstheviking)
Date: Sun, 1 Mar 2015 14:56:44 +0000
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <54F29851.9040202@arqux.com>
References: <54F29851.9040202@arqux.com>
Message-ID: <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>

I feel your pain...


> Confession: I abandoned Haskell two years ago because of frustration with
> cabal and hackage. I decided to get back to the language today, and to
> start with Haskell School of Expression. I immediately run into the problem
> that the first thing I try to install with cabal does not work.
>
> It is the same reason I have stopped using it. It's a real shame. Those
with better education and understanding than mine should be concerned that
the uptake of the language is stunted by its package manager.

I love Haskell. I have taught myself (the beginnings at least) of group
theory just to better comprehend the mindset of monads. For that alone I am
glad I learned Haskell as it has rekindled my interest in maths!



> Thanks for any help. Please tell me things are not just as bad now as they
> were when I left ;-)
>
> This mail seems to indicate to me that "cabal hell" is here for some time
to come...
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From r.wobben at home.nl  Sun Mar  1 18:08:22 2015
From: r.wobben at home.nl (Roelof Wobben)
Date: Sun, 01 Mar 2015 19:08:22 +0100
Subject: [Haskell-beginners] trees on Haskell : Do I understand it right
 ?
In-Reply-To: <CAEEzXAi=TN_+wmYqFqhN7puJVV0gc-nxt3G3KCU6oGw_FqtBkw@mail.gmail.com>
References: <54EF1DCF.40906@home.nl>
 <CAAbahfQO5o+Db_qy_B+vb3E6WWb1bdeQBmpAPa6E-5TwTjerjg@mail.gmail.com>
 <54EF2732.90005@home.nl>
 <CAAbahfRW8-XtPgGs1Md5-t4EjqKapsDrd0b4TnqdU0tQzU2NNg@mail.gmail.com>
 <54EF307A.40204@home.nl> <54EF66A5.5050107@home.nl>
 <CAEEzXAi=TN_+wmYqFqhN7puJVV0gc-nxt3G3KCU6oGw_FqtBkw@mail.gmail.com>
Message-ID: <54F35596.3060001@home.nl>

Hello,

Im looking for goofd online software to make the diagrams.
As soon as I find it . I will publish them on the ML.

Roelof

Joel Neely schreef op 1-3-2015 om 14:45:
> Roelof,
>
> Perhaps it would help for you to draw diagrams for the cases you're 
> working.
> If a leaf has nothing below it, and a node always has exactly three 
> things below it (the left child, the message, and the right child),
> what would your picture look like for 1, 2, and 3 nodes?
> And how would you write out constructor expressions that match those 
> shapes?
>
> Hope that helps,
> -jn-
>
>
> On Thu, Feb 26, 2015 at 12:32 PM, Roelof Wobben <r.wobben at home.nl 
> <mailto:r.wobben at home.nl>> wrote:
>
>     Oke,
>
>     I send it a second time but now in plain text.
>
>     So for 3 it will be like this :
>
>     Node = Node  (Node Leaf "msg1" Leaf) (Node Leaf "msg2") (Node 
>     "msg3" Leaf)  ???
>
>
>     Roelof
>
>
>
>>
>>
>>     Konstantine Rybnikov schreef op 26-2-2015 om 15:08:
>>>     In my second example you can see a minimal node with a message:
>>>
>>>     node = Node Leaf "msg" Leaf
>>>
>>>     Instead of either left or right Leaf you can put another value
>>>     of type MessageTree, for example:
>>>
>>>     node = Node Leaf "msg1" (Node Leaf "msg2" Leaf)
>>>
>>>     On Thu, Feb 26, 2015 at 4:01 PM, Roelof Wobben <r.wobben at home.nl
>>>     <mailto:r.wobben at home.nl>> wrote:
>>>
>>>         Oke,
>>>
>>>         So a leaf is a node which has no "branch"
>>>
>>>         I have made a exercise where I have to made the  logMessages.
>>>         Now I have to turn them into a tree
>>>
>>>         Where does the second entry goes then ?
>>>
>>>         Roelof
>>>
>>>
>>>         Konstantine Rybnikov schreef op 26-2-2015 om 14:56:
>>>>         Hi Roelof,
>>>>
>>>>         I think you misunderstood it.
>>>>
>>>>         There are two things here: types and values
>>>>         (value-constructors). They exist in different world, not
>>>>         touching each other.
>>>>
>>>>         In Haskell, you define a type as:
>>>>
>>>>         data <Type_Name> = <ValueConstructor_Name> <Type_Name>
>>>>         <Type_Name> <Type_Name>
>>>>
>>>>         You can create values as:
>>>>
>>>>         let varName = <ValueConstructor_Name> <Value> <Value> <Value>
>>>>
>>>>         You need to put <Value> of some type, not type name itself
>>>>         in place of those <Value>s.
>>>>
>>>>         So, with datatype you provided, you have two data-constructors:
>>>>
>>>>         Leaf
>>>>
>>>>         and
>>>>
>>>>         Node <val> <val> <val>
>>>>
>>>>         You can create a leaf:
>>>>
>>>>         let leaf = Leav
>>>>
>>>>         or a node:
>>>>
>>>>         let node = Node Leaf "msg" Leaf
>>>>
>>>>         You can see that Node is a data-constructor that takes 3
>>>>         values, not type-names as it's parameters.
>>>>
>>>>         Hope this helps.
>>>>
>>>>         On Thu, Feb 26, 2015 at 3:21 PM, Roelof Wobben
>>>>         <r.wobben at home.nl <mailto:r.wobben at home.nl>> wrote:
>>>>
>>>>             Hello,
>>>>
>>>>             Suppose we have this definition of a tree :
>>>>
>>>>             data MessageTree = Leaf
>>>>                              | Node MessageTree LogMessage MessageTree
>>>>               deriving (Show, Eq)
>>>>
>>>>             let Message  = LogMessage "E 1 this is a test error"
>>>>             let Message = LogMessage "e 2 this is the second test
>>>>             error "
>>>>
>>>>             As I understand it right I can make the first entry
>>>>             like this : first_entry = Node Messagetree  Message
>>>>             Messagetree
>>>>
>>>>             And the second one like this second_entry = Node
>>>>             Message Messagetree Message2 Messagetree ??
>>>>
>>>>             Roelof
>>>>
>>>>             _______________________________________________
>>>>             Beginners mailing list
>>>>             Beginners at haskell.org <mailto:Beginners at haskell.org>
>>>>             http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>>>
>>>>
>>>>         _______________________________________________
>>>>         Beginners mailing list
>>>>         Beginners at haskell.org  <mailto:Beginners at haskell.org>
>>>>         http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>>         _______________________________________________
>>>         Beginners mailing list
>>>         Beginners at haskell.org <mailto:Beginners at haskell.org>
>>>         http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>>
>>>
>>>     _______________________________________________
>>>     Beginners mailing list
>>>     Beginners at haskell.org  <mailto:Beginners at haskell.org>
>>>     http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
>
>     _______________________________________________
>     Beginners mailing list
>     Beginners at haskell.org <mailto:Beginners at haskell.org>
>     http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
>
>
> -- 
> Beauty of style and harmony and grace and good rhythm depend on 
> simplicity. - Plato
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners


From r.wobben at home.nl  Sun Mar  1 19:02:42 2015
From: r.wobben at home.nl (Roelof Wobben)
Date: Sun, 01 Mar 2015 20:02:42 +0100
Subject: [Haskell-beginners] trees on Haskell : Do I understand it right
 ?
In-Reply-To: <54F35596.3060001@home.nl>
References: <54EF1DCF.40906@home.nl>
 <CAAbahfQO5o+Db_qy_B+vb3E6WWb1bdeQBmpAPa6E-5TwTjerjg@mail.gmail.com>
 <54EF2732.90005@home.nl>
 <CAAbahfRW8-XtPgGs1Md5-t4EjqKapsDrd0b4TnqdU0tQzU2NNg@mail.gmail.com>
 <54EF307A.40204@home.nl> <54EF66A5.5050107@home.nl>
 <CAEEzXAi=TN_+wmYqFqhN7puJVV0gc-nxt3G3KCU6oGw_FqtBkw@mail.gmail.com>
 <54F35596.3060001@home.nl>
Message-ID: <54F36252.10809@home.nl>

Here the diagram for 1 messages : http://imgur.com/yuzXUNB

In haskell it would be  Node Message1 leaf leaf

Roelof


Roelof Wobben schreef op 1-3-2015 om 19:08:
> Hello,
>
> Im looking for goofd online software to make the diagrams.
> As soon as I find it . I will publish them on the ML.
>
> Roelof
>
> Joel Neely schreef op 1-3-2015 om 14:45:
>> Roelof,
>>
>> Perhaps it would help for you to draw diagrams for the cases you're 
>> working.
>> If a leaf has nothing below it, and a node always has exactly three 
>> things below it (the left child, the message, and the right child),
>> what would your picture look like for 1, 2, and 3 nodes?
>> And how would you write out constructor expressions that match those 
>> shapes?
>>
>> Hope that helps,
>> -jn-
>>
>>
>> On Thu, Feb 26, 2015 at 12:32 PM, Roelof Wobben <r.wobben at home.nl 
>> <mailto:r.wobben at home.nl>> wrote:
>>
>>     Oke,
>>
>>     I send it a second time but now in plain text.
>>
>>     So for 3 it will be like this :
>>
>>     Node = Node  (Node Leaf "msg1" Leaf) (Node Leaf "msg2") (Node     
>> "msg3" Leaf)  ???
>>
>>
>>     Roelof
>>
>>
>>
>>>
>>>
>>>     Konstantine Rybnikov schreef op 26-2-2015 om 15:08:
>>>>     In my second example you can see a minimal node with a message:
>>>>
>>>>     node = Node Leaf "msg" Leaf
>>>>
>>>>     Instead of either left or right Leaf you can put another value
>>>>     of type MessageTree, for example:
>>>>
>>>>     node = Node Leaf "msg1" (Node Leaf "msg2" Leaf)
>>>>
>>>>     On Thu, Feb 26, 2015 at 4:01 PM, Roelof Wobben <r.wobben at home.nl
>>>>     <mailto:r.wobben at home.nl>> wrote:
>>>>
>>>>         Oke,
>>>>
>>>>         So a leaf is a node which has no "branch"
>>>>
>>>>         I have made a exercise where I have to made the logMessages.
>>>>         Now I have to turn them into a tree
>>>>
>>>>         Where does the second entry goes then ?
>>>>
>>>>         Roelof
>>>>
>>>>
>>>>         Konstantine Rybnikov schreef op 26-2-2015 om 14:56:
>>>>>         Hi Roelof,
>>>>>
>>>>>         I think you misunderstood it.
>>>>>
>>>>>         There are two things here: types and values
>>>>>         (value-constructors). They exist in different world, not
>>>>>         touching each other.
>>>>>
>>>>>         In Haskell, you define a type as:
>>>>>
>>>>>         data <Type_Name> = <ValueConstructor_Name> <Type_Name>
>>>>>         <Type_Name> <Type_Name>
>>>>>
>>>>>         You can create values as:
>>>>>
>>>>>         let varName = <ValueConstructor_Name> <Value> <Value> <Value>
>>>>>
>>>>>         You need to put <Value> of some type, not type name itself
>>>>>         in place of those <Value>s.
>>>>>
>>>>>         So, with datatype you provided, you have two 
>>>>> data-constructors:
>>>>>
>>>>>         Leaf
>>>>>
>>>>>         and
>>>>>
>>>>>         Node <val> <val> <val>
>>>>>
>>>>>         You can create a leaf:
>>>>>
>>>>>         let leaf = Leav
>>>>>
>>>>>         or a node:
>>>>>
>>>>>         let node = Node Leaf "msg" Leaf
>>>>>
>>>>>         You can see that Node is a data-constructor that takes 3
>>>>>         values, not type-names as it's parameters.
>>>>>
>>>>>         Hope this helps.
>>>>>
>>>>>         On Thu, Feb 26, 2015 at 3:21 PM, Roelof Wobben
>>>>>         <r.wobben at home.nl <mailto:r.wobben at home.nl>> wrote:
>>>>>
>>>>>             Hello,
>>>>>
>>>>>             Suppose we have this definition of a tree :
>>>>>
>>>>>             data MessageTree = Leaf
>>>>>                              | Node MessageTree LogMessage 
>>>>> MessageTree
>>>>>               deriving (Show, Eq)
>>>>>
>>>>>             let Message  = LogMessage "E 1 this is a test error"
>>>>>             let Message = LogMessage "e 2 this is the second test
>>>>>             error "
>>>>>
>>>>>             As I understand it right I can make the first entry
>>>>>             like this : first_entry = Node Messagetree Message
>>>>>             Messagetree
>>>>>
>>>>>             And the second one like this second_entry = Node
>>>>>             Message Messagetree Message2 Messagetree ??
>>>>>
>>>>>             Roelof
>>>>>
>>>>> _______________________________________________
>>>>>             Beginners mailing list
>>>>>             Beginners at haskell.org <mailto:Beginners at haskell.org>
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>         _______________________________________________
>>>>>         Beginners mailing list
>>>>>         Beginners at haskell.org <mailto:Beginners at haskell.org>
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>>>         _______________________________________________
>>>>         Beginners mailing list
>>>>         Beginners at haskell.org <mailto:Beginners at haskell.org>
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>>>
>>>>
>>>>     _______________________________________________
>>>>     Beginners mailing list
>>>>     Beginners at haskell.org <mailto:Beginners at haskell.org>
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>
>>
>>     _______________________________________________
>>     Beginners mailing list
>>     Beginners at haskell.org <mailto:Beginners at haskell.org>
>>     http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>>
>>
>> -- 
>> Beauty of style and harmony and grace and good rhythm depend on 
>> simplicity. - Plato
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>


From jakep at arqux.com  Sun Mar  1 19:27:51 2015
From: jakep at arqux.com (DJ)
Date: Sun, 01 Mar 2015 14:27:51 -0500
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>
References: <54F29851.9040202@arqux.com>
 <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>
Message-ID: <54F36837.5090704@arqux.com>

Yeah. I think there are a lot of us out there. I won't rant (much) here, 
because I hope to get some help from time to time. Alienating people is 
not a good way to get help.

But:

There are many things I like about the language, but I have always been 
perplexed by the haskell ecosystem in a couple of ways. However good 
and/or interesting a language might be, it is not possible to do meat 
and potatoes development unless there is a good supply of 
packages/libraries.

My first problem was that I often just could not get stuff to install. 
The second problem was documentation. I hope it has improved a bit. I 
was always flabbergasted at the (apparent) belief that a few lines of 
API documentation coughed out by haddock are enough. But that's all 
there was in a lot of cases. That, and "refer to xxxx research paper" 
and "figure it out from the types". Hah. Any math paper that consisted 
of just the equations sorted in alphabetical order would not be well 
received, I think.

I really admired that fact that Michael Snoyman wrote a book about 
Yesod. For all I know he may not have kept the book up, but at least he 
realized that people needed an explanation of how to use his software.

Well, back to work. I am going to give haskell another try for sure. 
It's just too tempting to give up on. If it doesn't work, the next stop 
will be sml or ocaml.

Best,

- DJ -

On 15-03-01 09:56 AM, emacstheviking wrote:
> I feel your pain...
>
>     Confession: I abandoned Haskell two years ago because of
>     frustration with cabal and hackage. I decided to get back to the
>     language today, and to start with Haskell School of Expression. I
>     immediately run into the problem that the first thing I try to
>     install with cabal does not work.
>
> It is the same reason I have stopped using it. It's a real shame. 
> Those with better education and understanding than mine should be 
> concerned that the uptake of the language is stunted by its package 
> manager.
>
> I love Haskell. I have taught myself (the beginnings at least) of 
> group theory just to better comprehend the mindset of monads. For that 
> alone I am glad I learned Haskell as it has rekindled my interest in 
> maths!
>
>     Thanks for any help. Please tell me things are not just as bad now
>     as they were when I left ;-)
>
> This mail seems to indicate to me that "cabal hell" is here for some 
> time to come...
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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From jakep at arqux.com  Sun Mar  1 19:31:58 2015
From: jakep at arqux.com (DJ)
Date: Sun, 01 Mar 2015 14:31:58 -0500
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <CAKFCL4Urz9Tr-snqwNeXA_iGZg5Z1-xpa6XijtNSwRJjeWWsGA@mail.gmail.com>
References: <54F29851.9040202@arqux.com>
 <CAKFCL4Urz9Tr-snqwNeXA_iGZg5Z1-xpa6XijtNSwRJjeWWsGA@mail.gmail.com>
Message-ID: <54F3692E.40107@arqux.com>

Brandon: Thanks. I did have to install some gtk libraries. I am also 
working in a sandbox now. This moved things ahead somewhat. But I am 
getting this now from 'cabal install soegtk':

Graphics/SOE/Gtk.hs:94:18:
     Could not find module `System.Time'
     It is a member of the hidden package `old-time-1.1.0.1'.
     Perhaps you need to add `old-time' to the build-depends in your 
.cabal file.
     Use -v to see a list of the files searched for.
Failed to install soegtk-0.9.12.2

I have forgotten anything I once knew about using cabal. I apologize for 
not learning how to deal with the above on my own, but I am hoping to 
get over this hump as quickly and painlessly as possible so I don't lose 
my momentum getting back to Haskell.

So, can anyone give me a hint how to fix the above?

Best,

- DJ -


On 15-03-01 02:29 AM, Brandon Allbery wrote:
> On Sat, Feb 28, 2015 at 11:40 PM, DJ <jakep at arqux.com 
> <mailto:jakep at arqux.com>> wrote:
>
>     I am running linux mint 17 with the latest (as of Feb. 28) haskell
>     platform and cabal-install.
>     I did install gtk2hs-buildtools and put the .cabal/bin in my path.
>
>
> Do you have the devel libraries needed by gtk installed? cabal cannot 
> install them for you, and the runtime libraries are not sufficient. 
> You'll want libcairo2-dev for this, and others for other gtk 
> components (notably libgtk-3-dev but there will be others as well).
>
> -- 
> brandon s allbery kf8nh sine nomine associates
> allbery.b at gmail.com <mailto:allbery.b at gmail.com> 
> ballbery at sinenomine.net <mailto:ballbery at sinenomine.net>
> unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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From thbach at students.uni-mainz.de  Sun Mar  1 20:07:52 2015
From: thbach at students.uni-mainz.de (Thomas Bach)
Date: Sun, 1 Mar 2015 21:07:52 +0100
Subject: [Haskell-beginners] trying to install libraries for Haskell
	School of Expression
In-Reply-To: <54F3692E.40107@arqux.com> (DJ's message of "Sun, 1 Mar 2015
 14:31:58 -0500")
References: <54F29851.9040202@arqux.com>
 <CAKFCL4Urz9Tr-snqwNeXA_iGZg5Z1-xpa6XijtNSwRJjeWWsGA@mail.gmail.com>
 <54F3692E.40107@arqux.com>
Message-ID: <87h9u4qy5j.fsf@ilxwinb01.fritz.box>

DJ <jakep at arqux.com> writes:

> Graphics/SOE/Gtk.hs:94:18:
> Could not find module `System.Time'
> It is a member of the hidden package `old-time-1.1.0.1'.
> Perhaps you need to add `old-time' to the build-depends in your .cabal
> file.

That is actually all you have to do: in the cabal file of your package
you have either a 'library' section or an 'executable' section. These
should have a field 'build-depends' simply add 'old-time >= 1.1 && <
1.2' their.

See for example the cabal file of the lens package:
https://github.com/ekmett/lens/blob/master/lens.cabal

If you don't have a cabal file you can initialize one via 'cabal init'
or simply install the package in the sandbox via running 'cabal install
old-time-1.1' in your sandbox, i.e. the directory you ran 'cabal sandbox
init' beforehand.


Regards
        Thomas.

From allbery.b at gmail.com  Sun Mar  1 20:11:11 2015
From: allbery.b at gmail.com (Brandon Allbery)
Date: Sun, 1 Mar 2015 15:11:11 -0500
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <54F3692E.40107@arqux.com>
References: <54F29851.9040202@arqux.com>
 <CAKFCL4Urz9Tr-snqwNeXA_iGZg5Z1-xpa6XijtNSwRJjeWWsGA@mail.gmail.com>
 <54F3692E.40107@arqux.com>
Message-ID: <CAKFCL4V3S+X6znh9TeqLJJTX9p6o8pba30HE_3THeO6KuwDQ=A@mail.gmail.com>

On Sun, Mar 1, 2015 at 2:31 PM, DJ <jakep at arqux.com> wrote:

> Brandon: Thanks. I did have to install some gtk libraries. I am also
> working in a sandbox now. This moved things ahead somewhat. But I am
> getting this now from 'cabal install soegtk':
>
> Graphics/SOE/Gtk.hs:94:18:
>     Could not find module `System.Time'
>     It is a member of the hidden package `old-time-1.1.0.1'.
>

That would be an out of date [program referring to the old time component
(which wasn't very good, but was in base so didn't need dependencies). Just
adding a dependency on old-time to the cabal file should be sufficient to
get it to work.

I don't know why soegtk hasn't been updated to either reference that
package or use the modern time package.

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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From jakep at arqux.com  Sun Mar  1 21:43:53 2015
From: jakep at arqux.com (DJ)
Date: Sun, 01 Mar 2015 16:43:53 -0500
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <54F29851.9040202@arqux.com>
References: <54F29851.9040202@arqux.com>
Message-ID: <54F38819.3030304@arqux.com>

Thanks for the help, guys.

Life is short. I have spent the better part of two days trying to get 
soegtk to install. No luck. Same shit as when I left haskell behind two 
years ago.

I'm out.

- DJ -


On 15-02-28 11:40 PM, DJ wrote:
> Trying to install the package soegtk.
>
> I am running linux mint 17 with the latest (as of Feb. 28) haskell 
> platform and cabal-install.
>
> I did install gtk2hs-buildtools and put the .cabal/bin in my path.
>
> Cannot install soegtk:
>
> Failed to install glib-0.12.5.4
> cabal: Error: some packages failed to install:
> cairo-0.12.5.3 failed during the configure step. The exception was:
> ExitFailure 1
> gio-0.12.5.3 depends on glib-0.12.5.4 which failed to install.
> glib-0.12.5.4 failed during the configure step. The exception was:
> ExitFailure 1
> gtk-0.12.5.7 depends on glib-0.12.5.4 which failed to install.
> pango-0.12.5.3 depends on glib-0.12.5.4 which failed to install.
> soegtk-0.12.1 depends on glib-0.12.5.4 which failed to install.
>
> Confession: I abandoned Haskell two years ago because of frustration 
> with cabal and hackage. I decided to get back to the language today, 
> and to start with Haskell School of Expression. I immediately run into 
> the problem that the first thing I try to install with cabal does not 
> work.
>
> Thanks for any help. Please tell me things are not just as bad now as 
> they were when I left ;-)
>
> - DJP -
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>


From sean at objitsu.com  Mon Mar  2 11:46:05 2015
From: sean at objitsu.com (emacstheviking)
Date: Mon, 2 Mar 2015 11:46:05 +0000
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <54F36837.5090704@arqux.com>
References: <54F29851.9040202@arqux.com>
 <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>
 <54F36837.5090704@arqux.com>
Message-ID: <CAEiEuU+V6Ftp0sDVdU4cDYmyPcugrpU9BvNXNDGQdF8RopYKMQ@mail.gmail.com>

Agreed. I will "finish" with a mild semi-demi-rant too regarding the
assumption by a lot of posters that we are all academics with access to
papers on this and that. I am not and I don't. If you cite a paper, make
sure it's available to Joe Public please!

As for only sticking to plain vanialla simple Haskell, yes, that works BUT
there are some many great libraries that a real time savers...it's such a
shame when they won't play well together!

So, despite it all, tonight I will be hacking Haskell !

:)


On 1 March 2015 at 19:27, DJ <jakep at arqux.com> wrote:

>  Yeah. I think there are a lot of us out there. I won't rant (much) here,
> because I hope to get some help from time to time. Alienating people is not
> a good way to get help.
>
> But:
>
> There are many things I like about the language, but I have always been
> perplexed by the haskell ecosystem in a couple of ways. However good and/or
> interesting a language might be, it is not possible to do meat and potatoes
> development unless there is a good supply of packages/libraries.
>
> My first problem was that I often just could not get stuff to install. The
> second problem was documentation. I hope it has improved a bit. I was
> always flabbergasted at the (apparent) belief that a few lines of API
> documentation coughed out by haddock are enough. But that's all there was
> in a lot of cases. That, and "refer to xxxx research paper" and "figure it
> out from the types". Hah. Any math paper that consisted of just the
> equations sorted in alphabetical order would not be well received, I think.
>
> I really admired that fact that Michael Snoyman wrote a book about Yesod.
> For all I know he may not have kept the book up, but at least he realized
> that people needed an explanation of how to use his software.
>
> Well, back to work. I am going to give haskell another try for sure. It's
> just too tempting to give up on. If it doesn't work, the next stop will be
> sml or ocaml.
>
> Best,
>
> - DJ -
>
>
> On 15-03-01 09:56 AM, emacstheviking wrote:
>
>   I feel your pain...
>
>
>> Confession: I abandoned Haskell two years ago because of frustration with
>> cabal and hackage. I decided to get back to the language today, and to
>> start with Haskell School of Expression. I immediately run into the problem
>> that the first thing I try to install with cabal does not work.
>>
>>  It is the same reason I have stopped using it. It's a real shame. Those
> with better education and understanding than mine should be concerned that
> the uptake of the language is stunted by its package manager.
>
>  I love Haskell. I have taught myself (the beginnings at least) of group
> theory just to better comprehend the mindset of monads. For that alone I am
> glad I learned Haskell as it has rekindled my interest in maths!
>
>
>
>> Thanks for any help. Please tell me things are not just as bad now as
>> they were when I left ;-)
>>
>>  This mail seems to indicate to me that "cabal hell" is here for some
> time to come...
>
>
>
>
> _______________________________________________
> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From julian.birch at gmail.com  Mon Mar  2 12:10:20 2015
From: julian.birch at gmail.com (Julian Birch)
Date: Mon, 2 Mar 2015 12:10:20 +0000
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <CAEiEuU+V6Ftp0sDVdU4cDYmyPcugrpU9BvNXNDGQdF8RopYKMQ@mail.gmail.com>
References: <54F29851.9040202@arqux.com>
 <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>
 <54F36837.5090704@arqux.com>
 <CAEiEuU+V6Ftp0sDVdU4cDYmyPcugrpU9BvNXNDGQdF8RopYKMQ@mail.gmail.com>
Message-ID: <CAB0TuzDBdDrQ_8+ttdJy=eExCt2CfT5xxPeZa9G+hYGsHdKYGg@mail.gmail.com>

Just a small note: if you're suffering from cabal hell, LTS Haskell may
help you. I described how to use it here:

http://www.colourcoding.net/blog/archive/2015/02/22/hello-world-web-application-in-haskell.aspx

Hope someone finds it useful.

J

On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:

> Agreed. I will "finish" with a mild semi-demi-rant too regarding the
> assumption by a lot of posters that we are all academics with access to
> papers on this and that. I am not and I don't. If you cite a paper, make
> sure it's available to Joe Public please!
>
> As for only sticking to plain vanialla simple Haskell, yes, that works BUT
> there are some many great libraries that a real time savers...it's such a
> shame when they won't play well together!
>
> So, despite it all, tonight I will be hacking Haskell !
>
> :)
>
>
> On 1 March 2015 at 19:27, DJ <jakep at arqux.com
> <javascript:_e(%7B%7D,'cvml','jakep at arqux.com');>> wrote:
>
>>  Yeah. I think there are a lot of us out there. I won't rant (much) here,
>> because I hope to get some help from time to time. Alienating people is not
>> a good way to get help.
>>
>> But:
>>
>> There are many things I like about the language, but I have always been
>> perplexed by the haskell ecosystem in a couple of ways. However good and/or
>> interesting a language might be, it is not possible to do meat and potatoes
>> development unless there is a good supply of packages/libraries.
>>
>> My first problem was that I often just could not get stuff to install.
>> The second problem was documentation. I hope it has improved a bit. I was
>> always flabbergasted at the (apparent) belief that a few lines of API
>> documentation coughed out by haddock are enough. But that's all there was
>> in a lot of cases. That, and "refer to xxxx research paper" and "figure it
>> out from the types". Hah. Any math paper that consisted of just the
>> equations sorted in alphabetical order would not be well received, I think.
>>
>> I really admired that fact that Michael Snoyman wrote a book about Yesod.
>> For all I know he may not have kept the book up, but at least he realized
>> that people needed an explanation of how to use his software.
>>
>> Well, back to work. I am going to give haskell another try for sure. It's
>> just too tempting to give up on. If it doesn't work, the next stop will be
>> sml or ocaml.
>>
>> Best,
>>
>> - DJ -
>>
>>
>> On 15-03-01 09:56 AM, emacstheviking wrote:
>>
>>   I feel your pain...
>>
>>
>>> Confession: I abandoned Haskell two years ago because of frustration
>>> with cabal and hackage. I decided to get back to the language today, and to
>>> start with Haskell School of Expression. I immediately run into the problem
>>> that the first thing I try to install with cabal does not work.
>>>
>>>  It is the same reason I have stopped using it. It's a real shame.
>> Those with better education and understanding than mine should be concerned
>> that the uptake of the language is stunted by its package manager.
>>
>>  I love Haskell. I have taught myself (the beginnings at least) of group
>> theory just to better comprehend the mindset of monads. For that alone I am
>> glad I learned Haskell as it has rekindled my interest in maths!
>>
>>
>>
>>> Thanks for any help. Please tell me things are not just as bad now as
>>> they were when I left ;-)
>>>
>>>  This mail seems to indicate to me that "cabal hell" is here for some
>> time to come...
>>
>>
>>
>>
>> _______________________________________________
>> Beginners mailing listBeginners at haskell.org <javascript:_e(%7B%7D,'cvml','Beginners at haskell.org');>http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> <javascript:_e(%7B%7D,'cvml','Beginners at haskell.org');>
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>

-- 
Sent from an iPhone, please excuse brevity and typos.
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From sean at objitsu.com  Mon Mar  2 12:21:54 2015
From: sean at objitsu.com (emacstheviking)
Date: Mon, 2 Mar 2015 12:21:54 +0000
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <CAB0TuzDBdDrQ_8+ttdJy=eExCt2CfT5xxPeZa9G+hYGsHdKYGg@mail.gmail.com>
References: <54F29851.9040202@arqux.com>
 <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>
 <54F36837.5090704@arqux.com>
 <CAEiEuU+V6Ftp0sDVdU4cDYmyPcugrpU9BvNXNDGQdF8RopYKMQ@mail.gmail.com>
 <CAB0TuzDBdDrQ_8+ttdJy=eExCt2CfT5xxPeZa9G+hYGsHdKYGg@mail.gmail.com>
Message-ID: <CAEiEuUJo5cgCcemmto0thkmBDPqhdZ8w3pw=xbr6KJYkhz_iUg@mail.gmail.com>

Took a while to sink in....DEF trying this out tonight
Looks AWESOME if it does what it says!
Getting excited about Haskell again.
THANKS!!!! Yoohoo :)


On 2 March 2015 at 12:10, Julian Birch <julian.birch at gmail.com> wrote:

> Just a small note: if you're suffering from cabal hell, LTS Haskell may
> help you. I described how to use it here:
>
>
> http://www.colourcoding.net/blog/archive/2015/02/22/hello-world-web-application-in-haskell.aspx
>
> Hope someone finds it useful.
>
> J
>
>
> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>
>> Agreed. I will "finish" with a mild semi-demi-rant too regarding the
>> assumption by a lot of posters that we are all academics with access to
>> papers on this and that. I am not and I don't. If you cite a paper, make
>> sure it's available to Joe Public please!
>>
>> As for only sticking to plain vanialla simple Haskell, yes, that works
>> BUT there are some many great libraries that a real time savers...it's such
>> a shame when they won't play well together!
>>
>> So, despite it all, tonight I will be hacking Haskell !
>>
>> :)
>>
>>
>> On 1 March 2015 at 19:27, DJ <jakep at arqux.com> wrote:
>>
>>>  Yeah. I think there are a lot of us out there. I won't rant (much)
>>> here, because I hope to get some help from time to time. Alienating people
>>> is not a good way to get help.
>>>
>>> But:
>>>
>>> There are many things I like about the language, but I have always been
>>> perplexed by the haskell ecosystem in a couple of ways. However good and/or
>>> interesting a language might be, it is not possible to do meat and potatoes
>>> development unless there is a good supply of packages/libraries.
>>>
>>> My first problem was that I often just could not get stuff to install.
>>> The second problem was documentation. I hope it has improved a bit. I was
>>> always flabbergasted at the (apparent) belief that a few lines of API
>>> documentation coughed out by haddock are enough. But that's all there was
>>> in a lot of cases. That, and "refer to xxxx research paper" and "figure it
>>> out from the types". Hah. Any math paper that consisted of just the
>>> equations sorted in alphabetical order would not be well received, I think.
>>>
>>> I really admired that fact that Michael Snoyman wrote a book about
>>> Yesod. For all I know he may not have kept the book up, but at least he
>>> realized that people needed an explanation of how to use his software.
>>>
>>> Well, back to work. I am going to give haskell another try for sure.
>>> It's just too tempting to give up on. If it doesn't work, the next stop
>>> will be sml or ocaml.
>>>
>>> Best,
>>>
>>> - DJ -
>>>
>>>
>>> On 15-03-01 09:56 AM, emacstheviking wrote:
>>>
>>>   I feel your pain...
>>>
>>>
>>>> Confession: I abandoned Haskell two years ago because of frustration
>>>> with cabal and hackage. I decided to get back to the language today, and to
>>>> start with Haskell School of Expression. I immediately run into the problem
>>>> that the first thing I try to install with cabal does not work.
>>>>
>>>>  It is the same reason I have stopped using it. It's a real shame.
>>> Those with better education and understanding than mine should be concerned
>>> that the uptake of the language is stunted by its package manager.
>>>
>>>  I love Haskell. I have taught myself (the beginnings at least) of
>>> group theory just to better comprehend the mindset of monads. For that
>>> alone I am glad I learned Haskell as it has rekindled my interest in maths!
>>>
>>>
>>>
>>>> Thanks for any help. Please tell me things are not just as bad now as
>>>> they were when I left ;-)
>>>>
>>>>  This mail seems to indicate to me that "cabal hell" is here for some
>>> time to come...
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>
>
> --
> Sent from an iPhone, please excuse brevity and typos.
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From julian.birch at gmail.com  Mon Mar  2 12:34:07 2015
From: julian.birch at gmail.com (Julian Birch)
Date: Mon, 2 Mar 2015 12:34:07 +0000
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <CAEiEuUJo5cgCcemmto0thkmBDPqhdZ8w3pw=xbr6KJYkhz_iUg@mail.gmail.com>
References: <54F29851.9040202@arqux.com>
 <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>
 <54F36837.5090704@arqux.com>
 <CAEiEuU+V6Ftp0sDVdU4cDYmyPcugrpU9BvNXNDGQdF8RopYKMQ@mail.gmail.com>
 <CAB0TuzDBdDrQ_8+ttdJy=eExCt2CfT5xxPeZa9G+hYGsHdKYGg@mail.gmail.com>
 <CAEiEuUJo5cgCcemmto0thkmBDPqhdZ8w3pw=xbr6KJYkhz_iUg@mail.gmail.com>
Message-ID: <CAB0TuzBXL8ckz7f11r0jWGYDUTr-gO6Ubx6KBKQ=YijNmX08Dg@mail.gmail.com>

It gets better: they run a hoogle for the exact version, with every
library. :)

On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:

> Took a while to sink in....DEF trying this out tonight
> Looks AWESOME if it does what it says!
> Getting excited about Haskell again.
> THANKS!!!! Yoohoo :)
>
>
> On 2 March 2015 at 12:10, Julian Birch <julian.birch at gmail.com
> <javascript:_e(%7B%7D,'cvml','julian.birch at gmail.com');>> wrote:
>
>> Just a small note: if you're suffering from cabal hell, LTS Haskell may
>> help you. I described how to use it here:
>>
>>
>> http://www.colourcoding.net/blog/archive/2015/02/22/hello-world-web-application-in-haskell.aspx
>>
>> Hope someone finds it useful.
>>
>> J
>>
>>
>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com
>> <javascript:_e(%7B%7D,'cvml','sean at objitsu.com');>> wrote:
>>
>>> Agreed. I will "finish" with a mild semi-demi-rant too regarding the
>>> assumption by a lot of posters that we are all academics with access to
>>> papers on this and that. I am not and I don't. If you cite a paper, make
>>> sure it's available to Joe Public please!
>>>
>>> As for only sticking to plain vanialla simple Haskell, yes, that works
>>> BUT there are some many great libraries that a real time savers...it's such
>>> a shame when they won't play well together!
>>>
>>> So, despite it all, tonight I will be hacking Haskell !
>>>
>>> :)
>>>
>>>
>>> On 1 March 2015 at 19:27, DJ <jakep at arqux.com> wrote:
>>>
>>>>  Yeah. I think there are a lot of us out there. I won't rant (much)
>>>> here, because I hope to get some help from time to time. Alienating people
>>>> is not a good way to get help.
>>>>
>>>> But:
>>>>
>>>> There are many things I like about the language, but I have always been
>>>> perplexed by the haskell ecosystem in a couple of ways. However good and/or
>>>> interesting a language might be, it is not possible to do meat and potatoes
>>>> development unless there is a good supply of packages/libraries.
>>>>
>>>> My first problem was that I often just could not get stuff to install.
>>>> The second problem was documentation. I hope it has improved a bit. I was
>>>> always flabbergasted at the (apparent) belief that a few lines of API
>>>> documentation coughed out by haddock are enough. But that's all there was
>>>> in a lot of cases. That, and "refer to xxxx research paper" and "figure it
>>>> out from the types". Hah. Any math paper that consisted of just the
>>>> equations sorted in alphabetical order would not be well received, I think.
>>>>
>>>> I really admired that fact that Michael Snoyman wrote a book about
>>>> Yesod. For all I know he may not have kept the book up, but at least he
>>>> realized that people needed an explanation of how to use his software.
>>>>
>>>> Well, back to work. I am going to give haskell another try for sure.
>>>> It's just too tempting to give up on. If it doesn't work, the next stop
>>>> will be sml or ocaml.
>>>>
>>>> Best,
>>>>
>>>> - DJ -
>>>>
>>>>
>>>> On 15-03-01 09:56 AM, emacstheviking wrote:
>>>>
>>>>   I feel your pain...
>>>>
>>>>
>>>>> Confession: I abandoned Haskell two years ago because of frustration
>>>>> with cabal and hackage. I decided to get back to the language today, and to
>>>>> start with Haskell School of Expression. I immediately run into the problem
>>>>> that the first thing I try to install with cabal does not work.
>>>>>
>>>>>  It is the same reason I have stopped using it. It's a real shame.
>>>> Those with better education and understanding than mine should be concerned
>>>> that the uptake of the language is stunted by its package manager.
>>>>
>>>>  I love Haskell. I have taught myself (the beginnings at least) of
>>>> group theory just to better comprehend the mindset of monads. For that
>>>> alone I am glad I learned Haskell as it has rekindled my interest in maths!
>>>>
>>>>
>>>>
>>>>> Thanks for any help. Please tell me things are not just as bad now as
>>>>> they were when I left ;-)
>>>>>
>>>>>  This mail seems to indicate to me that "cabal hell" is here for some
>>>> time to come...
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
>>>> Beginners at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>>
>>
>> --
>> Sent from an iPhone, please excuse brevity and typos.
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> <javascript:_e(%7B%7D,'cvml','Beginners at haskell.org');>
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>

-- 
Sent from an iPhone, please excuse brevity and typos.
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From sean at objitsu.com  Mon Mar  2 12:44:38 2015
From: sean at objitsu.com (emacstheviking)
Date: Mon, 2 Mar 2015 12:44:38 +0000
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <CAB0TuzBXL8ckz7f11r0jWGYDUTr-gO6Ubx6KBKQ=YijNmX08Dg@mail.gmail.com>
References: <54F29851.9040202@arqux.com>
 <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>
 <54F36837.5090704@arqux.com>
 <CAEiEuU+V6Ftp0sDVdU4cDYmyPcugrpU9BvNXNDGQdF8RopYKMQ@mail.gmail.com>
 <CAB0TuzDBdDrQ_8+ttdJy=eExCt2CfT5xxPeZa9G+hYGsHdKYGg@mail.gmail.com>
 <CAEiEuUJo5cgCcemmto0thkmBDPqhdZ8w3pw=xbr6KJYkhz_iUg@mail.gmail.com>
 <CAB0TuzBXL8ckz7f11r0jWGYDUTr-gO6Ubx6KBKQ=YijNmX08Dg@mail.gmail.com>
Message-ID: <CAEiEuUJ+qzF85rwYK+CfLUuXgbuPM3_PYDGNZDFw3R-b-_bOxw@mail.gmail.com>

What does that do for me within LTS confines? I know Hoogle, but I don't
understand what you mean by "they run a hoogle for the exact version, with
every library." ...I just want to understand so I can be as excited as
everybody else right now! LOL :)


On 2 March 2015 at 12:34, Julian Birch <julian.birch at gmail.com> wrote:

>
> It gets better: they run a hoogle for the exact version, with every
> library. :)
>
> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>
>> Took a while to sink in....DEF trying this out tonight
>> Looks AWESOME if it does what it says!
>> Getting excited about Haskell again.
>> THANKS!!!! Yoohoo :)
>>
>>
>> On 2 March 2015 at 12:10, Julian Birch <julian.birch at gmail.com> wrote:
>>
>>> Just a small note: if you're suffering from cabal hell, LTS Haskell may
>>> help you. I described how to use it here:
>>>
>>>
>>> http://www.colourcoding.net/blog/archive/2015/02/22/hello-world-web-application-in-haskell.aspx
>>>
>>> Hope someone finds it useful.
>>>
>>> J
>>>
>>>
>>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>>>
>>>> Agreed. I will "finish" with a mild semi-demi-rant too regarding the
>>>> assumption by a lot of posters that we are all academics with access to
>>>> papers on this and that. I am not and I don't. If you cite a paper, make
>>>> sure it's available to Joe Public please!
>>>>
>>>> As for only sticking to plain vanialla simple Haskell, yes, that works
>>>> BUT there are some many great libraries that a real time savers...it's such
>>>> a shame when they won't play well together!
>>>>
>>>> So, despite it all, tonight I will be hacking Haskell !
>>>>
>>>> :)
>>>>
>>>>
>>>> On 1 March 2015 at 19:27, DJ <jakep at arqux.com> wrote:
>>>>
>>>>>  Yeah. I think there are a lot of us out there. I won't rant (much)
>>>>> here, because I hope to get some help from time to time. Alienating people
>>>>> is not a good way to get help.
>>>>>
>>>>> But:
>>>>>
>>>>> There are many things I like about the language, but I have always
>>>>> been perplexed by the haskell ecosystem in a couple of ways. However good
>>>>> and/or interesting a language might be, it is not possible to do meat and
>>>>> potatoes development unless there is a good supply of packages/libraries.
>>>>>
>>>>> My first problem was that I often just could not get stuff to install.
>>>>> The second problem was documentation. I hope it has improved a bit. I was
>>>>> always flabbergasted at the (apparent) belief that a few lines of API
>>>>> documentation coughed out by haddock are enough. But that's all there was
>>>>> in a lot of cases. That, and "refer to xxxx research paper" and "figure it
>>>>> out from the types". Hah. Any math paper that consisted of just the
>>>>> equations sorted in alphabetical order would not be well received, I think.
>>>>>
>>>>> I really admired that fact that Michael Snoyman wrote a book about
>>>>> Yesod. For all I know he may not have kept the book up, but at least he
>>>>> realized that people needed an explanation of how to use his software.
>>>>>
>>>>> Well, back to work. I am going to give haskell another try for sure.
>>>>> It's just too tempting to give up on. If it doesn't work, the next stop
>>>>> will be sml or ocaml.
>>>>>
>>>>> Best,
>>>>>
>>>>> - DJ -
>>>>>
>>>>>
>>>>> On 15-03-01 09:56 AM, emacstheviking wrote:
>>>>>
>>>>>   I feel your pain...
>>>>>
>>>>>
>>>>>> Confession: I abandoned Haskell two years ago because of frustration
>>>>>> with cabal and hackage. I decided to get back to the language today, and to
>>>>>> start with Haskell School of Expression. I immediately run into the problem
>>>>>> that the first thing I try to install with cabal does not work.
>>>>>>
>>>>>>  It is the same reason I have stopped using it. It's a real shame.
>>>>> Those with better education and understanding than mine should be concerned
>>>>> that the uptake of the language is stunted by its package manager.
>>>>>
>>>>>  I love Haskell. I have taught myself (the beginnings at least) of
>>>>> group theory just to better comprehend the mindset of monads. For that
>>>>> alone I am glad I learned Haskell as it has rekindled my interest in maths!
>>>>>
>>>>>
>>>>>
>>>>>> Thanks for any help. Please tell me things are not just as bad now as
>>>>>> they were when I left ;-)
>>>>>>
>>>>>>  This mail seems to indicate to me that "cabal hell" is here for
>>>>> some time to come...
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>>
>>>>
>>>
>>> --
>>> Sent from an iPhone, please excuse brevity and typos.
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>
>
> --
> Sent from an iPhone, please excuse brevity and typos.
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From julian.birch at gmail.com  Mon Mar  2 12:48:44 2015
From: julian.birch at gmail.com (Julian Birch)
Date: Mon, 2 Mar 2015 12:48:44 +0000
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <CAEiEuUJ+qzF85rwYK+CfLUuXgbuPM3_PYDGNZDFw3R-b-_bOxw@mail.gmail.com>
References: <54F29851.9040202@arqux.com>
 <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>
 <54F36837.5090704@arqux.com>
 <CAEiEuU+V6Ftp0sDVdU4cDYmyPcugrpU9BvNXNDGQdF8RopYKMQ@mail.gmail.com>
 <CAB0TuzDBdDrQ_8+ttdJy=eExCt2CfT5xxPeZa9G+hYGsHdKYGg@mail.gmail.com>
 <CAEiEuUJo5cgCcemmto0thkmBDPqhdZ8w3pw=xbr6KJYkhz_iUg@mail.gmail.com>
 <CAB0TuzBXL8ckz7f11r0jWGYDUTr-gO6Ubx6KBKQ=YijNmX08Dg@mail.gmail.com>
 <CAEiEuUJ+qzF85rwYK+CfLUuXgbuPM3_PYDGNZDFw3R-b-_bOxw@mail.gmail.com>
Message-ID: <CAB0TuzDR5ssqZ5C4Efxky8d10p4BqTcP1boPmx2tUQ2FYbvoOQ@mail.gmail.com>

If you go to www.stackage.com/lts/1.10, you'll find a hoogle search box.
It'll give you search results for everything in LTS 1.10.

On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:

> What does that do for me within LTS confines? I know Hoogle, but I don't
> understand what you mean by "they run a hoogle for the exact version,
> with every library." ...I just want to understand so I can be as excited as
> everybody else right now! LOL :)
>
>
> On 2 March 2015 at 12:34, Julian Birch <julian.birch at gmail.com
> <javascript:_e(%7B%7D,'cvml','julian.birch at gmail.com');>> wrote:
>
>>
>> It gets better: they run a hoogle for the exact version, with every
>> library. :)
>>
>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com
>> <javascript:_e(%7B%7D,'cvml','sean at objitsu.com');>> wrote:
>>
>>> Took a while to sink in....DEF trying this out tonight
>>> Looks AWESOME if it does what it says!
>>> Getting excited about Haskell again.
>>> THANKS!!!! Yoohoo :)
>>>
>>>
>>> On 2 March 2015 at 12:10, Julian Birch <julian.birch at gmail.com> wrote:
>>>
>>>> Just a small note: if you're suffering from cabal hell, LTS Haskell may
>>>> help you. I described how to use it here:
>>>>
>>>>
>>>> http://www.colourcoding.net/blog/archive/2015/02/22/hello-world-web-application-in-haskell.aspx
>>>>
>>>> Hope someone finds it useful.
>>>>
>>>> J
>>>>
>>>>
>>>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>>>>
>>>>> Agreed. I will "finish" with a mild semi-demi-rant too regarding the
>>>>> assumption by a lot of posters that we are all academics with access to
>>>>> papers on this and that. I am not and I don't. If you cite a paper, make
>>>>> sure it's available to Joe Public please!
>>>>>
>>>>> As for only sticking to plain vanialla simple Haskell, yes, that works
>>>>> BUT there are some many great libraries that a real time savers...it's such
>>>>> a shame when they won't play well together!
>>>>>
>>>>> So, despite it all, tonight I will be hacking Haskell !
>>>>>
>>>>> :)
>>>>>
>>>>>
>>>>> On 1 March 2015 at 19:27, DJ <jakep at arqux.com> wrote:
>>>>>
>>>>>>  Yeah. I think there are a lot of us out there. I won't rant (much)
>>>>>> here, because I hope to get some help from time to time. Alienating people
>>>>>> is not a good way to get help.
>>>>>>
>>>>>> But:
>>>>>>
>>>>>> There are many things I like about the language, but I have always
>>>>>> been perplexed by the haskell ecosystem in a couple of ways. However good
>>>>>> and/or interesting a language might be, it is not possible to do meat and
>>>>>> potatoes development unless there is a good supply of packages/libraries.
>>>>>>
>>>>>> My first problem was that I often just could not get stuff to
>>>>>> install. The second problem was documentation. I hope it has improved a
>>>>>> bit. I was always flabbergasted at the (apparent) belief that a few lines
>>>>>> of API documentation coughed out by haddock are enough. But that's all
>>>>>> there was in a lot of cases. That, and "refer to xxxx research paper" and
>>>>>> "figure it out from the types". Hah. Any math paper that consisted of just
>>>>>> the equations sorted in alphabetical order would not be well received, I
>>>>>> think.
>>>>>>
>>>>>> I really admired that fact that Michael Snoyman wrote a book about
>>>>>> Yesod. For all I know he may not have kept the book up, but at least he
>>>>>> realized that people needed an explanation of how to use his software.
>>>>>>
>>>>>> Well, back to work. I am going to give haskell another try for sure.
>>>>>> It's just too tempting to give up on. If it doesn't work, the next stop
>>>>>> will be sml or ocaml.
>>>>>>
>>>>>> Best,
>>>>>>
>>>>>> - DJ -
>>>>>>
>>>>>>
>>>>>> On 15-03-01 09:56 AM, emacstheviking wrote:
>>>>>>
>>>>>>   I feel your pain...
>>>>>>
>>>>>>
>>>>>>> Confession: I abandoned Haskell two years ago because of frustration
>>>>>>> with cabal and hackage. I decided to get back to the language today, and to
>>>>>>> start with Haskell School of Expression. I immediately run into the problem
>>>>>>> that the first thing I try to install with cabal does not work.
>>>>>>>
>>>>>>>  It is the same reason I have stopped using it. It's a real shame.
>>>>>> Those with better education and understanding than mine should be concerned
>>>>>> that the uptake of the language is stunted by its package manager.
>>>>>>
>>>>>>  I love Haskell. I have taught myself (the beginnings at least) of
>>>>>> group theory just to better comprehend the mindset of monads. For that
>>>>>> alone I am glad I learned Haskell as it has rekindled my interest in maths!
>>>>>>
>>>>>>
>>>>>>
>>>>>>> Thanks for any help. Please tell me things are not just as bad now
>>>>>>> as they were when I left ;-)
>>>>>>>
>>>>>>>  This mail seems to indicate to me that "cabal hell" is here for
>>>>>> some time to come...
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> Beginners mailing list
>>>>>> Beginners at haskell.org
>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>
>>>>>>
>>>>>
>>>>
>>>> --
>>>> Sent from an iPhone, please excuse brevity and typos.
>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
>>>> Beginners at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>>
>>
>> --
>> Sent from an iPhone, please excuse brevity and typos.
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> <javascript:_e(%7B%7D,'cvml','Beginners at haskell.org');>
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>

-- 
Sent from an iPhone, please excuse brevity and typos.
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From sean at objitsu.com  Mon Mar  2 13:04:34 2015
From: sean at objitsu.com (emacstheviking)
Date: Mon, 2 Mar 2015 13:04:34 +0000
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <CAB0TuzDR5ssqZ5C4Efxky8d10p4BqTcP1boPmx2tUQ2FYbvoOQ@mail.gmail.com>
References: <54F29851.9040202@arqux.com>
 <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>
 <54F36837.5090704@arqux.com>
 <CAEiEuU+V6Ftp0sDVdU4cDYmyPcugrpU9BvNXNDGQdF8RopYKMQ@mail.gmail.com>
 <CAB0TuzDBdDrQ_8+ttdJy=eExCt2CfT5xxPeZa9G+hYGsHdKYGg@mail.gmail.com>
 <CAEiEuUJo5cgCcemmto0thkmBDPqhdZ8w3pw=xbr6KJYkhz_iUg@mail.gmail.com>
 <CAB0TuzBXL8ckz7f11r0jWGYDUTr-gO6Ubx6KBKQ=YijNmX08Dg@mail.gmail.com>
 <CAEiEuUJ+qzF85rwYK+CfLUuXgbuPM3_PYDGNZDFw3R-b-_bOxw@mail.gmail.com>
 <CAB0TuzDR5ssqZ5C4Efxky8d10p4BqTcP1boPmx2tUQ2FYbvoOQ@mail.gmail.com>
Message-ID: <CAEiEuU+__riOPUwPiBXNJ564A2M6ieMTJop3gc-+RnwLXGUZdQ@mail.gmail.com>

Plunk! (penny drops)
I get it...now that is truly the icing on the cabal-hell free cake!
Thanks again Julian.


On 2 March 2015 at 12:48, Julian Birch <julian.birch at gmail.com> wrote:

> If you go to www.stackage.com/lts/1.10, you'll find a hoogle search box.
> It'll give you search results for everything in LTS 1.10.
>
>
> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>
>> What does that do for me within LTS confines? I know Hoogle, but I don't
>> understand what you mean by "they run a hoogle for the exact version,
>> with every library." ...I just want to understand so I can be as excited as
>> everybody else right now! LOL :)
>>
>>
>> On 2 March 2015 at 12:34, Julian Birch <julian.birch at gmail.com> wrote:
>>
>>>
>>> It gets better: they run a hoogle for the exact version, with every
>>> library. :)
>>>
>>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>>>
>>>> Took a while to sink in....DEF trying this out tonight
>>>> Looks AWESOME if it does what it says!
>>>> Getting excited about Haskell again.
>>>> THANKS!!!! Yoohoo :)
>>>>
>>>>
>>>> On 2 March 2015 at 12:10, Julian Birch <julian.birch at gmail.com> wrote:
>>>>
>>>>> Just a small note: if you're suffering from cabal hell, LTS Haskell
>>>>> may help you. I described how to use it here:
>>>>>
>>>>>
>>>>> http://www.colourcoding.net/blog/archive/2015/02/22/hello-world-web-application-in-haskell.aspx
>>>>>
>>>>> Hope someone finds it useful.
>>>>>
>>>>> J
>>>>>
>>>>>
>>>>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>>>>>
>>>>>> Agreed. I will "finish" with a mild semi-demi-rant too regarding the
>>>>>> assumption by a lot of posters that we are all academics with access to
>>>>>> papers on this and that. I am not and I don't. If you cite a paper, make
>>>>>> sure it's available to Joe Public please!
>>>>>>
>>>>>> As for only sticking to plain vanialla simple Haskell, yes, that
>>>>>> works BUT there are some many great libraries that a real time
>>>>>> savers...it's such a shame when they won't play well together!
>>>>>>
>>>>>> So, despite it all, tonight I will be hacking Haskell !
>>>>>>
>>>>>> :)
>>>>>>
>>>>>>
>>>>>> On 1 March 2015 at 19:27, DJ <jakep at arqux.com> wrote:
>>>>>>
>>>>>>>  Yeah. I think there are a lot of us out there. I won't rant (much)
>>>>>>> here, because I hope to get some help from time to time. Alienating people
>>>>>>> is not a good way to get help.
>>>>>>>
>>>>>>> But:
>>>>>>>
>>>>>>> There are many things I like about the language, but I have always
>>>>>>> been perplexed by the haskell ecosystem in a couple of ways. However good
>>>>>>> and/or interesting a language might be, it is not possible to do meat and
>>>>>>> potatoes development unless there is a good supply of packages/libraries.
>>>>>>>
>>>>>>> My first problem was that I often just could not get stuff to
>>>>>>> install. The second problem was documentation. I hope it has improved a
>>>>>>> bit. I was always flabbergasted at the (apparent) belief that a few lines
>>>>>>> of API documentation coughed out by haddock are enough. But that's all
>>>>>>> there was in a lot of cases. That, and "refer to xxxx research paper" and
>>>>>>> "figure it out from the types". Hah. Any math paper that consisted of just
>>>>>>> the equations sorted in alphabetical order would not be well received, I
>>>>>>> think.
>>>>>>>
>>>>>>> I really admired that fact that Michael Snoyman wrote a book about
>>>>>>> Yesod. For all I know he may not have kept the book up, but at least he
>>>>>>> realized that people needed an explanation of how to use his software.
>>>>>>>
>>>>>>> Well, back to work. I am going to give haskell another try for sure.
>>>>>>> It's just too tempting to give up on. If it doesn't work, the next stop
>>>>>>> will be sml or ocaml.
>>>>>>>
>>>>>>> Best,
>>>>>>>
>>>>>>> - DJ -
>>>>>>>
>>>>>>>
>>>>>>> On 15-03-01 09:56 AM, emacstheviking wrote:
>>>>>>>
>>>>>>>   I feel your pain...
>>>>>>>
>>>>>>>
>>>>>>>> Confession: I abandoned Haskell two years ago because of
>>>>>>>> frustration with cabal and hackage. I decided to get back to the language
>>>>>>>> today, and to start with Haskell School of Expression. I immediately run
>>>>>>>> into the problem that the first thing I try to install with cabal does not
>>>>>>>> work.
>>>>>>>>
>>>>>>>>  It is the same reason I have stopped using it. It's a real shame.
>>>>>>> Those with better education and understanding than mine should be concerned
>>>>>>> that the uptake of the language is stunted by its package manager.
>>>>>>>
>>>>>>>  I love Haskell. I have taught myself (the beginnings at least) of
>>>>>>> group theory just to better comprehend the mindset of monads. For that
>>>>>>> alone I am glad I learned Haskell as it has rekindled my interest in maths!
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>> Thanks for any help. Please tell me things are not just as bad now
>>>>>>>> as they were when I left ;-)
>>>>>>>>
>>>>>>>>  This mail seems to indicate to me that "cabal hell" is here for
>>>>>>> some time to come...
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> Beginners mailing list
>>>>>>> Beginners at haskell.org
>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>> --
>>>>> Sent from an iPhone, please excuse brevity and typos.
>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>>
>>>>
>>>
>>> --
>>> Sent from an iPhone, please excuse brevity and typos.
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>
>
> --
> Sent from an iPhone, please excuse brevity and typos.
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From sean at objitsu.com  Mon Mar  2 13:06:22 2015
From: sean at objitsu.com (emacstheviking)
Date: Mon, 2 Mar 2015 13:06:22 +0000
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <CAEiEuU+__riOPUwPiBXNJ564A2M6ieMTJop3gc-+RnwLXGUZdQ@mail.gmail.com>
References: <54F29851.9040202@arqux.com>
 <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>
 <54F36837.5090704@arqux.com>
 <CAEiEuU+V6Ftp0sDVdU4cDYmyPcugrpU9BvNXNDGQdF8RopYKMQ@mail.gmail.com>
 <CAB0TuzDBdDrQ_8+ttdJy=eExCt2CfT5xxPeZa9G+hYGsHdKYGg@mail.gmail.com>
 <CAEiEuUJo5cgCcemmto0thkmBDPqhdZ8w3pw=xbr6KJYkhz_iUg@mail.gmail.com>
 <CAB0TuzBXL8ckz7f11r0jWGYDUTr-gO6Ubx6KBKQ=YijNmX08Dg@mail.gmail.com>
 <CAEiEuUJ+qzF85rwYK+CfLUuXgbuPM3_PYDGNZDFw3R-b-_bOxw@mail.gmail.com>
 <CAB0TuzDR5ssqZ5C4Efxky8d10p4BqTcP1boPmx2tUQ2FYbvoOQ@mail.gmail.com>
 <CAEiEuU+__riOPUwPiBXNJ564A2M6ieMTJop3gc-+RnwLXGUZdQ@mail.gmail.com>
Message-ID: <CAEiEuULYj=VPkD3R+dYurm+5kDEPjd8WmOVBd8D+nnhrGKrwsw@mail.gmail.com>

PS: Did you mean stackage .ORG not .COM


On 2 March 2015 at 13:04, emacstheviking <sean at objitsu.com> wrote:

> Plunk! (penny drops)
> I get it...now that is truly the icing on the cabal-hell free cake!
> Thanks again Julian.
>
>
> On 2 March 2015 at 12:48, Julian Birch <julian.birch at gmail.com> wrote:
>
>> If you go to www.stackage.com/lts/1.10, you'll find a hoogle search box.
>> It'll give you search results for everything in LTS 1.10.
>>
>>
>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>>
>>> What does that do for me within LTS confines? I know Hoogle, but I don't
>>> understand what you mean by "they run a hoogle for the exact version,
>>> with every library." ...I just want to understand so I can be as excited as
>>> everybody else right now! LOL :)
>>>
>>>
>>> On 2 March 2015 at 12:34, Julian Birch <julian.birch at gmail.com> wrote:
>>>
>>>>
>>>> It gets better: they run a hoogle for the exact version, with every
>>>> library. :)
>>>>
>>>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>>>>
>>>>> Took a while to sink in....DEF trying this out tonight
>>>>> Looks AWESOME if it does what it says!
>>>>> Getting excited about Haskell again.
>>>>> THANKS!!!! Yoohoo :)
>>>>>
>>>>>
>>>>> On 2 March 2015 at 12:10, Julian Birch <julian.birch at gmail.com> wrote:
>>>>>
>>>>>> Just a small note: if you're suffering from cabal hell, LTS Haskell
>>>>>> may help you. I described how to use it here:
>>>>>>
>>>>>>
>>>>>> http://www.colourcoding.net/blog/archive/2015/02/22/hello-world-web-application-in-haskell.aspx
>>>>>>
>>>>>> Hope someone finds it useful.
>>>>>>
>>>>>> J
>>>>>>
>>>>>>
>>>>>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>>>>>>
>>>>>>> Agreed. I will "finish" with a mild semi-demi-rant too regarding the
>>>>>>> assumption by a lot of posters that we are all academics with access to
>>>>>>> papers on this and that. I am not and I don't. If you cite a paper, make
>>>>>>> sure it's available to Joe Public please!
>>>>>>>
>>>>>>> As for only sticking to plain vanialla simple Haskell, yes, that
>>>>>>> works BUT there are some many great libraries that a real time
>>>>>>> savers...it's such a shame when they won't play well together!
>>>>>>>
>>>>>>> So, despite it all, tonight I will be hacking Haskell !
>>>>>>>
>>>>>>> :)
>>>>>>>
>>>>>>>
>>>>>>> On 1 March 2015 at 19:27, DJ <jakep at arqux.com> wrote:
>>>>>>>
>>>>>>>>  Yeah. I think there are a lot of us out there. I won't rant (much)
>>>>>>>> here, because I hope to get some help from time to time. Alienating people
>>>>>>>> is not a good way to get help.
>>>>>>>>
>>>>>>>> But:
>>>>>>>>
>>>>>>>> There are many things I like about the language, but I have always
>>>>>>>> been perplexed by the haskell ecosystem in a couple of ways. However good
>>>>>>>> and/or interesting a language might be, it is not possible to do meat and
>>>>>>>> potatoes development unless there is a good supply of packages/libraries.
>>>>>>>>
>>>>>>>> My first problem was that I often just could not get stuff to
>>>>>>>> install. The second problem was documentation. I hope it has improved a
>>>>>>>> bit. I was always flabbergasted at the (apparent) belief that a few lines
>>>>>>>> of API documentation coughed out by haddock are enough. But that's all
>>>>>>>> there was in a lot of cases. That, and "refer to xxxx research paper" and
>>>>>>>> "figure it out from the types". Hah. Any math paper that consisted of just
>>>>>>>> the equations sorted in alphabetical order would not be well received, I
>>>>>>>> think.
>>>>>>>>
>>>>>>>> I really admired that fact that Michael Snoyman wrote a book about
>>>>>>>> Yesod. For all I know he may not have kept the book up, but at least he
>>>>>>>> realized that people needed an explanation of how to use his software.
>>>>>>>>
>>>>>>>> Well, back to work. I am going to give haskell another try for
>>>>>>>> sure. It's just too tempting to give up on. If it doesn't work, the next
>>>>>>>> stop will be sml or ocaml.
>>>>>>>>
>>>>>>>> Best,
>>>>>>>>
>>>>>>>> - DJ -
>>>>>>>>
>>>>>>>>
>>>>>>>> On 15-03-01 09:56 AM, emacstheviking wrote:
>>>>>>>>
>>>>>>>>   I feel your pain...
>>>>>>>>
>>>>>>>>
>>>>>>>>> Confession: I abandoned Haskell two years ago because of
>>>>>>>>> frustration with cabal and hackage. I decided to get back to the language
>>>>>>>>> today, and to start with Haskell School of Expression. I immediately run
>>>>>>>>> into the problem that the first thing I try to install with cabal does not
>>>>>>>>> work.
>>>>>>>>>
>>>>>>>>>  It is the same reason I have stopped using it. It's a real
>>>>>>>> shame. Those with better education and understanding than mine should be
>>>>>>>> concerned that the uptake of the language is stunted by its package manager.
>>>>>>>>
>>>>>>>>  I love Haskell. I have taught myself (the beginnings at least) of
>>>>>>>> group theory just to better comprehend the mindset of monads. For that
>>>>>>>> alone I am glad I learned Haskell as it has rekindled my interest in maths!
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>> Thanks for any help. Please tell me things are not just as bad now
>>>>>>>>> as they were when I left ;-)
>>>>>>>>>
>>>>>>>>>  This mail seems to indicate to me that "cabal hell" is here for
>>>>>>>> some time to come...
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> Beginners mailing list
>>>>>>>> Beginners at haskell.org
>>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>> --
>>>>>> Sent from an iPhone, please excuse brevity and typos.
>>>>>>
>>>>>> _______________________________________________
>>>>>> Beginners mailing list
>>>>>> Beginners at haskell.org
>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>
>>>>>>
>>>>>
>>>>
>>>> --
>>>> Sent from an iPhone, please excuse brevity and typos.
>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
>>>> Beginners at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>>
>>
>> --
>> Sent from an iPhone, please excuse brevity and typos.
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
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From julian.birch at gmail.com  Mon Mar  2 13:08:58 2015
From: julian.birch at gmail.com (Julian Birch)
Date: Mon, 2 Mar 2015 13:08:58 +0000
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <CAEiEuULYj=VPkD3R+dYurm+5kDEPjd8WmOVBd8D+nnhrGKrwsw@mail.gmail.com>
References: <54F29851.9040202@arqux.com>
 <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>
 <54F36837.5090704@arqux.com>
 <CAEiEuU+V6Ftp0sDVdU4cDYmyPcugrpU9BvNXNDGQdF8RopYKMQ@mail.gmail.com>
 <CAB0TuzDBdDrQ_8+ttdJy=eExCt2CfT5xxPeZa9G+hYGsHdKYGg@mail.gmail.com>
 <CAEiEuUJo5cgCcemmto0thkmBDPqhdZ8w3pw=xbr6KJYkhz_iUg@mail.gmail.com>
 <CAB0TuzBXL8ckz7f11r0jWGYDUTr-gO6Ubx6KBKQ=YijNmX08Dg@mail.gmail.com>
 <CAEiEuUJ+qzF85rwYK+CfLUuXgbuPM3_PYDGNZDFw3R-b-_bOxw@mail.gmail.com>
 <CAB0TuzDR5ssqZ5C4Efxky8d10p4BqTcP1boPmx2tUQ2FYbvoOQ@mail.gmail.com>
 <CAEiEuU+__riOPUwPiBXNJ564A2M6ieMTJop3gc-+RnwLXGUZdQ@mail.gmail.com>
 <CAEiEuULYj=VPkD3R+dYurm+5kDEPjd8WmOVBd8D+nnhrGKrwsw@mail.gmail.com>
Message-ID: <CAB0TuzB95xQSp0XaYhy5NK-uRrqypXvm2SN77BURuS_yQ=SagQ@mail.gmail.com>

Yes, sorry, the perils of typing URLs out on your phone... :)

On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:

> PS: Did you mean stackage .ORG not .COM
>
>
> On 2 March 2015 at 13:04, emacstheviking <sean at objitsu.com
> <javascript:_e(%7B%7D,'cvml','sean at objitsu.com');>> wrote:
>
>> Plunk! (penny drops)
>> I get it...now that is truly the icing on the cabal-hell free cake!
>> Thanks again Julian.
>>
>>
>> On 2 March 2015 at 12:48, Julian Birch <julian.birch at gmail.com
>> <javascript:_e(%7B%7D,'cvml','julian.birch at gmail.com');>> wrote:
>>
>>> If you go to www.stackage.com/lts/1.10, you'll find a hoogle search box.
>>> It'll give you search results for everything in LTS 1.10.
>>>
>>>
>>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com
>>> <javascript:_e(%7B%7D,'cvml','sean at objitsu.com');>> wrote:
>>>
>>>> What does that do for me within LTS confines? I know Hoogle, but I
>>>> don't understand what you mean by "they run a hoogle for the exact
>>>> version, with every library." ...I just want to understand so I can be as
>>>> excited as everybody else right now! LOL :)
>>>>
>>>>
>>>> On 2 March 2015 at 12:34, Julian Birch <julian.birch at gmail.com> wrote:
>>>>
>>>>>
>>>>> It gets better: they run a hoogle for the exact version, with every
>>>>> library. :)
>>>>>
>>>>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>>>>>
>>>>>> Took a while to sink in....DEF trying this out tonight
>>>>>> Looks AWESOME if it does what it says!
>>>>>> Getting excited about Haskell again.
>>>>>> THANKS!!!! Yoohoo :)
>>>>>>
>>>>>>
>>>>>> On 2 March 2015 at 12:10, Julian Birch <julian.birch at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>> Just a small note: if you're suffering from cabal hell, LTS Haskell
>>>>>>> may help you. I described how to use it here:
>>>>>>>
>>>>>>>
>>>>>>> http://www.colourcoding.net/blog/archive/2015/02/22/hello-world-web-application-in-haskell.aspx
>>>>>>>
>>>>>>> Hope someone finds it useful.
>>>>>>>
>>>>>>> J
>>>>>>>
>>>>>>>
>>>>>>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>>>>>>>
>>>>>>>> Agreed. I will "finish" with a mild semi-demi-rant too regarding
>>>>>>>> the assumption by a lot of posters that we are all academics with access to
>>>>>>>> papers on this and that. I am not and I don't. If you cite a paper, make
>>>>>>>> sure it's available to Joe Public please!
>>>>>>>>
>>>>>>>> As for only sticking to plain vanialla simple Haskell, yes, that
>>>>>>>> works BUT there are some many great libraries that a real time
>>>>>>>> savers...it's such a shame when they won't play well together!
>>>>>>>>
>>>>>>>> So, despite it all, tonight I will be hacking Haskell !
>>>>>>>>
>>>>>>>> :)
>>>>>>>>
>>>>>>>>
>>>>>>>> On 1 March 2015 at 19:27, DJ <jakep at arqux.com> wrote:
>>>>>>>>
>>>>>>>>>  Yeah. I think there are a lot of us out there. I won't rant
>>>>>>>>> (much) here, because I hope to get some help from time to time. Alienating
>>>>>>>>> people is not a good way to get help.
>>>>>>>>>
>>>>>>>>> But:
>>>>>>>>>
>>>>>>>>> There are many things I like about the language, but I have always
>>>>>>>>> been perplexed by the haskell ecosystem in a couple of ways. However good
>>>>>>>>> and/or interesting a language might be, it is not possible to do meat and
>>>>>>>>> potatoes development unless there is a good supply of packages/libraries.
>>>>>>>>>
>>>>>>>>> My first problem was that I often just could not get stuff to
>>>>>>>>> install. The second problem was documentation. I hope it has improved a
>>>>>>>>> bit. I was always flabbergasted at the (apparent) belief that a few lines
>>>>>>>>> of API documentation coughed out by haddock are enough. But that's all
>>>>>>>>> there was in a lot of cases. That, and "refer to xxxx research paper" and
>>>>>>>>> "figure it out from the types". Hah. Any math paper that consisted of just
>>>>>>>>> the equations sorted in alphabetical order would not be well received, I
>>>>>>>>> think.
>>>>>>>>>
>>>>>>>>> I really admired that fact that Michael Snoyman wrote a book about
>>>>>>>>> Yesod. For all I know he may not have kept the book up, but at least he
>>>>>>>>> realized that people needed an explanation of how to use his software.
>>>>>>>>>
>>>>>>>>> Well, back to work. I am going to give haskell another try for
>>>>>>>>> sure. It's just too tempting to give up on. If it doesn't work, the next
>>>>>>>>> stop will be sml or ocaml.
>>>>>>>>>
>>>>>>>>> Best,
>>>>>>>>>
>>>>>>>>> - DJ -
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 15-03-01 09:56 AM, emacstheviking wrote:
>>>>>>>>>
>>>>>>>>>   I feel your pain...
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> Confession: I abandoned Haskell two years ago because of
>>>>>>>>>> frustration with cabal and hackage. I decided to get back to the language
>>>>>>>>>> today, and to start with Haskell School of Expression. I immediately run
>>>>>>>>>> into the problem that the first thing I try to install with cabal does not
>>>>>>>>>> work.
>>>>>>>>>>
>>>>>>>>>>  It is the same reason I have stopped using it. It's a real
>>>>>>>>> shame. Those with better education and understanding than mine should be
>>>>>>>>> concerned that the uptake of the language is stunted by its package manager.
>>>>>>>>>
>>>>>>>>>  I love Haskell. I have taught myself (the beginnings at least)
>>>>>>>>> of group theory just to better comprehend the mindset of monads. For that
>>>>>>>>> alone I am glad I learned Haskell as it has rekindled my interest in maths!
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> Thanks for any help. Please tell me things are not just as bad
>>>>>>>>>> now as they were when I left ;-)
>>>>>>>>>>
>>>>>>>>>>  This mail seems to indicate to me that "cabal hell" is here for
>>>>>>>>> some time to come...
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> Beginners mailing list
>>>>>>>>> Beginners at haskell.org
>>>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> Sent from an iPhone, please excuse brevity and typos.
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> Beginners mailing list
>>>>>>> Beginners at haskell.org
>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>> --
>>>>> Sent from an iPhone, please excuse brevity and typos.
>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>>
>>>>
>>>
>>> --
>>> Sent from an iPhone, please excuse brevity and typos.
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> <javascript:_e(%7B%7D,'cvml','Beginners at haskell.org');>
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>
>

-- 
Sent from an iPhone, please excuse brevity and typos.
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From objitsu at gmail.com  Mon Mar  2 13:12:41 2015
From: objitsu at gmail.com (emacstheviking)
Date: Mon, 2 Mar 2015 13:12:41 +0000
Subject: [Haskell-beginners] trying to install libraries for Haskell
 School of Expression
In-Reply-To: <CAB0TuzB95xQSp0XaYhy5NK-uRrqypXvm2SN77BURuS_yQ=SagQ@mail.gmail.com>
References: <54F29851.9040202@arqux.com>
 <CAEiEuUJim7h=vYR0HnwoLvB_TEgYamikRz9uWsJbSGaM9B+pxQ@mail.gmail.com>
 <54F36837.5090704@arqux.com>
 <CAEiEuU+V6Ftp0sDVdU4cDYmyPcugrpU9BvNXNDGQdF8RopYKMQ@mail.gmail.com>
 <CAB0TuzDBdDrQ_8+ttdJy=eExCt2CfT5xxPeZa9G+hYGsHdKYGg@mail.gmail.com>
 <CAEiEuUJo5cgCcemmto0thkmBDPqhdZ8w3pw=xbr6KJYkhz_iUg@mail.gmail.com>
 <CAB0TuzBXL8ckz7f11r0jWGYDUTr-gO6Ubx6KBKQ=YijNmX08Dg@mail.gmail.com>
 <CAEiEuUJ+qzF85rwYK+CfLUuXgbuPM3_PYDGNZDFw3R-b-_bOxw@mail.gmail.com>
 <CAB0TuzDR5ssqZ5C4Efxky8d10p4BqTcP1boPmx2tUQ2FYbvoOQ@mail.gmail.com>
 <CAEiEuU+__riOPUwPiBXNJ564A2M6ieMTJop3gc-+RnwLXGUZdQ@mail.gmail.com>
 <CAEiEuULYj=VPkD3R+dYurm+5kDEPjd8WmOVBd8D+nnhrGKrwsw@mail.gmail.com>
 <CAB0TuzB95xQSp0XaYhy5NK-uRrqypXvm2SN77BURuS_yQ=SagQ@mail.gmail.com>
Message-ID: <CAEiEuU+GU0ZSbgRQVCZGbRw_urq27zuWzXQZrBRRdURWkzROkg@mail.gmail.com>

Coupled with auto-correction that you don't see until after you send...

On 2 March 2015 at 13:08, Julian Birch <julian.birch at gmail.com> wrote:

> Yes, sorry, the perils of typing URLs out on your phone... :)
>
>
> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>
>> PS: Did you mean stackage .ORG not .COM
>>
>>
>> On 2 March 2015 at 13:04, emacstheviking <sean at objitsu.com> wrote:
>>
>>> Plunk! (penny drops)
>>> I get it...now that is truly the icing on the cabal-hell free cake!
>>> Thanks again Julian.
>>>
>>>
>>> On 2 March 2015 at 12:48, Julian Birch <julian.birch at gmail.com> wrote:
>>>
>>>> If you go to www.stackage.com/lts/1.10, you'll find a hoogle search box.
>>>> It'll give you search results for everything in LTS 1.10.
>>>>
>>>>
>>>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>>>>
>>>>> What does that do for me within LTS confines? I know Hoogle, but I
>>>>> don't understand what you mean by "they run a hoogle for the exact
>>>>> version, with every library." ...I just want to understand so I can be as
>>>>> excited as everybody else right now! LOL :)
>>>>>
>>>>>
>>>>> On 2 March 2015 at 12:34, Julian Birch <julian.birch at gmail.com> wrote:
>>>>>
>>>>>>
>>>>>> It gets better: they run a hoogle for the exact version, with every
>>>>>> library. :)
>>>>>>
>>>>>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>>>>>>
>>>>>>> Took a while to sink in....DEF trying this out tonight
>>>>>>> Looks AWESOME if it does what it says!
>>>>>>> Getting excited about Haskell again.
>>>>>>> THANKS!!!! Yoohoo :)
>>>>>>>
>>>>>>>
>>>>>>> On 2 March 2015 at 12:10, Julian Birch <julian.birch at gmail.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> Just a small note: if you're suffering from cabal hell, LTS Haskell
>>>>>>>> may help you. I described how to use it here:
>>>>>>>>
>>>>>>>>
>>>>>>>> http://www.colourcoding.net/blog/archive/2015/02/22/hello-world-web-application-in-haskell.aspx
>>>>>>>>
>>>>>>>> Hope someone finds it useful.
>>>>>>>>
>>>>>>>> J
>>>>>>>>
>>>>>>>>
>>>>>>>> On Monday, March 2, 2015, emacstheviking <sean at objitsu.com> wrote:
>>>>>>>>
>>>>>>>>> Agreed. I will "finish" with a mild semi-demi-rant too regarding
>>>>>>>>> the assumption by a lot of posters that we are all academics with access to
>>>>>>>>> papers on this and that. I am not and I don't. If you cite a paper, make
>>>>>>>>> sure it's available to Joe Public please!
>>>>>>>>>
>>>>>>>>> As for only sticking to plain vanialla simple Haskell, yes, that
>>>>>>>>> works BUT there are some many great libraries that a real time
>>>>>>>>> savers...it's such a shame when they won't play well together!
>>>>>>>>>
>>>>>>>>> So, despite it all, tonight I will be hacking Haskell !
>>>>>>>>>
>>>>>>>>> :)
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 1 March 2015 at 19:27, DJ <jakep at arqux.com> wrote:
>>>>>>>>>
>>>>>>>>>>  Yeah. I think there are a lot of us out there. I won't rant
>>>>>>>>>> (much) here, because I hope to get some help from time to time. Alienating
>>>>>>>>>> people is not a good way to get help.
>>>>>>>>>>
>>>>>>>>>> But:
>>>>>>>>>>
>>>>>>>>>> There are many things I like about the language, but I have
>>>>>>>>>> always been perplexed by the haskell ecosystem in a couple of ways. However
>>>>>>>>>> good and/or interesting a language might be, it is not possible to do meat
>>>>>>>>>> and potatoes development unless there is a good supply of
>>>>>>>>>> packages/libraries.
>>>>>>>>>>
>>>>>>>>>> My first problem was that I often just could not get stuff to
>>>>>>>>>> install. The second problem was documentation. I hope it has improved a
>>>>>>>>>> bit. I was always flabbergasted at the (apparent) belief that a few lines
>>>>>>>>>> of API documentation coughed out by haddock are enough. But that's all
>>>>>>>>>> there was in a lot of cases. That, and "refer to xxxx research paper" and
>>>>>>>>>> "figure it out from the types". Hah. Any math paper that consisted of just
>>>>>>>>>> the equations sorted in alphabetical order would not be well received, I
>>>>>>>>>> think.
>>>>>>>>>>
>>>>>>>>>> I really admired that fact that Michael Snoyman wrote a book
>>>>>>>>>> about Yesod. For all I know he may not have kept the book up, but at least
>>>>>>>>>> he realized that people needed an explanation of how to use his software.
>>>>>>>>>>
>>>>>>>>>> Well, back to work. I am going to give haskell another try for
>>>>>>>>>> sure. It's just too tempting to give up on. If it doesn't work, the next
>>>>>>>>>> stop will be sml or ocaml.
>>>>>>>>>>
>>>>>>>>>> Best,
>>>>>>>>>>
>>>>>>>>>> - DJ -
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On 15-03-01 09:56 AM, emacstheviking wrote:
>>>>>>>>>>
>>>>>>>>>>   I feel your pain...
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>> Confession: I abandoned Haskell two years ago because of
>>>>>>>>>>> frustration with cabal and hackage. I decided to get back to the language
>>>>>>>>>>> today, and to start with Haskell School of Expression. I immediately run
>>>>>>>>>>> into the problem that the first thing I try to install with cabal does not
>>>>>>>>>>> work.
>>>>>>>>>>>
>>>>>>>>>>>  It is the same reason I have stopped using it. It's a real
>>>>>>>>>> shame. Those with better education and understanding than mine should be
>>>>>>>>>> concerned that the uptake of the language is stunted by its package manager.
>>>>>>>>>>
>>>>>>>>>>  I love Haskell. I have taught myself (the beginnings at least)
>>>>>>>>>> of group theory just to better comprehend the mindset of monads. For that
>>>>>>>>>> alone I am glad I learned Haskell as it has rekindled my interest in maths!
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>> Thanks for any help. Please tell me things are not just as bad
>>>>>>>>>>> now as they were when I left ;-)
>>>>>>>>>>>
>>>>>>>>>>>  This mail seems to indicate to me that "cabal hell" is here
>>>>>>>>>> for some time to come...
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> _______________________________________________
>>>>>>>>>> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> _______________________________________________
>>>>>>>>>> Beginners mailing list
>>>>>>>>>> Beginners at haskell.org
>>>>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>> --
>>>>>>>> Sent from an iPhone, please excuse brevity and typos.
>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> Beginners mailing list
>>>>>>>> Beginners at haskell.org
>>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>> --
>>>>>> Sent from an iPhone, please excuse brevity and typos.
>>>>>>
>>>>>> _______________________________________________
>>>>>> Beginners mailing list
>>>>>> Beginners at haskell.org
>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>
>>>>>>
>>>>>
>>>>
>>>> --
>>>> Sent from an iPhone, please excuse brevity and typos.
>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
>>>> Beginners at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>>
>>
>
> --
> Sent from an iPhone, please excuse brevity and typos.
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From hsyl20 at gmail.com  Mon Mar  2 15:20:00 2015
From: hsyl20 at gmail.com (Sylvain Henry)
Date: Mon, 2 Mar 2015 16:20:00 +0100
Subject: [Haskell-beginners] FFI Problems
In-Reply-To: <54F1C085.7020503@nyu.edu>
References: <54F1C085.7020503@nyu.edu>
Message-ID: <CAPmptcWKW0-nyttQnu_-n-WsWdmndE_Z1RcBKwn=NvhCwaVpuw@mail.gmail.com>

Hi,

1) You have a copy-paste error on the following line (s/bitrate/track)
 track <- ((#peek struct music_metadata, bitrate) a) :: IO Int

2) By looking at the values you get in hexadecimal, you are reading 64bits
values when your C struct contains 32 bits values. If you use: peek ... ::
IO Int32, it should be ok.

3) From a style point of view, you could use Applicative operators (from
Control.Applicative) to avoid temporary names:
peek a = MusicMetaData
        <$> liftM constantToCodec $ (((#peek struct music_metadata, codec)
a) :: IO Int32)
        <*> fmap fromIntegral (((#peek struct music_metadata, length) a) ::
IO Int32)
        <*> fmap fromIntegral (((#peek struct music_metadata, bitrate) a)
:: IO Int32)
        <*> fmap fromIntegral (((#peek struct music_metadata, channels) a)
:: IO Int32)
        <*> fmap fromIntegral (((#peek struct music_metadata, track) a) ::
IO Int32)
        <*> (peekCString =<< (#peek struct music_metadata, title) a)
        ...

4) In your constantToCodec function, you don't need the temporary names for
all the constants.

Best regards,
Sylvain

PS : recently I have been working on rewriting the FFI page on the wiki to
make it more beginner-friendly ;-). It is not totally finished but I'm open
to any comment. https://www.haskell.org/wiki/Foreign_Function_Interface

2015-02-28 14:20 GMT+01:00 Thomas Jakway <tjakway at nyu.edu>:

>  I'm very new to Haskell and am trying to write a "real" program to
> motivate myself to learn it better (so far I've only gotten through Project
> Euler problems after reading LYAH and most of RWH).  I'm using Taglib (
> https://github.com/taglib/taglib) to read the metadata from a music file
> and print it.  I have a struct C-side (with C linkage) serving as the
> bridge between Taglib's C++ and Haskell's FFI.  A small demo program
> (compiled with gcc and linked against the C++ object files) gives the
> correct results, but Haskell is weirdly only getting *some *of it right.
> Specifically, the C string fields are working but ints are not.
>
> The output from the C demo (what Haskell should be printing):
>
> music_metadata
> title: It's My Life,    artist: Bon Jovi,    album: Bon Jovi Greatest Hits
> - The Ultimate Collection
> comment: ,    genre: Rock,    track: 3,
> length: 224,    bitrate: 256,    channels: 2,
> codec: 768
>
> The output from Haskell:
>
> MusicMetadata {codec = UNKNOWN, length = 1099511628000, bitrate =
> 8589934848, channels = 12884901890, track = 8589934848, title = "It's My
> Life", artist = "Bon Jovi", album = "Bon Jovi Greatest Hits - The Ultimate
> Collection", comment = "", genre = "Rock"}
>
> I would have expected it to work or not work at all, but did not
> anticipate getting only some of it right.
>
> I was going to include snippets from my hsc file but given how new I am to
> Haskell I don't trust myself to assume where the problem is, so sorry if
> this is way too long:
>
> {-# LANGUAGE CPP, ForeignFunctionInterface #-}
>
> module MusicReader
> ( Codec,
>   MusicMetadata,
>   readMusicMetadata
> ) where
>
> import Control.Monad
> import Foreign
> import Foreign.C.Types
> import Foreign.C.String
> import System.IO.Unsafe as Unsafe
>
> #include "CodecDefines.h"
> #include "MusicReader.h"
>
> constantToCodec code
>                     | code == mp3 = MP3
>                     | code == flac = FLAC
>                     | code == ogg = OGG_VORBIS
>                     | code == mp4 = MP4
>                     | code == mpeg = MPEG
>                     | code == none = NONE
>                     | code == unknown = UNKNOWN
>                     | otherwise = UNKNOWN
>                        where mp3 = #const MP3_CODEC
>                              flac = #const FLAC_CODEC
>                              ogg = #const OGG_VORBIS_CODEC
>                              mp4 = #const MP4_CODEC
>                              mpeg = #const MPEG_CODEC
>                              none = #const NO_EXTENSION
>                              unknown = #const UNKNOWN_EXTENSION
>
> data Codec = MP3 | FLAC | OGG_VORBIS | MP4 | MPEG | NONE | UNKNOWN
> deriving (Show)
>
> data MusicMetadata = MusicMetadata { codec :: Codec,
>                       length :: Int,
>                       bitrate :: Int,
>                       channels :: Int,
>                       track :: Int,
>                       title :: String,
>                       artist :: String,
>                       album :: String,
>                       comment :: String,
>                       genre :: String } deriving (Show)
>
> instance Storable MusicMetadata where
>     sizeOf _ = (#size struct music_metadata)
>     alignment _ = alignment (undefined::CDouble)
>     peek a = do
>         codec <- liftM constantToCodec $ (((#peek struct music_metadata,
> codec) a) :: IO Int)
>         length <- ((#peek struct music_metadata, length) a) :: IO Int
>         bitrate <- ((#peek struct music_metadata, bitrate) a) :: IO Int
>         channels <- ((#peek struct music_metadata, channels) a) :: IO Int
>         track <- ((#peek struct music_metadata, bitrate) a) :: IO Int
>         title <-  ((#peek struct music_metadata, title) a) :: IO CString
>         artist <- ((#peek struct music_metadata, artist) a) :: IO CString
>         album <- ((#peek struct music_metadata, album) a) :: IO CString
>         comment <- ((#peek struct music_metadata, comment) a) :: IO CString
>         genre <- ((#peek struct music_metadata, genre) a) :: IO CString
>         --FIXME: find replacement for temporary names
>         marshalledTitle <- peekCString title
>         marshalledArtist <- peekCString artist
>         marshalledAlbum <- peekCString album
>         marshalledComment <- peekCString comment
>         marshalledGenre <- peekCString genre
>         return (MusicMetadata codec length bitrate channels track
> marshalledTitle marshalledArtist marshalledAlbum marshalledComment
> marshalledGenre)
>     poke a = undefined
>
> --This is the "primitive" FFI call--calls the C function and gets a pointer
> --in return
> --TODO: write a higher level function this module should export that calls
> --primReadMusicMetadata and converts the C Pointer into the Haskell data
> --MusicMetadata
> foreign import ccall unsafe "read_metadata" primReadMusicMetadata ::
> CString -> IO (Ptr MusicMetadata)
>
> --convert the Haskell string to a CString, call into the FFI then
> --dereference the resulting pointer
> readMusicMetadata a = join $ withCString a $ \cs -> ((liftM peek) $
> primReadMusicMetadata cs)
>
>
>
> Here's the struct in MusicReader.h (in an extern C block):
>     struct music_metadata
>     {
>         int codec;
>         int length,
>                 bitrate,
>                 channels;
>         int track;
>         char *title,
>              *artist,
>              *album,
>              *comment,
>              *genre;
>     };
>
>
> with the corresponding call:
>     struct music_metadata* read_metadata(char*);
>
>
> I've tried playing around with the alignment but it didn't do anything.  I
> also tried declaring the struct's fields as int32_t which also did nothing.
>
> The C demo in question is a very simple:
>
> #include <stdio.h>
> #include "MusicReader.h"
>
> #define FILENAME "Its_My_Life.m4a"
>
> int main()
> {
>     struct music_metadata* metadata = read_metadata(FILENAME);
>     printf("music_metadata\ntitle: %s,\tartist: %s,\talbum: %s\n",
>             metadata->title, metadata->artist, metadata->album);
>     printf("comment: %s,\tgenre: %s,\ttrack: %d,\n",
>             metadata->comment, metadata->genre, metadata->track);
>     printf("length: %d,\tbitrate: %d,\tchannels: %d,\n",
>             metadata->length, metadata->bitrate, metadata->channels);
>     printf("codec: %d\n");
>
> }
>
> It just reads the metadata into the struct and prints the fields.
>
> I've gotten the impression of the Haskell FFI being very
> beginner-unfriendly, which isn't surprising but still disappointing because
> it would be a great opportunity to replace some projects with Haskell.
>
> Any help is appreciated, including general feedback on my code!
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From hsyl20 at gmail.com  Mon Mar  2 15:30:27 2015
From: hsyl20 at gmail.com (Sylvain Henry)
Date: Mon, 2 Mar 2015 16:30:27 +0100
Subject: [Haskell-beginners] FFI Problems
In-Reply-To: <CAPmptcWKW0-nyttQnu_-n-WsWdmndE_Z1RcBKwn=NvhCwaVpuw@mail.gmail.com>
References: <54F1C085.7020503@nyu.edu>
 <CAPmptcWKW0-nyttQnu_-n-WsWdmndE_Z1RcBKwn=NvhCwaVpuw@mail.gmail.com>
Message-ID: <CAPmptcU6Ye_k+H8d_=eeHHOj0c1vz7kPdPcgPN1S5ykTDb+kyw@mail.gmail.com>

I forgot to comment this line from your code:
readMusicMetadata a = join $ withCString a $ \cs -> ((liftM peek) $
primReadMusicMetadata cs)

I would write:
readMusicMetadata a = withCString a $ \cs -> do
    p <- primReadMusicMetadata cs
    peek p

Or:
readMusicMetadata a = withCString a $ \cs -> (peek =<<
primReadMusicMetadata cs)

Or (with (<=<) from Control.Monad):
readMusicMetadata a = withCString a (peek <=< primReadMusicMetadata)

Best regards,
Sylvain



2015-03-02 16:20 GMT+01:00 Sylvain Henry <hsyl20 at gmail.com>:

> Hi,
>
> 1) You have a copy-paste error on the following line (s/bitrate/track)
>  track <- ((#peek struct music_metadata, bitrate) a) :: IO Int
>
> 2) By looking at the values you get in hexadecimal, you are reading 64bits
> values when your C struct contains 32 bits values. If you use: peek ... ::
> IO Int32, it should be ok.
>
> 3) From a style point of view, you could use Applicative operators (from
> Control.Applicative) to avoid temporary names:
> peek a = MusicMetaData
>         <$> liftM constantToCodec $ (((#peek struct music_metadata, codec)
> a) :: IO Int32)
>         <*> fmap fromIntegral (((#peek struct music_metadata, length) a)
> :: IO Int32)
>         <*> fmap fromIntegral (((#peek struct music_metadata, bitrate) a)
> :: IO Int32)
>         <*> fmap fromIntegral (((#peek struct music_metadata, channels) a)
> :: IO Int32)
>         <*> fmap fromIntegral (((#peek struct music_metadata, track) a) ::
> IO Int32)
>         <*> (peekCString =<< (#peek struct music_metadata, title) a)
>         ...
>
> 4) In your constantToCodec function, you don't need the temporary names
> for all the constants.
>
> Best regards,
> Sylvain
>
> PS : recently I have been working on rewriting the FFI page on the wiki to
> make it more beginner-friendly ;-). It is not totally finished but I'm open
> to any comment. https://www.haskell.org/wiki/Foreign_Function_Interface
>
> 2015-02-28 14:20 GMT+01:00 Thomas Jakway <tjakway at nyu.edu>:
>
>>  I'm very new to Haskell and am trying to write a "real" program to
>> motivate myself to learn it better (so far I've only gotten through Project
>> Euler problems after reading LYAH and most of RWH).  I'm using Taglib (
>> https://github.com/taglib/taglib) to read the metadata from a music file
>> and print it.  I have a struct C-side (with C linkage) serving as the
>> bridge between Taglib's C++ and Haskell's FFI.  A small demo program
>> (compiled with gcc and linked against the C++ object files) gives the
>> correct results, but Haskell is weirdly only getting *some *of it
>> right.  Specifically, the C string fields are working but ints are not.
>>
>> The output from the C demo (what Haskell should be printing):
>>
>> music_metadata
>> title: It's My Life,    artist: Bon Jovi,    album: Bon Jovi Greatest
>> Hits - The Ultimate Collection
>> comment: ,    genre: Rock,    track: 3,
>> length: 224,    bitrate: 256,    channels: 2,
>> codec: 768
>>
>> The output from Haskell:
>>
>> MusicMetadata {codec = UNKNOWN, length = 1099511628000, bitrate =
>> 8589934848, channels = 12884901890, track = 8589934848, title = "It's My
>> Life", artist = "Bon Jovi", album = "Bon Jovi Greatest Hits - The Ultimate
>> Collection", comment = "", genre = "Rock"}
>>
>> I would have expected it to work or not work at all, but did not
>> anticipate getting only some of it right.
>>
>> I was going to include snippets from my hsc file but given how new I am
>> to Haskell I don't trust myself to assume where the problem is, so sorry if
>> this is way too long:
>>
>> {-# LANGUAGE CPP, ForeignFunctionInterface #-}
>>
>> module MusicReader
>> ( Codec,
>>   MusicMetadata,
>>   readMusicMetadata
>> ) where
>>
>> import Control.Monad
>> import Foreign
>> import Foreign.C.Types
>> import Foreign.C.String
>> import System.IO.Unsafe as Unsafe
>>
>> #include "CodecDefines.h"
>> #include "MusicReader.h"
>>
>> constantToCodec code
>>                     | code == mp3 = MP3
>>                     | code == flac = FLAC
>>                     | code == ogg = OGG_VORBIS
>>                     | code == mp4 = MP4
>>                     | code == mpeg = MPEG
>>                     | code == none = NONE
>>                     | code == unknown = UNKNOWN
>>                     | otherwise = UNKNOWN
>>                        where mp3 = #const MP3_CODEC
>>                              flac = #const FLAC_CODEC
>>                              ogg = #const OGG_VORBIS_CODEC
>>                              mp4 = #const MP4_CODEC
>>                              mpeg = #const MPEG_CODEC
>>                              none = #const NO_EXTENSION
>>                              unknown = #const UNKNOWN_EXTENSION
>>
>> data Codec = MP3 | FLAC | OGG_VORBIS | MP4 | MPEG | NONE | UNKNOWN
>> deriving (Show)
>>
>> data MusicMetadata = MusicMetadata { codec :: Codec,
>>                       length :: Int,
>>                       bitrate :: Int,
>>                       channels :: Int,
>>                       track :: Int,
>>                       title :: String,
>>                       artist :: String,
>>                       album :: String,
>>                       comment :: String,
>>                       genre :: String } deriving (Show)
>>
>> instance Storable MusicMetadata where
>>     sizeOf _ = (#size struct music_metadata)
>>     alignment _ = alignment (undefined::CDouble)
>>     peek a = do
>>         codec <- liftM constantToCodec $ (((#peek struct music_metadata,
>> codec) a) :: IO Int)
>>         length <- ((#peek struct music_metadata, length) a) :: IO Int
>>         bitrate <- ((#peek struct music_metadata, bitrate) a) :: IO Int
>>         channels <- ((#peek struct music_metadata, channels) a) :: IO Int
>>         track <- ((#peek struct music_metadata, bitrate) a) :: IO Int
>>         title <-  ((#peek struct music_metadata, title) a) :: IO CString
>>         artist <- ((#peek struct music_metadata, artist) a) :: IO CString
>>         album <- ((#peek struct music_metadata, album) a) :: IO CString
>>         comment <- ((#peek struct music_metadata, comment) a) :: IO
>> CString
>>         genre <- ((#peek struct music_metadata, genre) a) :: IO CString
>>         --FIXME: find replacement for temporary names
>>         marshalledTitle <- peekCString title
>>         marshalledArtist <- peekCString artist
>>         marshalledAlbum <- peekCString album
>>         marshalledComment <- peekCString comment
>>         marshalledGenre <- peekCString genre
>>         return (MusicMetadata codec length bitrate channels track
>> marshalledTitle marshalledArtist marshalledAlbum marshalledComment
>> marshalledGenre)
>>     poke a = undefined
>>
>> --This is the "primitive" FFI call--calls the C function and gets a
>> pointer
>> --in return
>> --TODO: write a higher level function this module should export that calls
>> --primReadMusicMetadata and converts the C Pointer into the Haskell data
>> --MusicMetadata
>> foreign import ccall unsafe "read_metadata" primReadMusicMetadata ::
>> CString -> IO (Ptr MusicMetadata)
>>
>> --convert the Haskell string to a CString, call into the FFI then
>> --dereference the resulting pointer
>> readMusicMetadata a = join $ withCString a $ \cs -> ((liftM peek) $
>> primReadMusicMetadata cs)
>>
>>
>>
>> Here's the struct in MusicReader.h (in an extern C block):
>>     struct music_metadata
>>     {
>>         int codec;
>>         int length,
>>                 bitrate,
>>                 channels;
>>         int track;
>>         char *title,
>>              *artist,
>>              *album,
>>              *comment,
>>              *genre;
>>     };
>>
>>
>> with the corresponding call:
>>     struct music_metadata* read_metadata(char*);
>>
>>
>> I've tried playing around with the alignment but it didn't do anything.
>> I also tried declaring the struct's fields as int32_t which also did
>> nothing.
>>
>> The C demo in question is a very simple:
>>
>> #include <stdio.h>
>> #include "MusicReader.h"
>>
>> #define FILENAME "Its_My_Life.m4a"
>>
>> int main()
>> {
>>     struct music_metadata* metadata = read_metadata(FILENAME);
>>     printf("music_metadata\ntitle: %s,\tartist: %s,\talbum: %s\n",
>>             metadata->title, metadata->artist, metadata->album);
>>     printf("comment: %s,\tgenre: %s,\ttrack: %d,\n",
>>             metadata->comment, metadata->genre, metadata->track);
>>     printf("length: %d,\tbitrate: %d,\tchannels: %d,\n",
>>             metadata->length, metadata->bitrate, metadata->channels);
>>     printf("codec: %d\n");
>>
>> }
>>
>> It just reads the metadata into the struct and prints the fields.
>>
>> I've gotten the impression of the Haskell FFI being very
>> beginner-unfriendly, which isn't surprising but still disappointing because
>> it would be a great opportunity to replace some projects with Haskell.
>>
>> Any help is appreciated, including general feedback on my code!
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Mon Mar  2 20:43:15 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Tue, 3 Mar 2015 02:13:15 +0530
Subject: [Haskell-beginners] CodeWarriors -- Spreading the word
Message-ID: <CAJbEW8M_o65VjCPAscrJzRy==_Egany9TiqrzqU8XX2prOMHtA@mail.gmail.com>

I just found out about www.codewarriors.com, and wanted to spread the word.
It's a good place for beginners to get questions in increasing order of
complexity.

Best.

-- 
Regards

Sumit Sahrawat
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Mon Mar  2 20:44:07 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Tue, 3 Mar 2015 02:14:07 +0530
Subject: [Haskell-beginners] CodeWarriors -- Spreading the word
In-Reply-To: <CAJbEW8M_o65VjCPAscrJzRy==_Egany9TiqrzqU8XX2prOMHtA@mail.gmail.com>
References: <CAJbEW8M_o65VjCPAscrJzRy==_Egany9TiqrzqU8XX2prOMHtA@mail.gmail.com>
Message-ID: <CAJbEW8Mrx3xs5hVjb4BKbM-+wzGAYTcutsW-gCYzxiNJ5CiGyA@mail.gmail.com>

Sorry, I meant www.codewars.com.

On 3 March 2015 at 02:13, Sumit Sahrawat, Maths & Computing, IIT (BHU) <
sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

> I just found out about www.codewarriors.com, and wanted to spread the
> word.
> It's a good place for beginners to get questions in increasing order of
> complexity.
>
> Best.
>
> --
> Regards
>
> Sumit Sahrawat
>



-- 
Regards

Sumit Sahrawat
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From vxanica at gmail.com  Tue Mar  3 14:41:11 2015
From: vxanica at gmail.com (Song Zhang)
Date: Tue, 3 Mar 2015 22:41:11 +0800
Subject: [Haskell-beginners] "Template Haskell and higher order meta
	programming."
Message-ID: <CACGMEOn4-FUfKNhHpTUtTwydfF71_98JRXF_Mu65ZYtzLU0P9A@mail.gmail.com>

I generally have 2 questions. I am writing some functions to derive
typeclass instances with a package called derive. I just want to get rid of
using Derivation type with a stupid reason. I want use just Name type.

How can I defined a function with type

derivings :: Name -> Name -> Q [Dec]

by using function derive :: Derivation -> Name -> Q [Dec]
so that user can just write derivings ''Eq ''D instead of derive makeEq ''D?

> {-# LANGUAGE TemplateHaskell #-}

> import Data.DeriveTH -- in derive package
> import Language.Haskell.TH
> import qualified Language.Haskell.TH.Syntax as S
> data D = D
> derivings :: Name -> String -> Q Exp
> derivings cla typ = do
>               let makeClassName = mkName $ "make" ++ nameBase cla
>               a <-  [| derive makeClassName (mkName typ) |]
>               return a

> instance S.Lift Name where
>         lift x = varE x

Main> :t derivings ''Eq "D"
derivings ''Eq "D" :: Q Exp

Main> :t $(derivings ''Eq "D")
$(derivings ''Eq "D") :: Q [Dec]

There is no problem to quote twice in other file in order to get the
declaration. Maybe it should called second order meta programming. But the
following has problems:

> derivings'' :: Name -> Name -> Q Exp
> derivings'' cla typ = do
>               let makeClassName = mkName $ "make" ++ nameBase cla
>               a <-  [| derive makeClassName typ |]
>               return a

Main> :t derivings''
derivings'' :: Name -> Name -> Q Exp

Main> :t derivings'' ''Eq ''D
derivings'' ''Eq ''D :: Q Exp

Main> :t $(derivings'' ''Eq ''D)
<interactive>:1:3:
    Illegal variable name: ?D?
    When splicing a TH expression:
      Data.DeriveTH.derive makeEq (Language.Haskell.TH.Syntax.mkName Main.D)
    In the splice: $(derivings'' ''Eq ''D)

In the first case I used String, it works. however, it is not workiing if I
use D with a Name type. Cannot figure it out.
My second question is that if my return type is Q [Exp] while the [Exp] can
be further expanded into [Dec]. How can I do it to expand [Exp] into [Dec]?
Do we have a function for $ in $(thcode_with_Q_Exp) so that I do not need
to splice each Exp value?

Best wishes
Song
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From tjakway at nyu.edu  Tue Mar  3 19:27:08 2015
From: tjakway at nyu.edu (Thomas Jakway)
Date: Tue, 03 Mar 2015 14:27:08 -0500
Subject: [Haskell-beginners] FFI Problems
In-Reply-To: <CAPmptcWKW0-nyttQnu_-n-WsWdmndE_Z1RcBKwn=NvhCwaVpuw@mail.gmail.com>
References: <54F1C085.7020503@nyu.edu>
 <CAPmptcWKW0-nyttQnu_-n-WsWdmndE_Z1RcBKwn=NvhCwaVpuw@mail.gmail.com>
Message-ID: <54F60B0C.3070706@nyu.edu>

Thanks very much!  This helped a lot (and you were right on #2... I 
really should have caught that).

On 3/2/15 10:20 AM, Sylvain Henry wrote:
> Hi,
>
> 1) You have a copy-paste error on the following line (s/bitrate/track)
>  track <- ((#peek struct music_metadata, bitrate) a) :: IO Int
>
> 2) By looking at the values you get in hexadecimal, you are reading 
> 64bits values when your C struct contains 32 bits values. If you use: 
> peek ... :: IO Int32, it should be ok.
>
> 3) From a style point of view, you could use Applicative operators 
> (from Control.Applicative) to avoid temporary names:
> peek a = MusicMetaData
>         <$> liftM constantToCodec $ (((#peek struct music_metadata, 
> codec) a) :: IO Int32)
>         <*> fmap fromIntegral (((#peek struct music_metadata, length) 
> a) :: IO Int32)
>         <*> fmap fromIntegral (((#peek struct music_metadata, bitrate) 
> a) :: IO Int32)
>         <*> fmap fromIntegral (((#peek struct music_metadata, 
> channels) a) :: IO Int32)
>         <*> fmap fromIntegral (((#peek struct music_metadata, track) 
> a) :: IO Int32)
>         <*> (peekCString =<< (#peek struct music_metadata, title) a)
>         ...
>
> 4) In your constantToCodec function, you don't need the temporary 
> names for all the constants.
>
> Best regards,
> Sylvain
>
> PS : recently I have been working on rewriting the FFI page on the 
> wiki to make it more beginner-friendly ;-). It is not totally finished 
> but I'm open to any comment. 
> https://www.haskell.org/wiki/Foreign_Function_Interface
>
> 2015-02-28 14:20 GMT+01:00 Thomas Jakway <tjakway at nyu.edu 
> <mailto:tjakway at nyu.edu>>:
>
>     I'm very new to Haskell and am trying to write a "real" program to
>     motivate myself to learn it better (so far I've only gotten
>     through Project Euler problems after reading LYAH and most of
>     RWH).  I'm using Taglib (https://github.com/taglib/taglib) to read
>     the metadata from a music file and print it.  I have a struct
>     C-side (with C linkage) serving as the bridge between Taglib's C++
>     and Haskell's FFI.  A small demo program (compiled with gcc and
>     linked against the C++ object files) gives the correct results,
>     but Haskell is weirdly only getting /some /of it right.
>     Specifically, the C string fields are working but ints are not.
>
>     The output from the C demo (what Haskell should be printing):
>
>     music_metadata
>     title: It's My Life,    artist: Bon Jovi,    album: Bon Jovi
>     Greatest Hits - The Ultimate Collection
>     comment: ,    genre: Rock,    track: 3,
>     length: 224,    bitrate: 256,    channels: 2,
>     codec: 768
>
>     The output from Haskell:
>
>     MusicMetadata {codec = UNKNOWN, length = 1099511628000, bitrate =
>     8589934848 <tel:8589934848>, channels = 12884901890, track =
>     8589934848 <tel:8589934848>, title = "It's My Life", artist = "Bon
>     Jovi", album = "Bon Jovi Greatest Hits - The Ultimate Collection",
>     comment = "", genre = "Rock"}
>
>     I would have expected it to work or not work at all, but did not
>     anticipate getting only some of it right.
>
>     I was going to include snippets from my hsc file but given how new
>     I am to Haskell I don't trust myself to assume where the problem
>     is, so sorry if this is way too long:
>
>     {-# LANGUAGE CPP, ForeignFunctionInterface #-}
>
>     module MusicReader
>     ( Codec,
>       MusicMetadata,
>       readMusicMetadata
>     ) where
>
>     import Control.Monad
>     import Foreign
>     import Foreign.C.Types
>     import Foreign.C.String
>     import System.IO.Unsafe as Unsafe
>
>     #include "CodecDefines.h"
>     #include "MusicReader.h"
>
>     constantToCodec code
>                         | code == mp3 = MP3
>                         | code == flac = FLAC
>                         | code == ogg = OGG_VORBIS
>                         | code == mp4 = MP4
>                         | code == mpeg = MPEG
>                         | code == none = NONE
>                         | code == unknown = UNKNOWN
>                         | otherwise = UNKNOWN
>                            where mp3 = #const MP3_CODEC
>                                  flac = #const FLAC_CODEC
>                                  ogg = #const OGG_VORBIS_CODEC
>                                  mp4 = #const MP4_CODEC
>                                  mpeg = #const MPEG_CODEC
>                                  none = #const NO_EXTENSION
>                                  unknown = #const UNKNOWN_EXTENSION
>
>     data Codec = MP3 | FLAC | OGG_VORBIS | MP4 | MPEG | NONE | UNKNOWN
>     deriving (Show)
>
>     data MusicMetadata = MusicMetadata { codec :: Codec,
>                           length :: Int,
>                           bitrate :: Int,
>                           channels :: Int,
>                           track :: Int,
>                           title :: String,
>                           artist :: String,
>                           album :: String,
>                           comment :: String,
>                           genre :: String } deriving (Show)
>
>     instance Storable MusicMetadata where
>         sizeOf _ = (#size struct music_metadata)
>         alignment _ = alignment (undefined::CDouble)
>         peek a = do
>             codec <- liftM constantToCodec $ (((#peek struct
>     music_metadata, codec) a) :: IO Int)
>             length <- ((#peek struct music_metadata, length) a) :: IO Int
>             bitrate <- ((#peek struct music_metadata, bitrate) a) ::
>     IO Int
>             channels <- ((#peek struct music_metadata, channels) a) ::
>     IO Int
>             track <- ((#peek struct music_metadata, bitrate) a) :: IO Int
>             title <-  ((#peek struct music_metadata, title) a) :: IO
>     CString
>             artist <- ((#peek struct music_metadata, artist) a) :: IO
>     CString
>             album <- ((#peek struct music_metadata, album) a) :: IO
>     CString
>             comment <- ((#peek struct music_metadata, comment) a) ::
>     IO CString
>             genre <- ((#peek struct music_metadata, genre) a) :: IO
>     CString
>             --FIXME: find replacement for temporary names
>             marshalledTitle <- peekCString title
>             marshalledArtist <- peekCString artist
>             marshalledAlbum <- peekCString album
>             marshalledComment <- peekCString comment
>             marshalledGenre <- peekCString genre
>             return (MusicMetadata codec length bitrate channels track
>     marshalledTitle marshalledArtist marshalledAlbum marshalledComment
>     marshalledGenre)
>         poke a = undefined
>
>     --This is the "primitive" FFI call--calls the C function and gets
>     a pointer
>     --in return
>     --TODO: write a higher level function this module should export
>     that calls
>     --primReadMusicMetadata and converts the C Pointer into the
>     Haskell data
>     --MusicMetadata
>     foreign import ccall unsafe "read_metadata" primReadMusicMetadata
>     :: CString -> IO (Ptr MusicMetadata)
>
>     --convert the Haskell string to a CString, call into the FFI then
>     --dereference the resulting pointer
>     readMusicMetadata a = join $ withCString a $ \cs -> ((liftM peek)
>     $ primReadMusicMetadata cs)
>
>
>
>     Here's the struct in MusicReader.h (in an extern C block):
>         struct music_metadata
>         {
>             int codec;
>             int length,
>                     bitrate,
>                     channels;
>             int track;
>             char *title,
>                  *artist,
>                  *album,
>                  *comment,
>                  *genre;
>         };
>
>
>     with the corresponding call:
>         struct music_metadata* read_metadata(char*);
>
>
>     I've tried playing around with the alignment but it didn't do
>     anything.  I also tried declaring the struct's fields as int32_t
>     which also did nothing.
>
>     The C demo in question is a very simple:
>
>     #include <stdio.h>
>     #include "MusicReader.h"
>
>     #define FILENAME "Its_My_Life.m4a"
>
>     int main()
>     {
>         struct music_metadata* metadata = read_metadata(FILENAME);
>         printf("music_metadata\ntitle: %s,\tartist: %s,\talbum: %s\n",
>                 metadata->title, metadata->artist, metadata->album);
>         printf("comment: %s,\tgenre: %s,\ttrack: %d,\n",
>                 metadata->comment, metadata->genre, metadata->track);
>         printf("length: %d,\tbitrate: %d,\tchannels: %d,\n",
>                 metadata->length, metadata->bitrate, metadata->channels);
>         printf("codec: %d\n");
>
>     }
>
>     It just reads the metadata into the struct and prints the fields.
>
>     I've gotten the impression of the Haskell FFI being very
>     beginner-unfriendly, which isn't surprising but still
>     disappointing because it would be a great opportunity to replace
>     some projects with Haskell.
>
>     Any help is appreciated, including general feedback on my code!
>
>     _______________________________________________
>     Beginners mailing list
>     Beginners at haskell.org <mailto:Beginners at haskell.org>
>     http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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From objitsu at gmail.com  Tue Mar  3 20:19:36 2015
From: objitsu at gmail.com (emacstheviking)
Date: Tue, 3 Mar 2015 20:19:36 +0000
Subject: [Haskell-beginners] LTS is good but...
Message-ID: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>

I started an OpenGL project within an LTS sandbox as guided on another post
and, somewhat foolishly it would see, I did a "cabal install cabal-install"
when prompted that a new version was available.

The very next time I tried to work on my code.... I got this:

bash-3.2$ pwd
/Users/seancharles/Documents/Coding/haskell/lts1
bash-3.2$ cabal build
cabal: You need to re-run the 'configure' command. The version of Cabal
being
used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
bash-3.2$ cabal configure
Resolving dependencies...
Configuring lts1-0.1.0.0...
cabal: At least the following dependencies are missing:
base ==4.7.*
bash-3.2$ cabal build
cabal: You need to re-run the 'configure' command. The version of Cabal
being
used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
bash-3.2$ ls -l
total 112
-rw-r--r--  1 seancharles  staff     19 Mar  2 12:53 LICENCSE
-rw-r--r--  1 seancharles  staff    552 Mar  3 20:09 Main.hs
-rw-r--r--  1 seancharles  staff    477 Mar  3 20:07 Main.hs~
-rw-r--r--  1 seancharles  staff     46 Mar  2 12:52 Setup.hs
-rw-r--r--  1 seancharles  staff  30198 Mar  2 12:51 cabal.config
-rw-r--r--  1 seancharles  staff   1090 Mar  2 12:51 cabal.sandbox.config
drwxr-xr-x  5 seancharles  staff    170 Mar  2 12:53 dist
-rw-r--r--  1 seancharles  staff   1932 Mar  2 12:52 lts1.cabal
bash-3.2$

So, welcome to a different kind of hell, "cabal version hell" perhaps?

Can anybody help me get it running again?
Thanks.
Sean.
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From objitsu at gmail.com  Tue Mar  3 20:20:59 2015
From: objitsu at gmail.com (emacstheviking)
Date: Tue, 3 Mar 2015 20:20:59 +0000
Subject: [Haskell-beginners] LTS is good but...
In-Reply-To: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
Message-ID: <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>

Forgot to add that somehow, despite the upgrade, my cabal version has gone
backwards from 1.20.0.1 to 1.16.0

WTF?


On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:

> I started an OpenGL project within an LTS sandbox as guided on another
> post and, somewhat foolishly it would see, I did a "cabal install
> cabal-install" when prompted that a new version was available.
>
> The very next time I tried to work on my code.... I got this:
>
> bash-3.2$ pwd
> /Users/seancharles/Documents/Coding/haskell/lts1
> bash-3.2$ cabal build
> cabal: You need to re-run the 'configure' command. The version of Cabal
> being
> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
> bash-3.2$ cabal configure
> Resolving dependencies...
> Configuring lts1-0.1.0.0...
> cabal: At least the following dependencies are missing:
> base ==4.7.*
> bash-3.2$ cabal build
> cabal: You need to re-run the 'configure' command. The version of Cabal
> being
> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
> bash-3.2$ ls -l
> total 112
> -rw-r--r--  1 seancharles  staff     19 Mar  2 12:53 LICENCSE
> -rw-r--r--  1 seancharles  staff    552 Mar  3 20:09 Main.hs
> -rw-r--r--  1 seancharles  staff    477 Mar  3 20:07 Main.hs~
> -rw-r--r--  1 seancharles  staff     46 Mar  2 12:52 Setup.hs
> -rw-r--r--  1 seancharles  staff  30198 Mar  2 12:51 cabal.config
> -rw-r--r--  1 seancharles  staff   1090 Mar  2 12:51 cabal.sandbox.config
> drwxr-xr-x  5 seancharles  staff    170 Mar  2 12:53 dist
> -rw-r--r--  1 seancharles  staff   1932 Mar  2 12:52 lts1.cabal
> bash-3.2$
>
> So, welcome to a different kind of hell, "cabal version hell" perhaps?
>
> Can anybody help me get it running again?
> Thanks.
> Sean.
>
>
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From michael at snoyman.com  Tue Mar  3 20:43:34 2015
From: michael at snoyman.com (Michael Snoyman)
Date: Tue, 03 Mar 2015 20:43:34 +0000
Subject: [Haskell-beginners] LTS is good but...
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
Message-ID: <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>

Funny that there's an issue being discussed in the cabal issue tracker
right now[1] similar to this. But the report you're giving here is
particularly strange. Cabal-1.16 implies that you're using GHC 7.6, which
is supported by the claim that base 4.7 isn't available. I'd recommend
starting off with running the following commands and pasting the output:

ghc --version
which ghc
cabal --version
which cabal

[1] https://github.com/haskell/cabal/issues/2438

On Tue, Mar 3, 2015 at 10:21 PM emacstheviking <objitsu at gmail.com> wrote:

> Forgot to add that somehow, despite the upgrade, my cabal version has gone
> backwards from 1.20.0.1 to 1.16.0
>
> WTF?
>
>
> On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:
>
>> I started an OpenGL project within an LTS sandbox as guided on another
>> post and, somewhat foolishly it would see, I did a "cabal install
>> cabal-install" when prompted that a new version was available.
>>
>> The very next time I tried to work on my code.... I got this:
>>
>> bash-3.2$ pwd
>> /Users/seancharles/Documents/Coding/haskell/lts1
>> bash-3.2$ cabal build
>> cabal: You need to re-run the 'configure' command. The version of Cabal
>> being
>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>> bash-3.2$ cabal configure
>> Resolving dependencies...
>> Configuring lts1-0.1.0.0...
>> cabal: At least the following dependencies are missing:
>> base ==4.7.*
>> bash-3.2$ cabal build
>> cabal: You need to re-run the 'configure' command. The version of Cabal
>> being
>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>> bash-3.2$ ls -l
>> total 112
>> -rw-r--r--  1 seancharles  staff     19 Mar  2 12:53 LICENCSE
>> -rw-r--r--  1 seancharles  staff    552 Mar  3 20:09 Main.hs
>> -rw-r--r--  1 seancharles  staff    477 Mar  3 20:07 Main.hs~
>> -rw-r--r--  1 seancharles  staff     46 Mar  2 12:52 Setup.hs
>> -rw-r--r--  1 seancharles  staff  30198 Mar  2 12:51 cabal.config
>> -rw-r--r--  1 seancharles  staff   1090 Mar  2 12:51 cabal.sandbox.config
>> drwxr-xr-x  5 seancharles  staff    170 Mar  2 12:53 dist
>> -rw-r--r--  1 seancharles  staff   1932 Mar  2 12:52 lts1.cabal
>> bash-3.2$
>>
>> So, welcome to a different kind of hell, "cabal version hell" perhaps?
>>
>> Can anybody help me get it running again?
>> Thanks.
>> Sean.
>>
>>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From electriclightheads at gmail.com  Wed Mar  4 03:22:11 2015
From: electriclightheads at gmail.com ('2+)
Date: Wed, 4 Mar 2015 12:22:11 +0900
Subject: [Haskell-beginners] fractal x = fractal [y - head x + z | y <- x,
	z <- x] with take doesnt end
Message-ID: <4F515C47-6CAE-4DDB-B436-61CDD653F66B@gmail.com>

hi!
since my

fractal :: [Int] -> [Int]
fractal x = [y - head x + z | y <- x, z <- x]

seems to be working fine with something like:

take 300 $ cycle $ fractal $ fractal $ [11..17] ++ [55..77]

wanted to twist the function to something totally recursive
but:

fractal x = fractal [y - head x + z | y <- x, z <- x]

experimented with:

take 300 $ fractal $ [11..17] ++ [55..77]

doesnt end its process and eat up the RAM

any hints to solve this issue?

thank you

----- 2g ---
http://sarigama.namaste.jp



From sumit.sahrawat.apm13 at iitbhu.ac.in  Wed Mar  4 03:56:08 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Wed, 4 Mar 2015 09:26:08 +0530
Subject: [Haskell-beginners] fractal x = fractal [y - head x + z | y <-
 x, z <- x] with take doesnt end
In-Reply-To: <4F515C47-6CAE-4DDB-B436-61CDD653F66B@gmail.com>
References: <4F515C47-6CAE-4DDB-B436-61CDD653F66B@gmail.com>
Message-ID: <CAJbEW8M5NrOOPTx-rv+T40FHsHXq6pyP65RZtDX=Kddw2s2Ohw@mail.gmail.com>

Both the versions work for me. The one with two calls to fractal takes
~0.03 seconds, whereas the one with only one call takes ~0.01 seconds.
All timings produced using (:set +s) in ghci.

Also, that totally recursive definition is infinitely recursive.

If you provide some more context, it will be easier to answer your
question. Till now I don't exactly understand what the issue is.

On 4 March 2015 at 08:52, '2+ <electriclightheads at gmail.com> wrote:

> hi!
> since my
>
> fractal :: [Int] -> [Int]
> fractal x = [y - head x + z | y <- x, z <- x]
>
> seems to be working fine with something like:
>
> take 300 $ cycle $ fractal $ fractal $ [11..17] ++ [55..77]
>
> wanted to twist the function to something totally recursive
> but:
>
> fractal x = fractal [y - head x + z | y <- x, z <- x]
>
> experimented with:
>
> take 300 $ fractal $ [11..17] ++ [55..77]
>
> doesnt end its process and eat up the RAM
>
> any hints to solve this issue?
>
> thank you
>
> ----- 2g ---
> http://sarigama.namaste.jp
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>



-- 
Regards

Sumit Sahrawat
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From raguay at customct.com  Wed Mar  4 05:38:24 2015
From: raguay at customct.com (Richard Guay)
Date: Wed, 04 Mar 2015 12:38:24 +0700
Subject: [Haskell-beginners] Alfred Programs in Haskell
Message-ID: <54F69A50.4030308@customct.com>

Hi,

I am using Haskell to write Alfred programs, but I have run into an 
issue I just can not fix. The routine for reading a data file in a 
application specific location for Alfred never detects the file as 
non-existant. It then dies on the read. Here is the code:

getAlfredCacheFileContents :: String -> IO (String)
getAlfredCacheFileContents fileName = do
     h <- getHomeDirectory
     fExist <- doesFileExist $ h ++ cacheDirBasic ++ getBundleID ++ "/" 
++ fileName
     if fExist
     then do
         contents <- readFile $ h ++ cacheDirBasic ++ getBundleID ++ "/" 
++ fileName
         return contents
     else
         return ""

I just installed the latest version on my Mac Air. It always does the 
call to readFile, even when the file doesn't exist.
-- 
Sent with Postbox <http://www.getpostbox.com>
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From mike at proclivis.com  Wed Mar  4 06:50:43 2015
From: mike at proclivis.com (Michael Jones)
Date: Tue, 3 Mar 2015 23:50:43 -0700
Subject: [Haskell-beginners] Google Summer of Code
Message-ID: <C12467B5-D3B0-4E2A-93F5-694B78B763AD@proclivis.com>

If any students are interested in Google Summer of Code projects, there is a Minnow Board project here:

http://elinux.org/Minnowboard:GSoC2015

There is a Haskell project listed in the SMBus/PMBus section.

Mike

From objitsu at gmail.com  Wed Mar  4 08:39:56 2015
From: objitsu at gmail.com (emacstheviking)
Date: Wed, 4 Mar 2015 08:39:56 +0000
Subject: [Haskell-beginners] LTS is good but...
In-Reply-To: <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
Message-ID: <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>

As requested,

iMac:~ vosabristol$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.8.3

which ghc
/opt/local/bin/ghc

Hmm.... it shows a macports instsallation of GHC... but I am sure that's
how I git it... I *used* to use the OSX installation package but I thought
a clean start using MacPorts would somehow change the mood...

iMac:~ vosabristol$ cabal --version
cabal-install version 1.16.0.2
using version 1.16.0 of the Cabal library

iMac:~ vosabristol$ which cabal
/usr/bin/cabal

So... there is a possible case for *once again* performing a complete and
utter removal of Haskell and its partners in crime and doing a fresh
installation?


Out of frustration I built OCaml and OPAM from sources last night but I'd
still like to go with GHC!

:)


On 3 March 2015 at 20:43, Michael Snoyman <michael at snoyman.com> wrote:

> Funny that there's an issue being discussed in the cabal issue tracker
> right now[1] similar to this. But the report you're giving here is
> particularly strange. Cabal-1.16 implies that you're using GHC 7.6, which
> is supported by the claim that base 4.7 isn't available. I'd recommend
> starting off with running the following commands and pasting the output:
>
> ghc --version
> which ghc
> cabal --version
> which cabal
>
> [1] https://github.com/haskell/cabal/issues/2438
>
> On Tue, Mar 3, 2015 at 10:21 PM emacstheviking <objitsu at gmail.com> wrote:
>
>> Forgot to add that somehow, despite the upgrade, my cabal version has
>> gone backwards from 1.20.0.1 to 1.16.0
>>
>> WTF?
>>
>>
>> On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:
>>
>>> I started an OpenGL project within an LTS sandbox as guided on another
>>> post and, somewhat foolishly it would see, I did a "cabal install
>>> cabal-install" when prompted that a new version was available.
>>>
>>> The very next time I tried to work on my code.... I got this:
>>>
>>> bash-3.2$ pwd
>>> /Users/seancharles/Documents/Coding/haskell/lts1
>>> bash-3.2$ cabal build
>>> cabal: You need to re-run the 'configure' command. The version of Cabal
>>> being
>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>> bash-3.2$ cabal configure
>>> Resolving dependencies...
>>> Configuring lts1-0.1.0.0...
>>> cabal: At least the following dependencies are missing:
>>> base ==4.7.*
>>> bash-3.2$ cabal build
>>> cabal: You need to re-run the 'configure' command. The version of Cabal
>>> being
>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>> bash-3.2$ ls -l
>>> total 112
>>> -rw-r--r--  1 seancharles  staff     19 Mar  2 12:53 LICENCSE
>>> -rw-r--r--  1 seancharles  staff    552 Mar  3 20:09 Main.hs
>>> -rw-r--r--  1 seancharles  staff    477 Mar  3 20:07 Main.hs~
>>> -rw-r--r--  1 seancharles  staff     46 Mar  2 12:52 Setup.hs
>>> -rw-r--r--  1 seancharles  staff  30198 Mar  2 12:51 cabal.config
>>> -rw-r--r--  1 seancharles  staff   1090 Mar  2 12:51 cabal.sandbox.config
>>> drwxr-xr-x  5 seancharles  staff    170 Mar  2 12:53 dist
>>> -rw-r--r--  1 seancharles  staff   1932 Mar  2 12:52 lts1.cabal
>>> bash-3.2$
>>>
>>> So, welcome to a different kind of hell, "cabal version hell" perhaps?
>>>
>>> Can anybody help me get it running again?
>>> Thanks.
>>> Sean.
>>>
>>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From michael at snoyman.com  Wed Mar  4 09:13:41 2015
From: michael at snoyman.com (Michael Snoyman)
Date: Wed, 04 Mar 2015 09:13:41 +0000
Subject: [Haskell-beginners] LTS is good but...
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
Message-ID: <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>

I can't really explain the details of what's going on here. It seems that
in some cases different versions of the executables are being found, but I
don't know why. Wiping out your sandbox (cabal sandbox delete) and starting
over may be sufficient to recover.

On Wed, Mar 4, 2015 at 11:11 AM emacstheviking <objitsu at gmail.com> wrote:

> As requested,
>
> iMac:~ vosabristol$ ghc --version
> The Glorious Glasgow Haskell Compilation System, version 7.8.3
>
> which ghc
> /opt/local/bin/ghc
>
> Hmm.... it shows a macports instsallation of GHC... but I am sure that's
> how I git it... I *used* to use the OSX installation package but I thought
> a clean start using MacPorts would somehow change the mood...
>
> iMac:~ vosabristol$ cabal --version
> cabal-install version 1.16.0.2
> using version 1.16.0 of the Cabal library
>
> iMac:~ vosabristol$ which cabal
> /usr/bin/cabal
>
> So... there is a possible case for *once again* performing a complete and
> utter removal of Haskell and its partners in crime and doing a fresh
> installation?
>
>
> Out of frustration I built OCaml and OPAM from sources last night but I'd
> still like to go with GHC!
>
> :)
>
>
> On 3 March 2015 at 20:43, Michael Snoyman <michael at snoyman.com> wrote:
>
>> Funny that there's an issue being discussed in the cabal issue tracker
>> right now[1] similar to this. But the report you're giving here is
>> particularly strange. Cabal-1.16 implies that you're using GHC 7.6, which
>> is supported by the claim that base 4.7 isn't available. I'd recommend
>> starting off with running the following commands and pasting the output:
>>
>> ghc --version
>> which ghc
>> cabal --version
>> which cabal
>>
>> [1] https://github.com/haskell/cabal/issues/2438
>>
>> On Tue, Mar 3, 2015 at 10:21 PM emacstheviking <objitsu at gmail.com> wrote:
>>
>>> Forgot to add that somehow, despite the upgrade, my cabal version has
>>> gone backwards from 1.20.0.1 to 1.16.0
>>>
>>> WTF?
>>>
>>>
>>> On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:
>>>
>>>> I started an OpenGL project within an LTS sandbox as guided on another
>>>> post and, somewhat foolishly it would see, I did a "cabal install
>>>> cabal-install" when prompted that a new version was available.
>>>>
>>>> The very next time I tried to work on my code.... I got this:
>>>>
>>>> bash-3.2$ pwd
>>>> /Users/seancharles/Documents/Coding/haskell/lts1
>>>> bash-3.2$ cabal build
>>>> cabal: You need to re-run the 'configure' command. The version of Cabal
>>>> being
>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>> bash-3.2$ cabal configure
>>>> Resolving dependencies...
>>>> Configuring lts1-0.1.0.0...
>>>> cabal: At least the following dependencies are missing:
>>>> base ==4.7.*
>>>> bash-3.2$ cabal build
>>>> cabal: You need to re-run the 'configure' command. The version of Cabal
>>>> being
>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>> bash-3.2$ ls -l
>>>> total 112
>>>> -rw-r--r--  1 seancharles  staff     19 Mar  2 12:53 LICENCSE
>>>> -rw-r--r--  1 seancharles  staff    552 Mar  3 20:09 Main.hs
>>>> -rw-r--r--  1 seancharles  staff    477 Mar  3 20:07 Main.hs~
>>>> -rw-r--r--  1 seancharles  staff     46 Mar  2 12:52 Setup.hs
>>>> -rw-r--r--  1 seancharles  staff  30198 Mar  2 12:51 cabal.config
>>>> -rw-r--r--  1 seancharles  staff   1090 Mar  2 12:51
>>>> cabal.sandbox.config
>>>> drwxr-xr-x  5 seancharles  staff    170 Mar  2 12:53 dist
>>>> -rw-r--r--  1 seancharles  staff   1932 Mar  2 12:52 lts1.cabal
>>>> bash-3.2$
>>>>
>>>> So, welcome to a different kind of hell, "cabal version hell" perhaps?
>>>>
>>>> Can anybody help me get it running again?
>>>> Thanks.
>>>> Sean.
>>>>
>>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From d12frosted at icloud.com  Wed Mar  4 09:21:29 2015
From: d12frosted at icloud.com (Boris)
Date: Wed, 04 Mar 2015 11:21:29 +0200
Subject: [Haskell-beginners] LTS is good but...
In-Reply-To: <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
Message-ID: <etPan.54f6ce99.4e6afb66.45ae@d12frosted.local>

Hey,

Check your $PATH variable. It looks like the macports installation directory has precedence over /user/local/bin/.

But you also might want to remove the old version installed via macports.

~ d12frosted


On March 4, 2015 at 11:11:05, emacstheviking (objitsu at gmail.com) wrote:

As requested,

iMac:~ vosabristol$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.8.3

which ghc
/opt/local/bin/ghc

Hmm.... it shows a macports instsallation of GHC... but I am sure that's how I git it... I *used* to use the OSX installation package but I thought a clean start using MacPorts would somehow change the mood...

iMac:~ vosabristol$ cabal --version
cabal-install version 1.16.0.2
using version 1.16.0 of the Cabal library

iMac:~ vosabristol$ which cabal
/usr/bin/cabal

So... there is a possible case for *once again* performing a complete and utter removal of Haskell and its partners in crime and doing a fresh installation?


Out of frustration I built OCaml and OPAM from sources last night but I'd still like to go with GHC!

:)


On 3 March 2015 at 20:43, Michael Snoyman <michael at snoyman.com> wrote:
Funny that there's an issue being discussed in the cabal issue tracker right now[1] similar to this.?But the report you're giving here is particularly strange. Cabal-1.16 implies that you're using GHC 7.6, which is supported by the claim that base 4.7 isn't available. I'd recommend starting off with running the following commands and pasting the output:

ghc --version
which ghc
cabal --version
which cabal

[1]?https://github.com/haskell/cabal/issues/2438

On Tue, Mar 3, 2015 at 10:21 PM emacstheviking <objitsu at gmail.com> wrote:
Forgot to add that somehow, despite the upgrade, my cabal version has gone backwards from 1.20.0.1 to 1.16.0

WTF?


On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:
I started an OpenGL project within an LTS sandbox as guided on another post and, somewhat foolishly it would see, I did a "cabal install cabal-install" when prompted that a new version was available.

The very next time I tried to work on my code.... I got this:

bash-3.2$ pwd
/Users/seancharles/Documents/Coding/haskell/lts1
bash-3.2$ cabal build
cabal: You need to re-run the 'configure' command. The version of Cabal being
used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
bash-3.2$ cabal configure
Resolving dependencies...
Configuring lts1-0.1.0.0...
cabal: At least the following dependencies are missing:
base ==4.7.*
bash-3.2$ cabal build
cabal: You need to re-run the 'configure' command. The version of Cabal being
used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
bash-3.2$ ls -l
total 112
-rw-r--r-- ?1 seancharles ?staff ? ? 19 Mar ?2 12:53 LICENCSE
-rw-r--r-- ?1 seancharles ?staff ? ?552 Mar ?3 20:09 Main.hs
-rw-r--r-- ?1 seancharles ?staff ? ?477 Mar ?3 20:07 Main.hs~
-rw-r--r-- ?1 seancharles ?staff ? ? 46 Mar ?2 12:52 Setup.hs
-rw-r--r-- ?1 seancharles ?staff ?30198 Mar ?2 12:51 cabal.config
-rw-r--r-- ?1 seancharles ?staff ? 1090 Mar ?2 12:51 cabal.sandbox.config
drwxr-xr-x ?5 seancharles ?staff ? ?170 Mar ?2 12:53 dist
-rw-r--r-- ?1 seancharles ?staff ? 1932 Mar ?2 12:52 lts1.cabal
bash-3.2$?

So, welcome to a different kind of hell, "cabal version hell" perhaps?

Can anybody help me get it running again?
Thanks.
Sean.


_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners


_______________________________________________  
Beginners mailing list  
Beginners at haskell.org  
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners  
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From objitsu at gmail.com  Wed Mar  4 09:25:28 2015
From: objitsu at gmail.com (emacstheviking)
Date: Wed, 4 Mar 2015 09:25:28 +0000
Subject: [Haskell-beginners] LTS is good but...
In-Reply-To: <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
 <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
Message-ID: <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>

Agreed Michael!

I am currently wiping all Mac Ports packages and I will perform a clean up
of /Library/Haskell etc.

What is the *preferred* way to get the lastest for OSX: Using homebrew,
macports or the Haskell Platform. I *used* to use the platform installer
but for some reason (cabal I expect!) I started trying out other ways...

I think actually I will go the platform packager again...

On 4 March 2015 at 09:13, Michael Snoyman <michael at snoyman.com> wrote:

> I can't really explain the details of what's going on here. It seems that
> in some cases different versions of the executables are being found, but I
> don't know why. Wiping out your sandbox (cabal sandbox delete) and starting
> over may be sufficient to recover.
>
> On Wed, Mar 4, 2015 at 11:11 AM emacstheviking <objitsu at gmail.com> wrote:
>
>> As requested,
>>
>> iMac:~ vosabristol$ ghc --version
>> The Glorious Glasgow Haskell Compilation System, version 7.8.3
>>
>> which ghc
>> /opt/local/bin/ghc
>>
>> Hmm.... it shows a macports instsallation of GHC... but I am sure that's
>> how I git it... I *used* to use the OSX installation package but I thought
>> a clean start using MacPorts would somehow change the mood...
>>
>> iMac:~ vosabristol$ cabal --version
>> cabal-install version 1.16.0.2
>> using version 1.16.0 of the Cabal library
>>
>> iMac:~ vosabristol$ which cabal
>> /usr/bin/cabal
>>
>> So... there is a possible case for *once again* performing a complete and
>> utter removal of Haskell and its partners in crime and doing a fresh
>> installation?
>>
>>
>> Out of frustration I built OCaml and OPAM from sources last night but I'd
>> still like to go with GHC!
>>
>> :)
>>
>>
>> On 3 March 2015 at 20:43, Michael Snoyman <michael at snoyman.com> wrote:
>>
>>> Funny that there's an issue being discussed in the cabal issue tracker
>>> right now[1] similar to this. But the report you're giving here is
>>> particularly strange. Cabal-1.16 implies that you're using GHC 7.6, which
>>> is supported by the claim that base 4.7 isn't available. I'd recommend
>>> starting off with running the following commands and pasting the output:
>>>
>>> ghc --version
>>> which ghc
>>> cabal --version
>>> which cabal
>>>
>>> [1] https://github.com/haskell/cabal/issues/2438
>>>
>>> On Tue, Mar 3, 2015 at 10:21 PM emacstheviking <objitsu at gmail.com>
>>> wrote:
>>>
>>>> Forgot to add that somehow, despite the upgrade, my cabal version has
>>>> gone backwards from 1.20.0.1 to 1.16.0
>>>>
>>>> WTF?
>>>>
>>>>
>>>> On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:
>>>>
>>>>> I started an OpenGL project within an LTS sandbox as guided on another
>>>>> post and, somewhat foolishly it would see, I did a "cabal install
>>>>> cabal-install" when prompted that a new version was available.
>>>>>
>>>>> The very next time I tried to work on my code.... I got this:
>>>>>
>>>>> bash-3.2$ pwd
>>>>> /Users/seancharles/Documents/Coding/haskell/lts1
>>>>> bash-3.2$ cabal build
>>>>> cabal: You need to re-run the 'configure' command. The version of
>>>>> Cabal being
>>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>>> bash-3.2$ cabal configure
>>>>> Resolving dependencies...
>>>>> Configuring lts1-0.1.0.0...
>>>>> cabal: At least the following dependencies are missing:
>>>>> base ==4.7.*
>>>>> bash-3.2$ cabal build
>>>>> cabal: You need to re-run the 'configure' command. The version of
>>>>> Cabal being
>>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>>> bash-3.2$ ls -l
>>>>> total 112
>>>>> -rw-r--r--  1 seancharles  staff     19 Mar  2 12:53 LICENCSE
>>>>> -rw-r--r--  1 seancharles  staff    552 Mar  3 20:09 Main.hs
>>>>> -rw-r--r--  1 seancharles  staff    477 Mar  3 20:07 Main.hs~
>>>>> -rw-r--r--  1 seancharles  staff     46 Mar  2 12:52 Setup.hs
>>>>> -rw-r--r--  1 seancharles  staff  30198 Mar  2 12:51 cabal.config
>>>>> -rw-r--r--  1 seancharles  staff   1090 Mar  2 12:51
>>>>> cabal.sandbox.config
>>>>> drwxr-xr-x  5 seancharles  staff    170 Mar  2 12:53 dist
>>>>> -rw-r--r--  1 seancharles  staff   1932 Mar  2 12:52 lts1.cabal
>>>>> bash-3.2$
>>>>>
>>>>> So, welcome to a different kind of hell, "cabal version hell" perhaps?
>>>>>
>>>>> Can anybody help me get it running again?
>>>>> Thanks.
>>>>> Sean.
>>>>>
>>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
>>>> Beginners at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From michael at snoyman.com  Wed Mar  4 09:28:43 2015
From: michael at snoyman.com (Michael Snoyman)
Date: Wed, 04 Mar 2015 09:28:43 +0000
Subject: [Haskell-beginners] LTS is good but...
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
 <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
 <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>
Message-ID: <CAKA2JgKVtCHW1YbLcaBjdBqws6W0GmUrJvhWn6YrPcEHOdvfUw@mail.gmail.com>

I'd recommend looking at:

http://www.stackage.org/install#mac-os-x
https://github.com/bitemyapp/learnhaskell/blob/master/README.md#mac-os-x

I don't generally recommend using the platform installer, it ends up
pegging libraries to old versions which causes dependency problems, or in
some cases have known bugs.

On Wed, Mar 4, 2015 at 11:26 AM emacstheviking <objitsu at gmail.com> wrote:

> Agreed Michael!
>
> I am currently wiping all Mac Ports packages and I will perform a clean up
> of /Library/Haskell etc.
>
> What is the *preferred* way to get the lastest for OSX: Using homebrew,
> macports or the Haskell Platform. I *used* to use the platform installer
> but for some reason (cabal I expect!) I started trying out other ways...
>
> I think actually I will go the platform packager again...
>
> On 4 March 2015 at 09:13, Michael Snoyman <michael at snoyman.com> wrote:
>
>> I can't really explain the details of what's going on here. It seems that
>> in some cases different versions of the executables are being found, but I
>> don't know why. Wiping out your sandbox (cabal sandbox delete) and starting
>> over may be sufficient to recover.
>>
>> On Wed, Mar 4, 2015 at 11:11 AM emacstheviking <objitsu at gmail.com> wrote:
>>
>>> As requested,
>>>
>>> iMac:~ vosabristol$ ghc --version
>>> The Glorious Glasgow Haskell Compilation System, version 7.8.3
>>>
>>> which ghc
>>> /opt/local/bin/ghc
>>>
>>> Hmm.... it shows a macports instsallation of GHC... but I am sure that's
>>> how I git it... I *used* to use the OSX installation package but I thought
>>> a clean start using MacPorts would somehow change the mood...
>>>
>>> iMac:~ vosabristol$ cabal --version
>>> cabal-install version 1.16.0.2
>>> using version 1.16.0 of the Cabal library
>>>
>>> iMac:~ vosabristol$ which cabal
>>> /usr/bin/cabal
>>>
>>> So... there is a possible case for *once again* performing a complete
>>> and utter removal of Haskell and its partners in crime and doing a fresh
>>> installation?
>>>
>>>
>>> Out of frustration I built OCaml and OPAM from sources last night but
>>> I'd still like to go with GHC!
>>>
>>> :)
>>>
>>>
>>> On 3 March 2015 at 20:43, Michael Snoyman <michael at snoyman.com> wrote:
>>>
>>>> Funny that there's an issue being discussed in the cabal issue tracker
>>>> right now[1] similar to this. But the report you're giving here is
>>>> particularly strange. Cabal-1.16 implies that you're using GHC 7.6, which
>>>> is supported by the claim that base 4.7 isn't available. I'd recommend
>>>> starting off with running the following commands and pasting the output:
>>>>
>>>> ghc --version
>>>> which ghc
>>>> cabal --version
>>>> which cabal
>>>>
>>>> [1] https://github.com/haskell/cabal/issues/2438
>>>>
>>>> On Tue, Mar 3, 2015 at 10:21 PM emacstheviking <objitsu at gmail.com>
>>>> wrote:
>>>>
>>>>> Forgot to add that somehow, despite the upgrade, my cabal version has
>>>>> gone backwards from 1.20.0.1 to 1.16.0
>>>>>
>>>>> WTF?
>>>>>
>>>>>
>>>>> On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:
>>>>>
>>>>>> I started an OpenGL project within an LTS sandbox as guided on
>>>>>> another post and, somewhat foolishly it would see, I did a "cabal install
>>>>>> cabal-install" when prompted that a new version was available.
>>>>>>
>>>>>> The very next time I tried to work on my code.... I got this:
>>>>>>
>>>>>> bash-3.2$ pwd
>>>>>> /Users/seancharles/Documents/Coding/haskell/lts1
>>>>>> bash-3.2$ cabal build
>>>>>> cabal: You need to re-run the 'configure' command. The version of
>>>>>> Cabal being
>>>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>>>> bash-3.2$ cabal configure
>>>>>> Resolving dependencies...
>>>>>> Configuring lts1-0.1.0.0...
>>>>>> cabal: At least the following dependencies are missing:
>>>>>> base ==4.7.*
>>>>>> bash-3.2$ cabal build
>>>>>> cabal: You need to re-run the 'configure' command. The version of
>>>>>> Cabal being
>>>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>>>> bash-3.2$ ls -l
>>>>>> total 112
>>>>>> -rw-r--r--  1 seancharles  staff     19 Mar  2 12:53 LICENCSE
>>>>>> -rw-r--r--  1 seancharles  staff    552 Mar  3 20:09 Main.hs
>>>>>> -rw-r--r--  1 seancharles  staff    477 Mar  3 20:07 Main.hs~
>>>>>> -rw-r--r--  1 seancharles  staff     46 Mar  2 12:52 Setup.hs
>>>>>> -rw-r--r--  1 seancharles  staff  30198 Mar  2 12:51 cabal.config
>>>>>> -rw-r--r--  1 seancharles  staff   1090 Mar  2 12:51
>>>>>> cabal.sandbox.config
>>>>>> drwxr-xr-x  5 seancharles  staff    170 Mar  2 12:53 dist
>>>>>> -rw-r--r--  1 seancharles  staff   1932 Mar  2 12:52 lts1.cabal
>>>>>> bash-3.2$
>>>>>>
>>>>>> So, welcome to a different kind of hell, "cabal version hell" perhaps?
>>>>>>
>>>>>> Can anybody help me get it running again?
>>>>>> Thanks.
>>>>>> Sean.
>>>>>>
>>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
>>>> Beginners at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From objitsu at gmail.com  Wed Mar  4 09:31:02 2015
From: objitsu at gmail.com (emacstheviking)
Date: Wed, 4 Mar 2015 09:31:02 +0000
Subject: [Haskell-beginners] LTS is good but...
In-Reply-To: <CAKA2JgKVtCHW1YbLcaBjdBqws6W0GmUrJvhWn6YrPcEHOdvfUw@mail.gmail.com>
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
 <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
 <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>
 <CAKA2JgKVtCHW1YbLcaBjdBqws6W0GmUrJvhWn6YrPcEHOdvfUw@mail.gmail.com>
Message-ID: <CAEiEuUJLGZ070=whKSdvf_SjpNgXkFAPSCq2ho2e2UJVzkJ7mg@mail.gmail.com>

Aaaaaarrrrrrrrrggghhh!

LMAO. 30 years ago it was all simpler.... but we didn't have Haskell!

;)

Thanks. I will read that page and try again.


On 4 March 2015 at 09:28, Michael Snoyman <michael at snoyman.com> wrote:

> I'd recommend looking at:
>
> http://www.stackage.org/install#mac-os-x
> https://github.com/bitemyapp/learnhaskell/blob/master/README.md#mac-os-x
>
> I don't generally recommend using the platform installer, it ends up
> pegging libraries to old versions which causes dependency problems, or in
> some cases have known bugs.
>
> On Wed, Mar 4, 2015 at 11:26 AM emacstheviking <objitsu at gmail.com> wrote:
>
>> Agreed Michael!
>>
>> I am currently wiping all Mac Ports packages and I will perform a clean
>> up of /Library/Haskell etc.
>>
>> What is the *preferred* way to get the lastest for OSX: Using homebrew,
>> macports or the Haskell Platform. I *used* to use the platform installer
>> but for some reason (cabal I expect!) I started trying out other ways...
>>
>> I think actually I will go the platform packager again...
>>
>> On 4 March 2015 at 09:13, Michael Snoyman <michael at snoyman.com> wrote:
>>
>>> I can't really explain the details of what's going on here. It seems
>>> that in some cases different versions of the executables are being found,
>>> but I don't know why. Wiping out your sandbox (cabal sandbox delete) and
>>> starting over may be sufficient to recover.
>>>
>>> On Wed, Mar 4, 2015 at 11:11 AM emacstheviking <objitsu at gmail.com>
>>> wrote:
>>>
>>>> As requested,
>>>>
>>>> iMac:~ vosabristol$ ghc --version
>>>> The Glorious Glasgow Haskell Compilation System, version 7.8.3
>>>>
>>>> which ghc
>>>> /opt/local/bin/ghc
>>>>
>>>> Hmm.... it shows a macports instsallation of GHC... but I am sure
>>>> that's how I git it... I *used* to use the OSX installation package but I
>>>> thought a clean start using MacPorts would somehow change the mood...
>>>>
>>>> iMac:~ vosabristol$ cabal --version
>>>> cabal-install version 1.16.0.2
>>>> using version 1.16.0 of the Cabal library
>>>>
>>>> iMac:~ vosabristol$ which cabal
>>>> /usr/bin/cabal
>>>>
>>>> So... there is a possible case for *once again* performing a complete
>>>> and utter removal of Haskell and its partners in crime and doing a fresh
>>>> installation?
>>>>
>>>>
>>>> Out of frustration I built OCaml and OPAM from sources last night but
>>>> I'd still like to go with GHC!
>>>>
>>>> :)
>>>>
>>>>
>>>> On 3 March 2015 at 20:43, Michael Snoyman <michael at snoyman.com> wrote:
>>>>
>>>>> Funny that there's an issue being discussed in the cabal issue tracker
>>>>> right now[1] similar to this. But the report you're giving here is
>>>>> particularly strange. Cabal-1.16 implies that you're using GHC 7.6, which
>>>>> is supported by the claim that base 4.7 isn't available. I'd recommend
>>>>> starting off with running the following commands and pasting the output:
>>>>>
>>>>> ghc --version
>>>>> which ghc
>>>>> cabal --version
>>>>> which cabal
>>>>>
>>>>> [1] https://github.com/haskell/cabal/issues/2438
>>>>>
>>>>> On Tue, Mar 3, 2015 at 10:21 PM emacstheviking <objitsu at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> Forgot to add that somehow, despite the upgrade, my cabal version has
>>>>>> gone backwards from 1.20.0.1 to 1.16.0
>>>>>>
>>>>>> WTF?
>>>>>>
>>>>>>
>>>>>> On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:
>>>>>>
>>>>>>> I started an OpenGL project within an LTS sandbox as guided on
>>>>>>> another post and, somewhat foolishly it would see, I did a "cabal install
>>>>>>> cabal-install" when prompted that a new version was available.
>>>>>>>
>>>>>>> The very next time I tried to work on my code.... I got this:
>>>>>>>
>>>>>>> bash-3.2$ pwd
>>>>>>> /Users/seancharles/Documents/Coding/haskell/lts1
>>>>>>> bash-3.2$ cabal build
>>>>>>> cabal: You need to re-run the 'configure' command. The version of
>>>>>>> Cabal being
>>>>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>>>>> bash-3.2$ cabal configure
>>>>>>> Resolving dependencies...
>>>>>>> Configuring lts1-0.1.0.0...
>>>>>>> cabal: At least the following dependencies are missing:
>>>>>>> base ==4.7.*
>>>>>>> bash-3.2$ cabal build
>>>>>>> cabal: You need to re-run the 'configure' command. The version of
>>>>>>> Cabal being
>>>>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>>>>> bash-3.2$ ls -l
>>>>>>> total 112
>>>>>>> -rw-r--r--  1 seancharles  staff     19 Mar  2 12:53 LICENCSE
>>>>>>> -rw-r--r--  1 seancharles  staff    552 Mar  3 20:09 Main.hs
>>>>>>> -rw-r--r--  1 seancharles  staff    477 Mar  3 20:07 Main.hs~
>>>>>>> -rw-r--r--  1 seancharles  staff     46 Mar  2 12:52 Setup.hs
>>>>>>> -rw-r--r--  1 seancharles  staff  30198 Mar  2 12:51 cabal.config
>>>>>>> -rw-r--r--  1 seancharles  staff   1090 Mar  2 12:51
>>>>>>> cabal.sandbox.config
>>>>>>> drwxr-xr-x  5 seancharles  staff    170 Mar  2 12:53 dist
>>>>>>> -rw-r--r--  1 seancharles  staff   1932 Mar  2 12:52 lts1.cabal
>>>>>>> bash-3.2$
>>>>>>>
>>>>>>> So, welcome to a different kind of hell, "cabal version hell"
>>>>>>> perhaps?
>>>>>>>
>>>>>>> Can anybody help me get it running again?
>>>>>>> Thanks.
>>>>>>> Sean.
>>>>>>>
>>>>>>>
>>>>>> _______________________________________________
>>>>>> Beginners mailing list
>>>>>> Beginners at haskell.org
>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
>>>> Beginners at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From d12frosted at icloud.com  Wed Mar  4 09:31:54 2015
From: d12frosted at icloud.com (Boris)
Date: Wed, 04 Mar 2015 11:31:54 +0200
Subject: [Haskell-beginners] LTS is good but...
In-Reply-To: <CAKA2JgKVtCHW1YbLcaBjdBqws6W0GmUrJvhWn6YrPcEHOdvfUw@mail.gmail.com>
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
 <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
 <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>
 <CAKA2JgKVtCHW1YbLcaBjdBqws6W0GmUrJvhWn6YrPcEHOdvfUw@mail.gmail.com>
Message-ID: <etPan.54f6d10a.3f2dba31.45ae@d12frosted.local>

Hey,

Little addition to what Michael already sent you.

To get ?the latest? you should build from sources :D

But in case it?s ok for you to have ghc 7.8.3 and you don?t want to ./configure; make; make install, you might also check this installation script on github.



On March 4, 2015 at 11:28:50, Michael Snoyman (michael at snoyman.com) wrote:

I'd recommend looking at:

http://www.stackage.org/install#mac-os-x
https://github.com/bitemyapp/learnhaskell/blob/master/README.md#mac-os-x

I don't generally recommend using the platform installer, it ends up pegging libraries to old versions which causes dependency problems, or in some cases have known bugs.

On Wed, Mar 4, 2015 at 11:26 AM emacstheviking <objitsu at gmail.com> wrote:
Agreed Michael!

I am currently wiping all Mac Ports packages and I will perform a clean up of /Library/Haskell etc.

What is the *preferred* way to get the lastest for OSX: Using homebrew, macports or the Haskell Platform. I *used* to use the platform installer but for some reason (cabal I expect!) I started trying out other ways...

I think actually I will go the platform packager again...

On 4 March 2015 at 09:13, Michael Snoyman <michael at snoyman.com> wrote:
I can't really explain the details of what's going on here. It seems that in some cases different versions of the executables are being found, but I don't know why. Wiping out your sandbox (cabal sandbox delete) and starting over may be sufficient to recover.

On Wed, Mar 4, 2015 at 11:11 AM emacstheviking <objitsu at gmail.com> wrote:
As requested,

iMac:~ vosabristol$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.8.3

which ghc
/opt/local/bin/ghc

Hmm.... it shows a macports instsallation of GHC... but I am sure that's how I git it... I *used* to use the OSX installation package but I thought a clean start using MacPorts would somehow change the mood...

iMac:~ vosabristol$ cabal --version
cabal-install version 1.16.0.2
using version 1.16.0 of the Cabal library

iMac:~ vosabristol$ which cabal
/usr/bin/cabal

So... there is a possible case for *once again* performing a complete and utter removal of Haskell and its partners in crime and doing a fresh installation?


Out of frustration I built OCaml and OPAM from sources last night but I'd still like to go with GHC!

:)


On 3 March 2015 at 20:43, Michael Snoyman <michael at snoyman.com> wrote:
Funny that there's an issue being discussed in the cabal issue tracker right now[1] similar to this.?But the report you're giving here is particularly strange. Cabal-1.16 implies that you're using GHC 7.6, which is supported by the claim that base 4.7 isn't available. I'd recommend starting off with running the following commands and pasting the output:

ghc --version
which ghc
cabal --version
which cabal

[1]?https://github.com/haskell/cabal/issues/2438

On Tue, Mar 3, 2015 at 10:21 PM emacstheviking <objitsu at gmail.com> wrote:
Forgot to add that somehow, despite the upgrade, my cabal version has gone backwards from 1.20.0.1 to 1.16.0

WTF?


On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:
I started an OpenGL project within an LTS sandbox as guided on another post and, somewhat foolishly it would see, I did a "cabal install cabal-install" when prompted that a new version was available.

The very next time I tried to work on my code.... I got this:

bash-3.2$ pwd
/Users/seancharles/Documents/Coding/haskell/lts1
bash-3.2$ cabal build
cabal: You need to re-run the 'configure' command. The version of Cabal being
used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
bash-3.2$ cabal configure
Resolving dependencies...
Configuring lts1-0.1.0.0...
cabal: At least the following dependencies are missing:
base ==4.7.*
bash-3.2$ cabal build
cabal: You need to re-run the 'configure' command. The version of Cabal being
used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
bash-3.2$ ls -l
total 112
-rw-r--r-- ?1 seancharles ?staff ? ? 19 Mar ?2 12:53 LICENCSE
-rw-r--r-- ?1 seancharles ?staff ? ?552 Mar ?3 20:09 Main.hs
-rw-r--r-- ?1 seancharles ?staff ? ?477 Mar ?3 20:07 Main.hs~
-rw-r--r-- ?1 seancharles ?staff ? ? 46 Mar ?2 12:52 Setup.hs
-rw-r--r-- ?1 seancharles ?staff ?30198 Mar ?2 12:51 cabal.config
-rw-r--r-- ?1 seancharles ?staff ? 1090 Mar ?2 12:51 cabal.sandbox.config
drwxr-xr-x ?5 seancharles ?staff ? ?170 Mar ?2 12:53 dist
-rw-r--r-- ?1 seancharles ?staff ? 1932 Mar ?2 12:52 lts1.cabal
bash-3.2$?

So, welcome to a different kind of hell, "cabal version hell" perhaps?

Can anybody help me get it running again?
Thanks.
Sean.


_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners


_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners


_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
_______________________________________________  
Beginners mailing list  
Beginners at haskell.org  
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From haskell at elisehuard.be  Wed Mar  4 09:32:53 2015
From: haskell at elisehuard.be (Elise Huard)
Date: Wed, 4 Mar 2015 10:32:53 +0100
Subject: [Haskell-beginners] command-line interaction with running process
Message-ID: <CAHfyCqnaKuif5wtiPAfGOVRyOJ6hW57EwR7L6gwLRJS6k8Ty3A@mail.gmail.com>

Hi,

I'm trying to do the following: implementing a command-line interface
to directly interact with the state or the parameters of a
looping/running process (a game, to be specific).

My first thought is to fork a thread, passing in a TChan/TVar or
similar to enable communication, and to have the command-line
interface on the thread (getLine or haskelline ...).
Another option would be to have a bona fide server-client interface
(with sockets or others), to fork a thread again to run the server and
to connect using the client library - which would allow me to use ghci
and have transparent serialization under the hood.

Are there known use cases?  Am I missing something, for instance is it
possible to interact with a running process directly using ghci?
Any tips?

Thank you,

Elise

From sumit.sahrawat.apm13 at iitbhu.ac.in  Wed Mar  4 09:45:09 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Wed, 4 Mar 2015 15:15:09 +0530
Subject: [Haskell-beginners] command-line interaction with running
	process
In-Reply-To: <CAHfyCqnaKuif5wtiPAfGOVRyOJ6hW57EwR7L6gwLRJS6k8Ty3A@mail.gmail.com>
References: <CAHfyCqnaKuif5wtiPAfGOVRyOJ6hW57EwR7L6gwLRJS6k8Ty3A@mail.gmail.com>
Message-ID: <CAJbEW8Pde=bxrH9kR0jpBsJYmmVJW0i9t_khOC_-0-qK5-6ZmA@mail.gmail.com>

If you decice upon the server-client architecture, you should take a look
at zeromq.
It is used extensively in IHaskell, so you can peek at the code there for a
working example.

Also, I believe a client-server architecture will be better as I personally
enjoy games a lot more when they are multi-player.
It makes the conversion from single-player to multi-player a
non-destructive (or atleast less destructive) process.

On 4 March 2015 at 15:02, Elise Huard <haskell at elisehuard.be> wrote:

> Hi,
>
> I'm trying to do the following: implementing a command-line interface
> to directly interact with the state or the parameters of a
> looping/running process (a game, to be specific).
>
> My first thought is to fork a thread, passing in a TChan/TVar or
> similar to enable communication, and to have the command-line
> interface on the thread (getLine or haskelline ...).
> Another option would be to have a bona fide server-client interface
> (with sockets or others), to fork a thread again to run the server and
> to connect using the client library - which would allow me to use ghci
> and have transparent serialization under the hood.
>
> Are there known use cases?  Am I missing something, for instance is it
> possible to interact with a running process directly using ghci?
> Any tips?
>
> Thank you,
>
> Elise
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>



-- 
Regards

Sumit Sahrawat
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From haskell at elisehuard.be  Wed Mar  4 09:58:26 2015
From: haskell at elisehuard.be (Elise Huard)
Date: Wed, 4 Mar 2015 10:58:26 +0100
Subject: [Haskell-beginners] command-line interaction with running
	process
In-Reply-To: <CAJbEW8Pde=bxrH9kR0jpBsJYmmVJW0i9t_khOC_-0-qK5-6ZmA@mail.gmail.com>
References: <CAHfyCqnaKuif5wtiPAfGOVRyOJ6hW57EwR7L6gwLRJS6k8Ty3A@mail.gmail.com>
 <CAJbEW8Pde=bxrH9kR0jpBsJYmmVJW0i9t_khOC_-0-qK5-6ZmA@mail.gmail.com>
Message-ID: <CAHfyCqk_3fPY_7yZ46Ytb5BKaJb4OB6tv5J+r5S5ic2kT7ufvA@mail.gmail.com>

Thanks for this answer - that's a good point.
At this point it's mostly for (interactive) game/level design and
testing though :)

On 4 March 2015 at 10:45, Sumit Sahrawat, Maths & Computing, IIT (BHU)
<sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:
> If you decice upon the server-client architecture, you should take a look at
> zeromq.
> It is used extensively in IHaskell, so you can peek at the code there for a
> working example.
>
> Also, I believe a client-server architecture will be better as I personally
> enjoy games a lot more when they are multi-player.
> It makes the conversion from single-player to multi-player a non-destructive
> (or atleast less destructive) process.
>
> On 4 March 2015 at 15:02, Elise Huard <haskell at elisehuard.be> wrote:
>>
>> Hi,
>>
>> I'm trying to do the following: implementing a command-line interface
>> to directly interact with the state or the parameters of a
>> looping/running process (a game, to be specific).
>>
>> My first thought is to fork a thread, passing in a TChan/TVar or
>> similar to enable communication, and to have the command-line
>> interface on the thread (getLine or haskelline ...).
>> Another option would be to have a bona fide server-client interface
>> (with sockets or others), to fork a thread again to run the server and
>> to connect using the client library - which would allow me to use ghci
>> and have transparent serialization under the hood.
>>
>> Are there known use cases?  Am I missing something, for instance is it
>> possible to interact with a running process directly using ghci?
>> Any tips?
>>
>> Thank you,
>>
>> Elise
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
>
>
> --
> Regards
>
> Sumit Sahrawat
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>

From objitsu at gmail.com  Wed Mar  4 10:44:45 2015
From: objitsu at gmail.com (emacstheviking)
Date: Wed, 4 Mar 2015 10:44:45 +0000
Subject: [Haskell-beginners] command-line interaction with running
	process
In-Reply-To: <CAHfyCqnaKuif5wtiPAfGOVRyOJ6hW57EwR7L6gwLRJS6k8Ty3A@mail.gmail.com>
References: <CAHfyCqnaKuif5wtiPAfGOVRyOJ6hW57EwR7L6gwLRJS6k8Ty3A@mail.gmail.com>
Message-ID: <CAEiEuUKU1w1gM87xApGUNCOaKDzX-ND2g7Pd7VkP+0u5OJaDXQ@mail.gmail.com>

I did something similar a while back using a socket based "terminal
session" into an OpenGL program so I could change things as it ran.

It's viable. I used TVar I think and didn't encounter any issues.
Sadly, the code has vanished but it worked well and wasn't that much code
actually thanks to the libraries being good.


On 4 March 2015 at 09:32, Elise Huard <haskell at elisehuard.be> wrote:

> Hi,
>
> I'm trying to do the following: implementing a command-line interface
> to directly interact with the state or the parameters of a
> looping/running process (a game, to be specific).
>
> My first thought is to fork a thread, passing in a TChan/TVar or
> similar to enable communication, and to have the command-line
> interface on the thread (getLine or haskelline ...).
> Another option would be to have a bona fide server-client interface
> (with sockets or others), to fork a thread again to run the server and
> to connect using the client library - which would allow me to use ghci
> and have transparent serialization under the hood.
>
> Are there known use cases?  Am I missing something, for instance is it
> possible to interact with a running process directly using ghci?
> Any tips?
>
> Thank you,
>
> Elise
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From objitsu at gmail.com  Wed Mar  4 10:45:39 2015
From: objitsu at gmail.com (emacstheviking)
Date: Wed, 4 Mar 2015 10:45:39 +0000
Subject: [Haskell-beginners] LTS is good but...
In-Reply-To: <etPan.54f6d10a.3f2dba31.45ae@d12frosted.local>
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
 <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
 <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>
 <CAKA2JgKVtCHW1YbLcaBjdBqws6W0GmUrJvhWn6YrPcEHOdvfUw@mail.gmail.com>
 <etPan.54f6d10a.3f2dba31.45ae@d12frosted.local>
Message-ID: <CAEiEuU+ekyG_W9aMhYSJ6TFGX-aK9k2QpNR9fv0KCVXZMgFgkg@mail.gmail.com>

I am a 30+ years hacker Boris, building from source might just tempt me
still!! HAHAHA
My machine is now "clean" so I will decide how to play it!

Thanks again everybody.


On 4 March 2015 at 09:31, Boris <d12frosted at icloud.com> wrote:

> Hey,
>
> Little addition to what Michael already sent you.
>
> To get ?the latest? you should build from sources :D
>
> But in case it?s ok for you to have ghc 7.8.3 and you don?t want to ./configure;
> make; make install, you might also check this installation script on
> github <https://github.com/yogsototh/install-haskell>.
>
>
>
> On March 4, 2015 at 11:28:50, Michael Snoyman (michael at snoyman.com) wrote:
>
> I'd recommend looking at:
>
> http://www.stackage.org/install#mac-os-x
>  https://github.com/bitemyapp/learnhaskell/blob/master/README.md#mac-os-x
>
> I don't generally recommend using the platform installer, it ends up
> pegging libraries to old versions which causes dependency problems, or in
> some cases have known bugs.
>
> On Wed, Mar 4, 2015 at 11:26 AM emacstheviking <objitsu at gmail.com> wrote:
>
>> Agreed Michael!
>>
>> I am currently wiping all Mac Ports packages and I will perform a clean
>> up of /Library/Haskell etc.
>>
>> What is the *preferred* way to get the lastest for OSX: Using homebrew,
>> macports or the Haskell Platform. I *used* to use the platform installer
>> but for some reason (cabal I expect!) I started trying out other ways...
>>
>> I think actually I will go the platform packager again...
>>
>> On 4 March 2015 at 09:13, Michael Snoyman <michael at snoyman.com> wrote:
>>
>>> I can't really explain the details of what's going on here. It seems
>>> that in some cases different versions of the executables are being found,
>>> but I don't know why. Wiping out your sandbox (cabal sandbox delete) and
>>> starting over may be sufficient to recover.
>>>
>>> On Wed, Mar 4, 2015 at 11:11 AM emacstheviking <objitsu at gmail.com>
>>> wrote:
>>>
>>>>  As requested,
>>>>
>>>> iMac:~ vosabristol$ ghc --version
>>>> The Glorious Glasgow Haskell Compilation System, version 7.8.3
>>>>
>>>> which ghc
>>>> /opt/local/bin/ghc
>>>>
>>>> Hmm.... it shows a macports instsallation of GHC... but I am sure
>>>> that's how I git it... I *used* to use the OSX installation package but I
>>>> thought a clean start using MacPorts would somehow change the mood...
>>>>
>>>>  iMac:~ vosabristol$ cabal --version
>>>> cabal-install version 1.16.0.2
>>>> using version 1.16.0 of the Cabal library
>>>>
>>>>  iMac:~ vosabristol$ which cabal
>>>> /usr/bin/cabal
>>>>
>>>> So... there is a possible case for *once again* performing a complete
>>>> and utter removal of Haskell and its partners in crime and doing a fresh
>>>> installation?
>>>>
>>>>
>>>> Out of frustration I built OCaml and OPAM from sources last night but
>>>> I'd still like to go with GHC!
>>>>
>>>> :)
>>>>
>>>>
>>>> On 3 March 2015 at 20:43, Michael Snoyman <michael at snoyman.com> wrote:
>>>>
>>>>> Funny that there's an issue being discussed in the cabal issue tracker
>>>>> right now[1] similar to this. But the report you're giving here is
>>>>> particularly strange. Cabal-1.16 implies that you're using GHC 7.6, which
>>>>> is supported by the claim that base 4.7 isn't available. I'd recommend
>>>>> starting off with running the following commands and pasting the output:
>>>>>
>>>>> ghc --version
>>>>> which ghc
>>>>> cabal --version
>>>>> which cabal
>>>>>
>>>>> [1] https://github.com/haskell/cabal/issues/2438
>>>>>
>>>>>  On Tue, Mar 3, 2015 at 10:21 PM emacstheviking <objitsu at gmail.com>
>>>>> wrote:
>>>>>
>>>>>>  Forgot to add that somehow, despite the upgrade, my cabal version
>>>>>> has gone backwards from 1.20.0.1 to 1.16.0
>>>>>>
>>>>>> WTF?
>>>>>>
>>>>>>
>>>>>> On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:
>>>>>>
>>>>>>> I started an OpenGL project within an LTS sandbox as guided on
>>>>>>> another post and, somewhat foolishly it would see, I did a "cabal install
>>>>>>> cabal-install" when prompted that a new version was available.
>>>>>>>
>>>>>>> The very next time I tried to work on my code.... I got this:
>>>>>>>
>>>>>>>  bash-3.2$ pwd
>>>>>>> /Users/seancharles/Documents/Coding/haskell/lts1
>>>>>>> bash-3.2$ cabal build
>>>>>>> cabal: You need to re-run the 'configure' command. The version of
>>>>>>> Cabal being
>>>>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>>>>> bash-3.2$ cabal configure
>>>>>>> Resolving dependencies...
>>>>>>> Configuring lts1-0.1.0.0...
>>>>>>> cabal: At least the following dependencies are missing:
>>>>>>> base ==4.7.*
>>>>>>> bash-3.2$ cabal build
>>>>>>> cabal: You need to re-run the 'configure' command. The version of
>>>>>>> Cabal being
>>>>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>>>>> bash-3.2$ ls -l
>>>>>>> total 112
>>>>>>> -rw-r--r--  1 seancharles  staff     19 Mar  2 12:53 LICENCSE
>>>>>>> -rw-r--r--  1 seancharles  staff    552 Mar  3 20:09 Main.hs
>>>>>>> -rw-r--r--  1 seancharles  staff    477 Mar  3 20:07 Main.hs~
>>>>>>> -rw-r--r--  1 seancharles  staff     46 Mar  2 12:52 Setup.hs
>>>>>>> -rw-r--r--  1 seancharles  staff  30198 Mar  2 12:51 cabal.config
>>>>>>> -rw-r--r--  1 seancharles  staff   1090 Mar  2 12:51
>>>>>>> cabal.sandbox.config
>>>>>>> drwxr-xr-x  5 seancharles  staff    170 Mar  2 12:53 dist
>>>>>>> -rw-r--r--  1 seancharles  staff   1932 Mar  2 12:52 lts1.cabal
>>>>>>> bash-3.2$
>>>>>>>
>>>>>>> So, welcome to a different kind of hell, "cabal version hell"
>>>>>>> perhaps?
>>>>>>>
>>>>>>> Can anybody help me get it running again?
>>>>>>> Thanks.
>>>>>>> Sean.
>>>>>>>
>>>>>>>
>>>>>>  _______________________________________________
>>>>>> Beginners mailing list
>>>>>> Beginners at haskell.org
>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
>>>> Beginners at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>  _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From objitsu at gmail.com  Wed Mar  4 11:24:57 2015
From: objitsu at gmail.com (emacstheviking)
Date: Wed, 4 Mar 2015 11:24:57 +0000
Subject: [Haskell-beginners] LTS is good but...
In-Reply-To: <CAEiEuU+ekyG_W9aMhYSJ6TFGX-aK9k2QpNR9fv0KCVXZMgFgkg@mail.gmail.com>
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
 <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
 <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>
 <CAKA2JgKVtCHW1YbLcaBjdBqws6W0GmUrJvhWn6YrPcEHOdvfUw@mail.gmail.com>
 <etPan.54f6d10a.3f2dba31.45ae@d12frosted.local>
 <CAEiEuU+ekyG_W9aMhYSJ6TFGX-aK9k2QpNR9fv0KCVXZMgFgkg@mail.gmail.com>
Message-ID: <CAEiEuUJ-dttpY+MYgOonmmuxROS8h4PFrpxWr=m037YkoLc0RQ@mail.gmail.com>

Well, I have port installed 7.8.3 as it needs an existing GHC, then I
unpacked 7.8.4 from tarball and...............

checking whether bootstrap compiler is affected by bug 9439... You are
using a new version of LLVM that hasn't been tested yet!
We will try though...

/var/folders/15/fqzwrj1n32gfgx8yy5t8v1bc0000gq/T/ghc7339_0/ghc7339_6.s:3:2:
     error: unknown directive
            .macosx_version_min 10, 0
            ^

<no location info>:
    Error running clang! you need clang installed to use theLLVM backend
    (or GHC tried to execute clang incorrectly)
failed to compile

Is that a problem??? It seems to have run the configure script just fine
for all of that.


On 4 March 2015 at 10:45, emacstheviking <objitsu at gmail.com> wrote:

> I am a 30+ years hacker Boris, building from source might just tempt me
> still!! HAHAHA
> My machine is now "clean" so I will decide how to play it!
>
> Thanks again everybody.
>
>
> On 4 March 2015 at 09:31, Boris <d12frosted at icloud.com> wrote:
>
>> Hey,
>>
>> Little addition to what Michael already sent you.
>>
>> To get ?the latest? you should build from sources :D
>>
>> But in case it?s ok for you to have ghc 7.8.3 and you don?t want to ./configure;
>> make; make install, you might also check this installation script on
>> github <https://github.com/yogsototh/install-haskell>.
>>
>>
>>
>> On March 4, 2015 at 11:28:50, Michael Snoyman (michael at snoyman.com)
>> wrote:
>>
>> I'd recommend looking at:
>>
>> http://www.stackage.org/install#mac-os-x
>>  https://github.com/bitemyapp/learnhaskell/blob/master/README.md#mac-os-x
>>
>> I don't generally recommend using the platform installer, it ends up
>> pegging libraries to old versions which causes dependency problems, or in
>> some cases have known bugs.
>>
>> On Wed, Mar 4, 2015 at 11:26 AM emacstheviking <objitsu at gmail.com> wrote:
>>
>>> Agreed Michael!
>>>
>>> I am currently wiping all Mac Ports packages and I will perform a clean
>>> up of /Library/Haskell etc.
>>>
>>> What is the *preferred* way to get the lastest for OSX: Using homebrew,
>>> macports or the Haskell Platform. I *used* to use the platform installer
>>> but for some reason (cabal I expect!) I started trying out other ways...
>>>
>>> I think actually I will go the platform packager again...
>>>
>>> On 4 March 2015 at 09:13, Michael Snoyman <michael at snoyman.com> wrote:
>>>
>>>> I can't really explain the details of what's going on here. It seems
>>>> that in some cases different versions of the executables are being found,
>>>> but I don't know why. Wiping out your sandbox (cabal sandbox delete) and
>>>> starting over may be sufficient to recover.
>>>>
>>>> On Wed, Mar 4, 2015 at 11:11 AM emacstheviking <objitsu at gmail.com>
>>>> wrote:
>>>>
>>>>>  As requested,
>>>>>
>>>>> iMac:~ vosabristol$ ghc --version
>>>>> The Glorious Glasgow Haskell Compilation System, version 7.8.3
>>>>>
>>>>> which ghc
>>>>> /opt/local/bin/ghc
>>>>>
>>>>> Hmm.... it shows a macports instsallation of GHC... but I am sure
>>>>> that's how I git it... I *used* to use the OSX installation package but I
>>>>> thought a clean start using MacPorts would somehow change the mood...
>>>>>
>>>>>  iMac:~ vosabristol$ cabal --version
>>>>> cabal-install version 1.16.0.2
>>>>> using version 1.16.0 of the Cabal library
>>>>>
>>>>>  iMac:~ vosabristol$ which cabal
>>>>> /usr/bin/cabal
>>>>>
>>>>> So... there is a possible case for *once again* performing a complete
>>>>> and utter removal of Haskell and its partners in crime and doing a fresh
>>>>> installation?
>>>>>
>>>>>
>>>>> Out of frustration I built OCaml and OPAM from sources last night but
>>>>> I'd still like to go with GHC!
>>>>>
>>>>> :)
>>>>>
>>>>>
>>>>> On 3 March 2015 at 20:43, Michael Snoyman <michael at snoyman.com> wrote:
>>>>>
>>>>>> Funny that there's an issue being discussed in the cabal issue
>>>>>> tracker right now[1] similar to this. But the report you're giving
>>>>>> here is particularly strange. Cabal-1.16 implies that you're using GHC 7.6,
>>>>>> which is supported by the claim that base 4.7 isn't available. I'd
>>>>>> recommend starting off with running the following commands and pasting the
>>>>>> output:
>>>>>>
>>>>>> ghc --version
>>>>>> which ghc
>>>>>> cabal --version
>>>>>> which cabal
>>>>>>
>>>>>> [1] https://github.com/haskell/cabal/issues/2438
>>>>>>
>>>>>>  On Tue, Mar 3, 2015 at 10:21 PM emacstheviking <objitsu at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>>  Forgot to add that somehow, despite the upgrade, my cabal version
>>>>>>> has gone backwards from 1.20.0.1 to 1.16.0
>>>>>>>
>>>>>>> WTF?
>>>>>>>
>>>>>>>
>>>>>>> On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:
>>>>>>>
>>>>>>>> I started an OpenGL project within an LTS sandbox as guided on
>>>>>>>> another post and, somewhat foolishly it would see, I did a "cabal install
>>>>>>>> cabal-install" when prompted that a new version was available.
>>>>>>>>
>>>>>>>> The very next time I tried to work on my code.... I got this:
>>>>>>>>
>>>>>>>>  bash-3.2$ pwd
>>>>>>>> /Users/seancharles/Documents/Coding/haskell/lts1
>>>>>>>> bash-3.2$ cabal build
>>>>>>>> cabal: You need to re-run the 'configure' command. The version of
>>>>>>>> Cabal being
>>>>>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>>>>>> bash-3.2$ cabal configure
>>>>>>>> Resolving dependencies...
>>>>>>>> Configuring lts1-0.1.0.0...
>>>>>>>> cabal: At least the following dependencies are missing:
>>>>>>>> base ==4.7.*
>>>>>>>> bash-3.2$ cabal build
>>>>>>>> cabal: You need to re-run the 'configure' command. The version of
>>>>>>>> Cabal being
>>>>>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>>>>>> bash-3.2$ ls -l
>>>>>>>> total 112
>>>>>>>> -rw-r--r--  1 seancharles  staff     19 Mar  2 12:53 LICENCSE
>>>>>>>> -rw-r--r--  1 seancharles  staff    552 Mar  3 20:09 Main.hs
>>>>>>>> -rw-r--r--  1 seancharles  staff    477 Mar  3 20:07 Main.hs~
>>>>>>>> -rw-r--r--  1 seancharles  staff     46 Mar  2 12:52 Setup.hs
>>>>>>>> -rw-r--r--  1 seancharles  staff  30198 Mar  2 12:51 cabal.config
>>>>>>>> -rw-r--r--  1 seancharles  staff   1090 Mar  2 12:51
>>>>>>>> cabal.sandbox.config
>>>>>>>> drwxr-xr-x  5 seancharles  staff    170 Mar  2 12:53 dist
>>>>>>>> -rw-r--r--  1 seancharles  staff   1932 Mar  2 12:52 lts1.cabal
>>>>>>>> bash-3.2$
>>>>>>>>
>>>>>>>> So, welcome to a different kind of hell, "cabal version hell"
>>>>>>>> perhaps?
>>>>>>>>
>>>>>>>> Can anybody help me get it running again?
>>>>>>>> Thanks.
>>>>>>>> Sean.
>>>>>>>>
>>>>>>>>
>>>>>>>  _______________________________________________
>>>>>>> Beginners mailing list
>>>>>>> Beginners at haskell.org
>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> Beginners mailing list
>>>>>> Beginners at haskell.org
>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>
>>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
>>>> Beginners at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>  _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
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From objitsu at gmail.com  Wed Mar  4 11:25:28 2015
From: objitsu at gmail.com (emacstheviking)
Date: Wed, 4 Mar 2015 11:25:28 +0000
Subject: [Haskell-beginners] LTS is good but...
In-Reply-To: <CAEiEuUJ-dttpY+MYgOonmmuxROS8h4PFrpxWr=m037YkoLc0RQ@mail.gmail.com>
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
 <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
 <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>
 <CAKA2JgKVtCHW1YbLcaBjdBqws6W0GmUrJvhWn6YrPcEHOdvfUw@mail.gmail.com>
 <etPan.54f6d10a.3f2dba31.45ae@d12frosted.local>
 <CAEiEuU+ekyG_W9aMhYSJ6TFGX-aK9k2QpNR9fv0KCVXZMgFgkg@mail.gmail.com>
 <CAEiEuUJ-dttpY+MYgOonmmuxROS8h4PFrpxWr=m037YkoLc0RQ@mail.gmail.com>
Message-ID: <CAEiEuUKThCKVF9obAedwf9Y9rJTaaivqUZYk95Pyf-ZVRW2hOA@mail.gmail.com>

Forgot this:
----------------------------------------------------------------------
Configure completed successfully.

   Building GHC version  : 7.8.4

   Build platform        : x86_64-apple-darwin
   Host platform         : x86_64-apple-darwin
   Target platform       : x86_64-apple-darwin

   Bootstrapping using   : /opt/local/bin/ghc
      which is version   : 7.8.3

   Using clang               : /usr/bin/gcc
      which is version       : 6.0
   Building a cross compiler : NO
   cpp       : /usr/bin/gcc
   cpp-flags : -E -undef -traditional -Wno-invalid-pp-token -Wno-unicode
-Wno-trigraphs
   ld        : /opt/local/bin/ld
   Happy     :  ()
   Alex      :  ()
   Perl      : /opt/local/bin/perl
   dblatex   :
   xsltproc  : /opt/local/bin/xsltproc

   Using LLVM tools
      llc   :
      opt   :
   HsColour : /opt/local/bin/HsColour

   Building DocBook HTML documentation : YES
   Building DocBook PS documentation   : NO
   Building DocBook PDF documentation  : NO
----------------------------------------------------------------------


On 4 March 2015 at 11:24, emacstheviking <objitsu at gmail.com> wrote:

> Well, I have port installed 7.8.3 as it needs an existing GHC, then I
> unpacked 7.8.4 from tarball and...............
>
> checking whether bootstrap compiler is affected by bug 9439... You are
> using a new version of LLVM that hasn't been tested yet!
> We will try though...
>
> /var/folders/15/fqzwrj1n32gfgx8yy5t8v1bc0000gq/T/ghc7339_0/ghc7339_6.s:3:2:
>      error: unknown directive
>             .macosx_version_min 10, 0
>             ^
>
> <no location info>:
>     Error running clang! you need clang installed to use theLLVM backend
>     (or GHC tried to execute clang incorrectly)
> failed to compile
>
> Is that a problem??? It seems to have run the configure script just fine
> for all of that.
>
>
> On 4 March 2015 at 10:45, emacstheviking <objitsu at gmail.com> wrote:
>
>> I am a 30+ years hacker Boris, building from source might just tempt me
>> still!! HAHAHA
>> My machine is now "clean" so I will decide how to play it!
>>
>> Thanks again everybody.
>>
>>
>> On 4 March 2015 at 09:31, Boris <d12frosted at icloud.com> wrote:
>>
>>> Hey,
>>>
>>> Little addition to what Michael already sent you.
>>>
>>> To get ?the latest? you should build from sources :D
>>>
>>> But in case it?s ok for you to have ghc 7.8.3 and you don?t want to ./configure;
>>> make; make install, you might also check this installation script on
>>> github <https://github.com/yogsototh/install-haskell>.
>>>
>>>
>>>
>>> On March 4, 2015 at 11:28:50, Michael Snoyman (michael at snoyman.com)
>>> wrote:
>>>
>>> I'd recommend looking at:
>>>
>>> http://www.stackage.org/install#mac-os-x
>>>  https://github.com/bitemyapp/learnhaskell/blob/master/README.md#mac-os-x
>>>
>>> I don't generally recommend using the platform installer, it ends up
>>> pegging libraries to old versions which causes dependency problems, or in
>>> some cases have known bugs.
>>>
>>> On Wed, Mar 4, 2015 at 11:26 AM emacstheviking <objitsu at gmail.com>
>>> wrote:
>>>
>>>> Agreed Michael!
>>>>
>>>> I am currently wiping all Mac Ports packages and I will perform a clean
>>>> up of /Library/Haskell etc.
>>>>
>>>> What is the *preferred* way to get the lastest for OSX: Using homebrew,
>>>> macports or the Haskell Platform. I *used* to use the platform installer
>>>> but for some reason (cabal I expect!) I started trying out other ways...
>>>>
>>>> I think actually I will go the platform packager again...
>>>>
>>>> On 4 March 2015 at 09:13, Michael Snoyman <michael at snoyman.com> wrote:
>>>>
>>>>> I can't really explain the details of what's going on here. It seems
>>>>> that in some cases different versions of the executables are being found,
>>>>> but I don't know why. Wiping out your sandbox (cabal sandbox delete) and
>>>>> starting over may be sufficient to recover.
>>>>>
>>>>> On Wed, Mar 4, 2015 at 11:11 AM emacstheviking <objitsu at gmail.com>
>>>>> wrote:
>>>>>
>>>>>>  As requested,
>>>>>>
>>>>>> iMac:~ vosabristol$ ghc --version
>>>>>> The Glorious Glasgow Haskell Compilation System, version 7.8.3
>>>>>>
>>>>>> which ghc
>>>>>> /opt/local/bin/ghc
>>>>>>
>>>>>> Hmm.... it shows a macports instsallation of GHC... but I am sure
>>>>>> that's how I git it... I *used* to use the OSX installation package but I
>>>>>> thought a clean start using MacPorts would somehow change the mood...
>>>>>>
>>>>>>  iMac:~ vosabristol$ cabal --version
>>>>>> cabal-install version 1.16.0.2
>>>>>> using version 1.16.0 of the Cabal library
>>>>>>
>>>>>>  iMac:~ vosabristol$ which cabal
>>>>>> /usr/bin/cabal
>>>>>>
>>>>>> So... there is a possible case for *once again* performing a complete
>>>>>> and utter removal of Haskell and its partners in crime and doing a fresh
>>>>>> installation?
>>>>>>
>>>>>>
>>>>>> Out of frustration I built OCaml and OPAM from sources last night but
>>>>>> I'd still like to go with GHC!
>>>>>>
>>>>>> :)
>>>>>>
>>>>>>
>>>>>> On 3 March 2015 at 20:43, Michael Snoyman <michael at snoyman.com>
>>>>>> wrote:
>>>>>>
>>>>>>> Funny that there's an issue being discussed in the cabal issue
>>>>>>> tracker right now[1] similar to this. But the report you're giving
>>>>>>> here is particularly strange. Cabal-1.16 implies that you're using GHC 7.6,
>>>>>>> which is supported by the claim that base 4.7 isn't available. I'd
>>>>>>> recommend starting off with running the following commands and pasting the
>>>>>>> output:
>>>>>>>
>>>>>>> ghc --version
>>>>>>> which ghc
>>>>>>> cabal --version
>>>>>>> which cabal
>>>>>>>
>>>>>>> [1] https://github.com/haskell/cabal/issues/2438
>>>>>>>
>>>>>>>  On Tue, Mar 3, 2015 at 10:21 PM emacstheviking <objitsu at gmail.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>>  Forgot to add that somehow, despite the upgrade, my cabal version
>>>>>>>> has gone backwards from 1.20.0.1 to 1.16.0
>>>>>>>>
>>>>>>>> WTF?
>>>>>>>>
>>>>>>>>
>>>>>>>> On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:
>>>>>>>>
>>>>>>>>> I started an OpenGL project within an LTS sandbox as guided on
>>>>>>>>> another post and, somewhat foolishly it would see, I did a "cabal install
>>>>>>>>> cabal-install" when prompted that a new version was available.
>>>>>>>>>
>>>>>>>>> The very next time I tried to work on my code.... I got this:
>>>>>>>>>
>>>>>>>>>  bash-3.2$ pwd
>>>>>>>>> /Users/seancharles/Documents/Coding/haskell/lts1
>>>>>>>>> bash-3.2$ cabal build
>>>>>>>>> cabal: You need to re-run the 'configure' command. The version of
>>>>>>>>> Cabal being
>>>>>>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>>>>>>> bash-3.2$ cabal configure
>>>>>>>>> Resolving dependencies...
>>>>>>>>> Configuring lts1-0.1.0.0...
>>>>>>>>> cabal: At least the following dependencies are missing:
>>>>>>>>> base ==4.7.*
>>>>>>>>> bash-3.2$ cabal build
>>>>>>>>> cabal: You need to re-run the 'configure' command. The version of
>>>>>>>>> Cabal being
>>>>>>>>> used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
>>>>>>>>> bash-3.2$ ls -l
>>>>>>>>> total 112
>>>>>>>>> -rw-r--r--  1 seancharles  staff     19 Mar  2 12:53 LICENCSE
>>>>>>>>> -rw-r--r--  1 seancharles  staff    552 Mar  3 20:09 Main.hs
>>>>>>>>> -rw-r--r--  1 seancharles  staff    477 Mar  3 20:07 Main.hs~
>>>>>>>>> -rw-r--r--  1 seancharles  staff     46 Mar  2 12:52 Setup.hs
>>>>>>>>> -rw-r--r--  1 seancharles  staff  30198 Mar  2 12:51 cabal.config
>>>>>>>>> -rw-r--r--  1 seancharles  staff   1090 Mar  2 12:51
>>>>>>>>> cabal.sandbox.config
>>>>>>>>> drwxr-xr-x  5 seancharles  staff    170 Mar  2 12:53 dist
>>>>>>>>> -rw-r--r--  1 seancharles  staff   1932 Mar  2 12:52 lts1.cabal
>>>>>>>>> bash-3.2$
>>>>>>>>>
>>>>>>>>> So, welcome to a different kind of hell, "cabal version hell"
>>>>>>>>> perhaps?
>>>>>>>>>
>>>>>>>>> Can anybody help me get it running again?
>>>>>>>>> Thanks.
>>>>>>>>> Sean.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>  _______________________________________________
>>>>>>>> Beginners mailing list
>>>>>>>> Beginners at haskell.org
>>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> Beginners mailing list
>>>>>>> Beginners at haskell.org
>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>
>>>>>>>
>>>>>> _______________________________________________
>>>>>> Beginners mailing list
>>>>>> Beginners at haskell.org
>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
>>>> Beginners at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>  _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>
>
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From d12frosted at icloud.com  Wed Mar  4 11:28:37 2015
From: d12frosted at icloud.com (Boris)
Date: Wed, 04 Mar 2015 13:28:37 +0200
Subject: [Haskell-beginners] LTS is good but...
In-Reply-To: <CAEiEuUKThCKVF9obAedwf9Y9rJTaaivqUZYk95Pyf-ZVRW2hOA@mail.gmail.com>
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
 <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
 <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>
 <CAKA2JgKVtCHW1YbLcaBjdBqws6W0GmUrJvhWn6YrPcEHOdvfUw@mail.gmail.com>
 <etPan.54f6d10a.3f2dba31.45ae@d12frosted.local>
 <CAEiEuU+ekyG_W9aMhYSJ6TFGX-aK9k2QpNR9fv0KCVXZMgFgkg@mail.gmail.com>
 <CAEiEuUJ-dttpY+MYgOonmmuxROS8h4PFrpxWr=m037YkoLc0RQ@mail.gmail.com>
 <CAEiEuUKThCKVF9obAedwf9Y9rJTaaivqUZYk95Pyf-ZVRW2hOA@mail.gmail.com>
Message-ID: <etPan.54f6ec66.6b8b4567.72f@d12frosted.local>

Have you installed xcode command-line tools?

And what which clang returns?


On March 4, 2015 at 13:26:14, emacstheviking (objitsu at gmail.com) wrote:

Forgot this:
----------------------------------------------------------------------
Configure completed successfully.

? ?Building GHC version ?: 7.8.4

? ?Build platform ? ? ? ?: x86_64-apple-darwin
? ?Host platform ? ? ? ? : x86_64-apple-darwin
? ?Target platform ? ? ? : x86_64-apple-darwin

? ?Bootstrapping using ? : /opt/local/bin/ghc
? ? ? which is version ? : 7.8.3

? ?Using clang ? ? ? ? ? ? ? : /usr/bin/gcc
? ? ? which is version ? ? ? : 6.0
? ?Building a cross compiler : NO
? ?cpp ? ? ? : /usr/bin/gcc
? ?cpp-flags : -E -undef -traditional -Wno-invalid-pp-token -Wno-unicode -Wno-trigraphs
? ?ld ? ? ? ?: /opt/local/bin/ld
? ?Happy ? ? : ?()
? ?Alex ? ? ?: ?()
? ?Perl ? ? ?: /opt/local/bin/perl
? ?dblatex ? :
? ?xsltproc ?: /opt/local/bin/xsltproc

? ?Using LLVM tools
? ? ? llc ? :
? ? ? opt ? :
? ?HsColour : /opt/local/bin/HsColour

? ?Building DocBook HTML documentation : YES
? ?Building DocBook PS documentation ? : NO
? ?Building DocBook PDF documentation ?: NO
----------------------------------------------------------------------


On 4 March 2015 at 11:24, emacstheviking <objitsu at gmail.com> wrote:
Well, I have port installed 7.8.3 as it needs an existing GHC, then I unpacked 7.8.4 from tarball and...............

checking whether bootstrap compiler is affected by bug 9439... You are using a new version of LLVM that hasn't been tested yet!
We will try though...

/var/folders/15/fqzwrj1n32gfgx8yy5t8v1bc0000gq/T/ghc7339_0/ghc7339_6.s:3:2:
? ? ?error: unknown directive
? ? ? ? ? ? .macosx_version_min 10, 0
? ? ? ? ? ? ^

<no location info>:
? ? Error running clang! you need clang installed to use theLLVM backend
? ? (or GHC tried to execute clang incorrectly)
failed to compile

Is that a problem??? It seems to have run the configure script just fine for all of that.


On 4 March 2015 at 10:45, emacstheviking <objitsu at gmail.com> wrote:
I am a 30+ years hacker Boris, building from source might just tempt me still!! HAHAHA
My machine is now "clean" so I will decide how to play it!

Thanks again everybody.


On 4 March 2015 at 09:31, Boris <d12frosted at icloud.com> wrote:
Hey,

Little addition to what Michael already sent you.

To get ?the latest? you should build from sources :D

But in case it?s ok for you to have ghc 7.8.3 and you don?t want to ./configure; make; make install, you might also check this installation script on github.



On March 4, 2015 at 11:28:50, Michael Snoyman (michael at snoyman.com) wrote:

I'd recommend looking at:

http://www.stackage.org/install#mac-os-x
https://github.com/bitemyapp/learnhaskell/blob/master/README.md#mac-os-x

I don't generally recommend using the platform installer, it ends up pegging libraries to old versions which causes dependency problems, or in some cases have known bugs.

On Wed, Mar 4, 2015 at 11:26 AM emacstheviking <objitsu at gmail.com> wrote:
Agreed Michael!

I am currently wiping all Mac Ports packages and I will perform a clean up of /Library/Haskell etc.

What is the *preferred* way to get the lastest for OSX: Using homebrew, macports or the Haskell Platform. I *used* to use the platform installer but for some reason (cabal I expect!) I started trying out other ways...

I think actually I will go the platform packager again...

On 4 March 2015 at 09:13, Michael Snoyman <michael at snoyman.com> wrote:
I can't really explain the details of what's going on here. It seems that in some cases different versions of the executables are being found, but I don't know why. Wiping out your sandbox (cabal sandbox delete) and starting over may be sufficient to recover.

On Wed, Mar 4, 2015 at 11:11 AM emacstheviking <objitsu at gmail.com> wrote:
As requested,

iMac:~ vosabristol$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.8.3

which ghc
/opt/local/bin/ghc

Hmm.... it shows a macports instsallation of GHC... but I am sure that's how I git it... I *used* to use the OSX installation package but I thought a clean start using MacPorts would somehow change the mood...

iMac:~ vosabristol$ cabal --version
cabal-install version 1.16.0.2
using version 1.16.0 of the Cabal library

iMac:~ vosabristol$ which cabal
/usr/bin/cabal

So... there is a possible case for *once again* performing a complete and utter removal of Haskell and its partners in crime and doing a fresh installation?


Out of frustration I built OCaml and OPAM from sources last night but I'd still like to go with GHC!

:)


On 3 March 2015 at 20:43, Michael Snoyman <michael at snoyman.com> wrote:
Funny that there's an issue being discussed in the cabal issue tracker right now[1] similar to this.?But the report you're giving here is particularly strange. Cabal-1.16 implies that you're using GHC 7.6, which is supported by the claim that base 4.7 isn't available. I'd recommend starting off with running the following commands and pasting the output:

ghc --version
which ghc
cabal --version
which cabal

[1]?https://github.com/haskell/cabal/issues/2438

On Tue, Mar 3, 2015 at 10:21 PM emacstheviking <objitsu at gmail.com> wrote:
Forgot to add that somehow, despite the upgrade, my cabal version has gone backwards from 1.20.0.1 to 1.16.0  

WTF?


On 3 March 2015 at 20:19, emacstheviking <objitsu at gmail.com> wrote:
I started an OpenGL project within an LTS sandbox as guided on another post and, somewhat foolishly it would see, I did a "cabal install cabal-install" when prompted that a new version was available.

The very next time I tried to work on my code.... I got this:

bash-3.2$ pwd
/Users/seancharles/Documents/Coding/haskell/lts1
bash-3.2$ cabal build
cabal: You need to re-run the 'configure' command. The version of Cabal being
used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
bash-3.2$ cabal configure
Resolving dependencies...
Configuring lts1-0.1.0.0...
cabal: At least the following dependencies are missing:
base ==4.7.*
bash-3.2$ cabal build
cabal: You need to re-run the 'configure' command. The version of Cabal being
used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0).
bash-3.2$ ls -l
total 112
-rw-r--r-- ?1 seancharles ?staff ? ? 19 Mar ?2 12:53 LICENCSE
-rw-r--r-- ?1 seancharles ?staff ? ?552 Mar ?3 20:09 Main.hs
-rw-r--r-- ?1 seancharles ?staff ? ?477 Mar ?3 20:07 Main.hs~
-rw-r--r-- ?1 seancharles ?staff ? ? 46 Mar ?2 12:52 Setup.hs
-rw-r--r-- ?1 seancharles ?staff ?30198 Mar ?2 12:51 cabal.config
-rw-r--r-- ?1 seancharles ?staff ? 1090 Mar ?2 12:51 cabal.sandbox.config
drwxr-xr-x ?5 seancharles ?staff ? ?170 Mar ?2 12:53 dist
-rw-r--r-- ?1 seancharles ?staff ? 1932 Mar ?2 12:52 lts1.cabal
bash-3.2$?

So, welcome to a different kind of hell, "cabal version hell" perhaps?

Can anybody help me get it running again?
Thanks.
Sean.


_______________________________________________
Beginners mailing list
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_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners


_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners


_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners




_______________________________________________  
Beginners mailing list  
Beginners at haskell.org  
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From ky3 at atamo.com  Wed Mar  4 12:10:49 2015
From: ky3 at atamo.com (Kim-Ee Yeoh)
Date: Wed, 4 Mar 2015 19:10:49 +0700
Subject: [Haskell-beginners] Alfred Programs in Haskell
In-Reply-To: <54F69A50.4030308@customct.com>
References: <54F69A50.4030308@customct.com>
Message-ID: <CAPY+ZdS08TtCqpnD8Q9oC6x=iPYUAT=4XXmN77Px_Gf59wdLZg@mail.gmail.com>

On Wed, Mar 4, 2015 at 12:38 PM, Richard Guay <raguay at customct.com> wrote:

>     fExist <- doesFileExist $ h ++ cacheDirBasic ++ getBundleID ++ "/" ++
> fileName
>     if fExist
>     then do
>         contents <- readFile $ h ++ cacheDirBasic ++ getBundleID ++ "/" ++
> fileName
>

First of all, let's refactor that into a let binding, e.g.

    let pathName = h ++ cacheDirBasic ++ getBundleID ++ "/" ++ fileName
    fExist <- doesFileExist pathName
    if fExist then readFile pathName else return ""

The advantage of keeping DRY here should be obvious. Defining the same
thing in 2 places can lead to both getting out of lockstep.

Now as for debugging the actual problem, have you tried the REPL? That is,
launch ghci, load the appropriate libraries, and see what happens when you
enter

doesFileExist "/home/me/myfile"

and similarly for readFile. That way you'd rule things out like a file
permissions problem, etc.


-- Kim-Ee
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From objitsu at gmail.com  Wed Mar  4 12:51:43 2015
From: objitsu at gmail.com (emacstheviking)
Date: Wed, 4 Mar 2015 12:51:43 +0000
Subject: [Haskell-beginners] GHC 7.8.4 source build fails on OSX...
Message-ID: <CAEiEuU+QD=rQ5OvGUZv8yVkJSN7SLwx-B3p6WrUVA9h1NtruSw@mail.gmail.com>

ld: couldn't dlopen() /usr/lib/libdtrace.dylib:
dlopen(/usr/lib/libdtrace.dylib, 1): Symbol not found: _CGLGetCurrentContext
  Referenced from:
/System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo
  Expected in: /System/Library/Frameworks/OpenGL.framework/Versions/A/OpenGL
 in /System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo for
architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see
invocation)
make[1]: *** [rts/dist/build/libHSrts-ghc7.8.4.dylib] Error 1
make: *** [all] Error 2
iMac:ghc-7.8.4 vosabristol$

Perhaps I need to tweak the configure parameters and try again...
stackoverflow has this page:
http://stackoverflow.com/questions/19480099/compiling-macos-application-with-cgl

There is no CGL framework in OS X (CGL is included in the OpenGL framework,
you need only #include OpenGL/OpenGL.h), and you do not use -lXXX to link
against a framework either. Now, depending on how your Makefile is
structured, you may also have a property known as LD_FLAGS for a separate
linking stage - you *also* need to include -framework OpenGL there. This
directive does *both*, include path resolution and linker path resolution
for frameworks. ?  Andon M. Coleman
<http://stackoverflow.com/users/2423205/andon-m-coleman> Oct 20 '13 at 19:36
<http://stackoverflow.com/questions/19480099/compiling-macos-application-with-cgl#comment28894036_19480099>
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From raguay at customct.com  Wed Mar  4 14:01:36 2015
From: raguay at customct.com (Richard Guay)
Date: Wed, 04 Mar 2015 21:01:36 +0700
Subject: [Haskell-beginners] Alfred Programs in Haskell
In-Reply-To: <CAPY+ZdS08TtCqpnD8Q9oC6x=iPYUAT=4XXmN77Px_Gf59wdLZg@mail.gmail.com>
References: <54F69A50.4030308@customct.com>
 <CAPY+ZdS08TtCqpnD8Q9oC6x=iPYUAT=4XXmN77Px_Gf59wdLZg@mail.gmail.com>
Message-ID: <54F71040.40609@customct.com>

It's not a file permission problem because it works great when the file 
exists. But, when the file doesn't exist, it still tries to read the 
file. It acts like the file is there when it isn't. In ghci, it returns 
the true boolean when the file is there and a false when it isn't. But, 
in the program, it is always executing the readFile.

Kim-Ee Yeoh wrote:
>
>
> On Wed, Mar 4, 2015 at 12:38 PM, Richard Guay <raguay at customct.com
> <mailto:raguay at customct.com>> wrote:
>
> fExist <- doesFileExist $ h ++ cacheDirBasic ++ getBundleID ++
> "/" ++ fileName
> if fExist
> then do
> contents <- readFile $ h ++ cacheDirBasic ++ getBundleID
> ++ "/" ++ fileName
>
>
> First of all, let's refactor that into a let binding, e.g.
>
> let pathName = h ++ cacheDirBasic ++ getBundleID ++ "/" ++ fileName
> fExist <- doesFileExist pathName
> if fExist then readFile pathName else return ""
>
> The advantage of keeping DRY here should be obvious. Defining the same
> thing in 2 places can lead to both getting out of lockstep.
>
> Now as for debugging the actual problem, have you tried the REPL? That
> is, launch ghci, load the appropriate libraries, and see what happens
> when you enter
>
> doesFileExist "/home/me/myfile"
>
> and similarly for readFile. That way you'd rule things out like a file
> permissions problem, etc.
>
>
> -- Kim-Ee
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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From objitsu at gmail.com  Wed Mar  4 14:05:24 2015
From: objitsu at gmail.com (emacstheviking)
Date: Wed, 4 Mar 2015 14:05:24 +0000
Subject: [Haskell-beginners] GHC 7.8.4 source build fails on OSX...
In-Reply-To: <CAEiEuU+QD=rQ5OvGUZv8yVkJSN7SLwx-B3p6WrUVA9h1NtruSw@mail.gmail.com>
References: <CAEiEuU+QD=rQ5OvGUZv8yVkJSN7SLwx-B3p6WrUVA9h1NtruSw@mail.gmail.com>
Message-ID: <CAEiEuUJLd66nNhBuMreoSMptwDLS1+TSHPSJj7WU4ZXtcAGcYA@mail.gmail.com>

tried adding:

LIBS="-framework OpenGL" ./configure

same result.... bummer.

On 4 March 2015 at 12:51, emacstheviking <objitsu at gmail.com> wrote:

>
> ld: couldn't dlopen() /usr/lib/libdtrace.dylib:
> dlopen(/usr/lib/libdtrace.dylib, 1): Symbol not found: _CGLGetCurrentContext
>   Referenced from:
> /System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo
>   Expected in:
> /System/Library/Frameworks/OpenGL.framework/Versions/A/OpenGL
>  in /System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo
> for architecture x86_64
> clang: error: linker command failed with exit code 1 (use -v to see
> invocation)
> make[1]: *** [rts/dist/build/libHSrts-ghc7.8.4.dylib] Error 1
> make: *** [all] Error 2
> iMac:ghc-7.8.4 vosabristol$
>
> Perhaps I need to tweak the configure parameters and try again...
> stackoverflow has this page:
>
> http://stackoverflow.com/questions/19480099/compiling-macos-application-with-cgl
>
> There is no CGL framework in OS X (CGL is included in the OpenGL
> framework, you need only #include OpenGL/OpenGL.h), and you do not use
> -lXXX to link against a framework either. Now, depending on how your
> Makefile is structured, you may also have a property known as LD_FLAGS for
> a separate linking stage - you *also* need to include -framework OpenGL there.
> This directive does *both*, include path resolution and linker path
> resolution for frameworks. ?  Andon M. Coleman
> <http://stackoverflow.com/users/2423205/andon-m-coleman> Oct 20 '13 at
> 19:36
> <http://stackoverflow.com/questions/19480099/compiling-macos-application-with-cgl#comment28894036_19480099>
>
>
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From objitsu at gmail.com  Wed Mar  4 14:12:44 2015
From: objitsu at gmail.com (emacstheviking)
Date: Wed, 4 Mar 2015 14:12:44 +0000
Subject: [Haskell-beginners] GHC 7.8.4 source build fails on OSX...
In-Reply-To: <CAEiEuUJLd66nNhBuMreoSMptwDLS1+TSHPSJj7WU4ZXtcAGcYA@mail.gmail.com>
References: <CAEiEuU+QD=rQ5OvGUZv8yVkJSN7SLwx-B3p6WrUVA9h1NtruSw@mail.gmail.com>
 <CAEiEuUJLd66nNhBuMreoSMptwDLS1+TSHPSJj7WU4ZXtcAGcYA@mail.gmail.com>
Message-ID: <CAEiEuUJ60DH=fM-QRGMhc5TXbVgVLefstJHKMDH8g6=vroOMTQ@mail.gmail.com>

The Gods provided me with serendipity;

https://ghcformacosx.github.io/

JOB DONE, FLAWLESS!

Many thanks to those concerned!

:)


On 4 March 2015 at 14:05, emacstheviking <objitsu at gmail.com> wrote:

> tried adding:
>
> LIBS="-framework OpenGL" ./configure
>
> same result.... bummer.
>
> On 4 March 2015 at 12:51, emacstheviking <objitsu at gmail.com> wrote:
>
>>
>> ld: couldn't dlopen() /usr/lib/libdtrace.dylib:
>> dlopen(/usr/lib/libdtrace.dylib, 1): Symbol not found: _CGLGetCurrentContext
>>   Referenced from:
>> /System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo
>>   Expected in:
>> /System/Library/Frameworks/OpenGL.framework/Versions/A/OpenGL
>>  in /System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo
>> for architecture x86_64
>> clang: error: linker command failed with exit code 1 (use -v to see
>> invocation)
>> make[1]: *** [rts/dist/build/libHSrts-ghc7.8.4.dylib] Error 1
>> make: *** [all] Error 2
>> iMac:ghc-7.8.4 vosabristol$
>>
>> Perhaps I need to tweak the configure parameters and try again...
>> stackoverflow has this page:
>>
>> http://stackoverflow.com/questions/19480099/compiling-macos-application-with-cgl
>>
>> There is no CGL framework in OS X (CGL is included in the OpenGL
>> framework, you need only #include OpenGL/OpenGL.h), and you do not use
>> -lXXX to link against a framework either. Now, depending on how your
>> Makefile is structured, you may also have a property known as LD_FLAGS for
>> a separate linking stage - you *also* need to include -framework OpenGL there.
>> This directive does *both*, include path resolution and linker path
>> resolution for frameworks. ?  Andon M. Coleman
>> <http://stackoverflow.com/users/2423205/andon-m-coleman> Oct 20 '13 at
>> 19:36
>> <http://stackoverflow.com/questions/19480099/compiling-macos-application-with-cgl#comment28894036_19480099>
>>
>>
>
>
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From d12frosted at icloud.com  Wed Mar  4 14:18:45 2015
From: d12frosted at icloud.com (Boris)
Date: Wed, 04 Mar 2015 16:18:45 +0200
Subject: [Haskell-beginners] GHC 7.8.4 source build fails on OSX...
In-Reply-To: <CAEiEuUJ60DH=fM-QRGMhc5TXbVgVLefstJHKMDH8g6=vroOMTQ@mail.gmail.com>
References: <CAEiEuU+QD=rQ5OvGUZv8yVkJSN7SLwx-B3p6WrUVA9h1NtruSw@mail.gmail.com>
 <CAEiEuUJLd66nNhBuMreoSMptwDLS1+TSHPSJj7WU4ZXtcAGcYA@mail.gmail.com>
 <CAEiEuUJ60DH=fM-QRGMhc5TXbVgVLefstJHKMDH8g6=vroOMTQ@mail.gmail.com>
Message-ID: <etPan.54f71446.3d1b58ba.72f@d12frosted.local>

Yeah, one might call Michael Snoyman god :D

Writing this because it is Michael who provided you this link
On March 4, 2015 at 11:28:50, Michael Snoyman (michael at snoyman.com) wrote:

I'd recommend looking at:

http://www.stackage.org/install#mac-os-x
https://github.com/bitemyapp/learnhaskell/blob/master/README.md#mac-os-x

I don't generally recommend using the platform installer, it ends up pegging libraries to old versions which causes dependency problems, or in some cases have known bugs.
On March 4, 2015 at 16:13:43, emacstheviking (objitsu at gmail.com) wrote:

The Gods provided me with serendipity;

https://ghcformacosx.github.io/

JOB DONE, FLAWLESS!

Many thanks to those concerned!

:)


On 4 March 2015 at 14:05, emacstheviking <objitsu at gmail.com> wrote:
tried adding:

LIBS="-framework OpenGL" ./configure

same result.... bummer.

On 4 March 2015 at 12:51, emacstheviking <objitsu at gmail.com> wrote:

ld: couldn't dlopen() /usr/lib/libdtrace.dylib: dlopen(/usr/lib/libdtrace.dylib, 1): Symbol not found: _CGLGetCurrentContext
? Referenced from: /System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo
? Expected in: /System/Library/Frameworks/OpenGL.framework/Versions/A/OpenGL
?in /System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[1]: *** [rts/dist/build/libHSrts-ghc7.8.4.dylib] Error 1
make: *** [all] Error 2
iMac:ghc-7.8.4 vosabristol$

Perhaps I need to tweak the configure parameters and try again... stackoverflow has this page:
http://stackoverflow.com/questions/19480099/compiling-macos-application-with-cgl

There is no CGL framework in OS X (CGL is included in the OpenGL framework, you need only?#include OpenGL/OpenGL.h), and you do not use?-lXXX?to link against a framework either. Now, depending on how your Makefile is structured, you may also have a property known as LD_FLAGS for a separate linking stage - you?also?need to include?-framework OpenGL?there. This directive does?both, include path resolution and linker path resolution for frameworks.????Andon M. Coleman?Oct 20 '13 at 19:36?


_______________________________________________  
Beginners mailing list  
Beginners at haskell.org  
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners  
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From allbery.b at gmail.com  Wed Mar  4 14:32:50 2015
From: allbery.b at gmail.com (Brandon Allbery)
Date: Wed, 4 Mar 2015 09:32:50 -0500
Subject: [Haskell-beginners] LTS is good but...
In-Reply-To: <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
 <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
 <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>
Message-ID: <CAKFCL4Xn_QUn=1+Sseorpc36j+YXsNRNJquseP=AAm_p-Een0w@mail.gmail.com>

On Wed, Mar 4, 2015 at 4:25 AM, emacstheviking <objitsu at gmail.com> wrote:

> What is the *preferred* way to get the lastest for OSX: Using homebrew,
> macports or the Haskell Platform. I *used* to use the platform installer
> but for some reason (cabal I expect!) I started trying out other ways...


Depends on who you ask. I use MacPorts and keep it updated, but
please-root-me (aka Homebrew) is preferred by most people these days. You
will find people arguing that you should use the Platform and people who
insist you should always install just ghc and cabal, or people who insist
you shouldn't ever look beyond Stackage (great if Stackage bothers to
acknowledge that the library you need exists).

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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From thbach at students.uni-mainz.de  Wed Mar  4 14:43:12 2015
From: thbach at students.uni-mainz.de (Thomas Bach)
Date: Wed, 4 Mar 2015 15:43:12 +0100
Subject: [Haskell-beginners] Alfred Programs in Haskell
In-Reply-To: <54F71040.40609@customct.com> (Richard Guay's message of "Wed, 4
 Mar 2015 21:01:36 +0700")
References: <54F69A50.4030308@customct.com>
 <CAPY+ZdS08TtCqpnD8Q9oC6x=iPYUAT=4XXmN77Px_Gf59wdLZg@mail.gmail.com>
 <54F71040.40609@customct.com>
Message-ID: <87sidklt6n.fsf@ilxwinb01.fritz.box>

Richard Guay <raguay at customct.com> writes:

> It's not a file permission problem because it works great when the
> file exists. But, when the file doesn't exist, it still tries to read
> the file.

Is this also the case with the version provided by Kim-Ee Yeoh?

> It acts like the file is there when it isn't. In ghci, it returns the
> true boolean when the file is there and a false when it isn't. But, in
> the program, it is always executing the readFile.

This is most unlikely a problem of ghc, i.e. I think there is some
problem in your programs logic. Can you provide further code possibly
restricting it to a minimal breaking example?

Also: have you tried running the function in question in ghci?

Regards

        Thomas.

From michael at snoyman.com  Wed Mar  4 15:31:02 2015
From: michael at snoyman.com (Michael Snoyman)
Date: Wed, 04 Mar 2015 15:31:02 +0000
Subject: [Haskell-beginners] LTS is good but...
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
 <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
 <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>
 <CAKFCL4Xn_QUn=1+Sseorpc36j+YXsNRNJquseP=AAm_p-Een0w@mail.gmail.com>
Message-ID: <CAKA2JgKG3RkNvrCBO4NC14d5LRV5FUcLYnEH=b-8qdqVyLaj5Q@mail.gmail.com>

On Wed, Mar 4, 2015 at 4:32 PM Brandon Allbery <allbery.b at gmail.com> wrote:

> On Wed, Mar 4, 2015 at 4:25 AM, emacstheviking <objitsu at gmail.com> wrote:
>
>> What is the *preferred* way to get the lastest for OSX: Using homebrew,
>> macports or the Haskell Platform. I *used* to use the platform installer
>> but for some reason (cabal I expect!) I started trying out other ways...
>
>
> Depends on who you ask. I use MacPorts and keep it updated, but
> please-root-me (aka Homebrew) is preferred by most people these days. You
> will find people arguing that you should use the Platform and people who
> insist you should always install just ghc and cabal, or people who insist
> you shouldn't ever look beyond Stackage (great if Stackage bothers to
> acknowledge that the library you need exists).
>
>
>
Just to clarify, because there seems to be some confusion in your comments
on Stackage. When someone sets up a project to use Stackage[1], you still
have full access to all packages on Hackage, there just isn't any way to be
certain the packages will compile reliably (which is the case with
installing from Hackage as well). And getting a package into Stackage is a
simple, open procedure[2] which anyone is allowed to participate in.

In other words: Stackage acknowledges the existence of every package on
Hackage, and can provide build guarantees for any package that is added to
its build-constraints file.

[1] Minimal procedure: run `wget http://www.stackage.org/lts/cabal.config`
[2] https://github.com/fpco/stackage#get-your-package-included
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From raguay at customct.com  Wed Mar  4 15:36:01 2015
From: raguay at customct.com (Richard Guay)
Date: Wed, 04 Mar 2015 22:36:01 +0700
Subject: [Haskell-beginners] Alfred Programs in Haskell
In-Reply-To: <87sidklt6n.fsf@ilxwinb01.fritz.box>
References: <54F69A50.4030308@customct.com>
 <CAPY+ZdS08TtCqpnD8Q9oC6x=iPYUAT=4XXmN77Px_Gf59wdLZg@mail.gmail.com>
 <54F71040.40609@customct.com> <87sidklt6n.fsf@ilxwinb01.fritz.box>
Message-ID: <54F72661.1030409@customct.com>

Hi,

Sorry. I found my mistake. I figured my newness in the language was 
showing, so I asked for help. But, to my surprise, I have been modifying 
the function for the cache file and testing the data file. :Palm Plant:. 
Sorry for wasting your time. It is working.

Richard

Thomas Bach wrote:
>
> Richard Guay<raguay at customct.com> writes:
>
>>
>> It's not a file permission problem because it works great when the
>> file exists. But, when the file doesn't exist, it still tries to read
>> the file.
>
>
> Is this also the case with the version provided by Kim-Ee Yeoh?
>
>>
>> It acts like the file is there when it isn't. In ghci, it returns the
>> true boolean when the file is there and a false when it isn't. But, in
>> the program, it is always executing the readFile.
>
>
> This is most unlikely a problem of ghc, i.e. I think there is some
> problem in your programs logic. Can you provide further code possibly
> restricting it to a minimal breaking example?
>
> Also: have you tried running the function in question in ghci?
>
> Regards
>
> Thomas.
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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From sean at objitsu.com  Wed Mar  4 16:09:59 2015
From: sean at objitsu.com (emacstheviking)
Date: Wed, 4 Mar 2015 16:09:59 +0000
Subject: [Haskell-beginners] LTS is good but...
In-Reply-To: <CAKA2JgKG3RkNvrCBO4NC14d5LRV5FUcLYnEH=b-8qdqVyLaj5Q@mail.gmail.com>
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
 <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
 <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>
 <CAKFCL4Xn_QUn=1+Sseorpc36j+YXsNRNJquseP=AAm_p-Een0w@mail.gmail.com>
 <CAKA2JgKG3RkNvrCBO4NC14d5LRV5FUcLYnEH=b-8qdqVyLaj5Q@mail.gmail.com>
Message-ID: <CAEiEuUKoWveJNfnZwR6ubdJj7x1AJo-jOLtZd_BUW8A4a_PWiw@mail.gmail.com>

OK, having installed it, am I now at liberty to use the LTS instructions to
once again attempt to start an OpenGL project?



On 4 March 2015 at 15:31, Michael Snoyman <michael at snoyman.com> wrote:

>
>
> On Wed, Mar 4, 2015 at 4:32 PM Brandon Allbery <allbery.b at gmail.com>
> wrote:
>
>> On Wed, Mar 4, 2015 at 4:25 AM, emacstheviking <objitsu at gmail.com> wrote:
>>
>>> What is the *preferred* way to get the lastest for OSX: Using homebrew,
>>> macports or the Haskell Platform. I *used* to use the platform installer
>>> but for some reason (cabal I expect!) I started trying out other ways...
>>
>>
>> Depends on who you ask. I use MacPorts and keep it updated, but
>> please-root-me (aka Homebrew) is preferred by most people these days. You
>> will find people arguing that you should use the Platform and people who
>> insist you should always install just ghc and cabal, or people who insist
>> you shouldn't ever look beyond Stackage (great if Stackage bothers to
>> acknowledge that the library you need exists).
>>
>>
>>
> Just to clarify, because there seems to be some confusion in your comments
> on Stackage. When someone sets up a project to use Stackage[1], you still
> have full access to all packages on Hackage, there just isn't any way to be
> certain the packages will compile reliably (which is the case with
> installing from Hackage as well). And getting a package into Stackage is a
> simple, open procedure[2] which anyone is allowed to participate in.
>
> In other words: Stackage acknowledges the existence of every package on
> Hackage, and can provide build guarantees for any package that is added to
> its build-constraints file.
>
> [1] Minimal procedure: run `wget http://www.stackage.org/lts/cabal.config`
> <http://www.stackage.org/lts/cabal.config>
> [2] https://github.com/fpco/stackage#get-your-package-included
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From michael at snoyman.com  Wed Mar  4 16:16:29 2015
From: michael at snoyman.com (Michael Snoyman)
Date: Wed, 04 Mar 2015 16:16:29 +0000
Subject: [Haskell-beginners] LTS is good but...
References: <CAEiEuUKkqD6b7uz0Zb=PpJAJzG8n9US=2f3H1mL7VBTCwgkN6Q@mail.gmail.com>
 <CAEiEuUJZeJ4LEYCwB81o_crFiSJUx9d4abLUeZprBgMRGm362A@mail.gmail.com>
 <CAKA2JgKTkTuwEA=EeAd2DAt6xQU2PfHgovunuC+mZ3E+Y++wEg@mail.gmail.com>
 <CAEiEuULQzspFZ76nYy1hx+eVObw=A3koj201AvKM2ogcptxQYQ@mail.gmail.com>
 <CAKA2JgLFCUyvTHV-53bx1Vc7PAfWUbfnw6QMHe4qLosx2nO8bQ@mail.gmail.com>
 <CAEiEuUK==P-y4kRPd=PYJ2Yr9yw-pCTe2y1LGBkV+8WkZD_Q4g@mail.gmail.com>
 <CAKFCL4Xn_QUn=1+Sseorpc36j+YXsNRNJquseP=AAm_p-Een0w@mail.gmail.com>
 <CAKA2JgKG3RkNvrCBO4NC14d5LRV5FUcLYnEH=b-8qdqVyLaj5Q@mail.gmail.com>
 <CAEiEuUKoWveJNfnZwR6ubdJj7x1AJo-jOLtZd_BUW8A4a_PWiw@mail.gmail.com>
Message-ID: <CAKA2JgL00Oj2hpDCZuDh-fOm39n768MF1C9exJDUYfBi8_8nsg@mail.gmail.com>

Yes.

On Wed, Mar 4, 2015, 6:10 PM emacstheviking <sean at objitsu.com> wrote:

> OK, having installed it, am I now at liberty to use the LTS instructions
> to once again attempt to start an OpenGL project?
>
>
>
> On 4 March 2015 at 15:31, Michael Snoyman <michael at snoyman.com> wrote:
>
>>
>>
>> On Wed, Mar 4, 2015 at 4:32 PM Brandon Allbery <allbery.b at gmail.com>
>> wrote:
>>
>>> On Wed, Mar 4, 2015 at 4:25 AM, emacstheviking <objitsu at gmail.com>
>>> wrote:
>>>
>>>> What is the *preferred* way to get the lastest for OSX: Using homebrew,
>>>> macports or the Haskell Platform. I *used* to use the platform installer
>>>> but for some reason (cabal I expect!) I started trying out other ways...
>>>
>>>
>>> Depends on who you ask. I use MacPorts and keep it updated, but
>>> please-root-me (aka Homebrew) is preferred by most people these days. You
>>> will find people arguing that you should use the Platform and people who
>>> insist you should always install just ghc and cabal, or people who insist
>>> you shouldn't ever look beyond Stackage (great if Stackage bothers to
>>> acknowledge that the library you need exists).
>>>
>>>
>>>
>> Just to clarify, because there seems to be some confusion in your
>> comments on Stackage. When someone sets up a project to use Stackage[1],
>> you still have full access to all packages on Hackage, there just isn't any
>> way to be certain the packages will compile reliably (which is the case
>> with installing from Hackage as well). And getting a package into Stackage
>> is a simple, open procedure[2] which anyone is allowed to participate in.
>>
>> In other words: Stackage acknowledges the existence of every package on
>> Hackage, and can provide build guarantees for any package that is added to
>> its build-constraints file.
>>
>> [1] Minimal procedure: run `wget
>> http://www.stackage.org/lts/cabal.config`
>> <http://www.stackage.org/lts/cabal.config>
>> [2] https://github.com/fpco/stackage#get-your-package-included
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From konstantin.saveljev at gmail.com  Thu Mar  5 13:43:30 2015
From: konstantin.saveljev at gmail.com (Konstantin Saveljev)
Date: Thu, 5 Mar 2015 15:43:30 +0200
Subject: [Haskell-beginners] cyclic dependency problem
Message-ID: <CAKncEjpiFcf2Ma_ePfWhCUMCES_jTPiMGGMgbzqLpOcqatUprQ@mail.gmail.com>

Hello,

I'm having some trouble with cyclic dependency.

My simplified version of hierarchy:

    module A where

    import MyState

    data A a = A (StateT MyState IO a) deriving (...)

Now there is a module MyState:

    module MyState where

    import SomeType

    data MyState = MyState { st :: SomeType, ... }

Finally the module that introduces cyclic dependency:

    module SomeType where

    import A

    data SomeType = SomeType { f :: A (), ... }

I have been suggested to move the types into one module and then import it
wherever needed. But the problem is that even if I put them into one file
those three data types still form a cyclic dependency and compiler throws
errors saying "i don't know about this"

So if I have a single module:

    data A a = A (StateT MyState IO a) deriving (...)
    data MyState = MyState { st :: SomeType, ... }
    data SomeType = SomeType { f :: A (), ... }

With this setup the compiler is gonna tell us it doesn't know about
MyState. And no matter how we shuffle these types within a file we are
still getting some "unknowns"

How would one approach such a problem?

Best regards,
Konstantin
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From fa-ml at ariis.it  Thu Mar  5 13:54:13 2015
From: fa-ml at ariis.it (Francesco Ariis)
Date: Thu, 5 Mar 2015 14:54:13 +0100
Subject: [Haskell-beginners] cyclic dependency problem
In-Reply-To: <CAKncEjpiFcf2Ma_ePfWhCUMCES_jTPiMGGMgbzqLpOcqatUprQ@mail.gmail.com>
References: <CAKncEjpiFcf2Ma_ePfWhCUMCES_jTPiMGGMgbzqLpOcqatUprQ@mail.gmail.com>
Message-ID: <20150305135413.GA11695@x60s.casa>

On Thu, Mar 05, 2015 at 03:43:30PM +0200, Konstantin Saveljev wrote:
> I'm having some trouble with cyclic dependency.
> 
> [..]
> 
> So if I have a single module:
> 
>     data A a = A (StateT MyState IO a) deriving (...)
>     data MyState = MyState { st :: SomeType, ... }
>     data SomeType = SomeType { f :: A (), ... }
> 
> With this setup the compiler is gonna tell us it doesn't know about
> MyState. And no matter how we shuffle these types within a file we are
> still getting some "unknowns"

Hello Konstantin. I just tried to feed this to ghci:

    data StateT a b = StateT a b
    data A a = A (StateT MyState (IO a))
    data MyState = MyState { st :: SomeType }
    data SomeType = SomeType { f :: A () }

and it works. I think the deriving clause is causing problems (not
surprisingly); writing the instance out yourself should solve it.

From amindfv at gmail.com  Thu Mar  5 14:25:03 2015
From: amindfv at gmail.com (amindfv at gmail.com)
Date: Thu, 5 Mar 2015 09:25:03 -0500
Subject: [Haskell-beginners] cyclic dependency problem
In-Reply-To: <CAKncEjpiFcf2Ma_ePfWhCUMCES_jTPiMGGMgbzqLpOcqatUprQ@mail.gmail.com>
References: <CAKncEjpiFcf2Ma_ePfWhCUMCES_jTPiMGGMgbzqLpOcqatUprQ@mail.gmail.com>
Message-ID: <D2C35182-B034-44F6-893C-288067C7B1B7@gmail.com>



El Mar 5, 2015, a las 8:43, Konstantin Saveljev <konstantin.saveljev at gmail.com> escribi?:

> Hello,
> 
> I'm having some trouble with cyclic dependency.
> 
> My simplified version of hierarchy:
> 
>     module A where
> 
>     import MyState
> 
>     data A a = A (StateT MyState IO a) deriving (...)
> 
> Now there is a module MyState:
> 
>     module MyState where
> 
>     import SomeType
> 
>     data MyState = MyState { st :: SomeType, ... }
> 
> Finally the module that introduces cyclic dependency:
> 
>     module SomeType where
> 
>     import A
> 
>     data SomeType = SomeType { f :: A (), ... }
> 
> I have been suggested to move the types into one module and then import it wherever needed. But the problem is that even if I put them into one file those three data types still form a cyclic dependency and compiler throws errors saying "i don't know about this"
> 
> So if I have a single module:
> 
>     data A a = A (StateT MyState IO a) deriving (...)
>     data MyState = MyState { st :: SomeType, ... }
>     data SomeType = SomeType { f :: A (), ... }
> 
> With this setup the compiler is gonna tell us it doesn't know about MyState. And no matter how we shuffle these types within a file we are still getting some "unknowns"
> 

Did you try it? It should work fine -- haskell doesn't care about the order of data declarations within a file.

Tom


> How would one approach such a problem? 
> 
> Best regards,
> Konstantin
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

From konstantin.saveljev at gmail.com  Thu Mar  5 14:37:02 2015
From: konstantin.saveljev at gmail.com (Konstantin Saveljev)
Date: Thu, 5 Mar 2015 16:37:02 +0200
Subject: [Haskell-beginners] cyclic dependency problem
In-Reply-To: <D2C35182-B034-44F6-893C-288067C7B1B7@gmail.com>
References: <CAKncEjpiFcf2Ma_ePfWhCUMCES_jTPiMGGMgbzqLpOcqatUprQ@mail.gmail.com>
 <D2C35182-B034-44F6-893C-288067C7B1B7@gmail.com>
Message-ID: <CAKncEjrZWtZMypQcxqXY3R=H+1T2D=RYnE4FQ4NkFp3pZ7vkxw@mail.gmail.com>

I believe Francesco has a point here. I do have "deriving (MonadState
MyState)" which is probably causing the problem.

Unfortunately I'm not capable of writing the instance myself. Maybe someone
could help me?

My full declaration looks something like this:

    newtype X a = X (StateT MyState (ExceptT String IO) a)
                    deriving (Functor, Applicative, Monad, MonadIO,
MonadError String, MonadState MyState)

If I was to remove "MonadState MyState" from the "deriving" section, how
would one implement it?


Konstantin

On Thu, Mar 5, 2015 at 4:25 PM, <amindfv at gmail.com> wrote:

>
>
> El Mar 5, 2015, a las 8:43, Konstantin Saveljev <
> konstantin.saveljev at gmail.com> escribi?:
>
> > Hello,
> >
> > I'm having some trouble with cyclic dependency.
> >
> > My simplified version of hierarchy:
> >
> >     module A where
> >
> >     import MyState
> >
> >     data A a = A (StateT MyState IO a) deriving (...)
> >
> > Now there is a module MyState:
> >
> >     module MyState where
> >
> >     import SomeType
> >
> >     data MyState = MyState { st :: SomeType, ... }
> >
> > Finally the module that introduces cyclic dependency:
> >
> >     module SomeType where
> >
> >     import A
> >
> >     data SomeType = SomeType { f :: A (), ... }
> >
> > I have been suggested to move the types into one module and then import
> it wherever needed. But the problem is that even if I put them into one
> file those three data types still form a cyclic dependency and compiler
> throws errors saying "i don't know about this"
> >
> > So if I have a single module:
> >
> >     data A a = A (StateT MyState IO a) deriving (...)
> >     data MyState = MyState { st :: SomeType, ... }
> >     data SomeType = SomeType { f :: A (), ... }
> >
> > With this setup the compiler is gonna tell us it doesn't know about
> MyState. And no matter how we shuffle these types within a file we are
> still getting some "unknowns"
> >
>
> Did you try it? It should work fine -- haskell doesn't care about the
> order of data declarations within a file.
>
> Tom
>
>
> > How would one approach such a problem?
> >
> > Best regards,
> > Konstantin
> > _______________________________________________
> > Beginners mailing list
> > Beginners at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From konstantin.saveljev at gmail.com  Thu Mar  5 14:50:43 2015
From: konstantin.saveljev at gmail.com (Konstantin Saveljev)
Date: Thu, 5 Mar 2015 16:50:43 +0200
Subject: [Haskell-beginners] cyclic dependency problem
In-Reply-To: <CAKncEjrZWtZMypQcxqXY3R=H+1T2D=RYnE4FQ4NkFp3pZ7vkxw@mail.gmail.com>
References: <CAKncEjpiFcf2Ma_ePfWhCUMCES_jTPiMGGMgbzqLpOcqatUprQ@mail.gmail.com>
 <D2C35182-B034-44F6-893C-288067C7B1B7@gmail.com>
 <CAKncEjrZWtZMypQcxqXY3R=H+1T2D=RYnE4FQ4NkFp3pZ7vkxw@mail.gmail.com>
Message-ID: <CAKncEjr8-x8zeVA3pkW0=MOs_VTuep7EZsn53tYTNvLptp_1kA@mail.gmail.com>

Would this be a correct implementation?:

{-# LANGUAGE MultiParamTypeClasses #-}     -- for some reason compiler says
to use it ?

import qualified Control.Monad.State.Lazy as Lazy

instance MonadState MyState X where
    get = X Lazy.get
    put s = X $ Lazy.put s


Konstantin

On Thu, Mar 5, 2015 at 4:37 PM, Konstantin Saveljev <
konstantin.saveljev at gmail.com> wrote:

> I believe Francesco has a point here. I do have "deriving (MonadState
> MyState)" which is probably causing the problem.
>
> Unfortunately I'm not capable of writing the instance myself. Maybe
> someone could help me?
>
> My full declaration looks something like this:
>
>     newtype X a = X (StateT MyState (ExceptT String IO) a)
>                     deriving (Functor, Applicative, Monad, MonadIO,
> MonadError String, MonadState MyState)
>
> If I was to remove "MonadState MyState" from the "deriving" section, how
> would one implement it?
>
>
> Konstantin
>
> On Thu, Mar 5, 2015 at 4:25 PM, <amindfv at gmail.com> wrote:
>
>>
>>
>> El Mar 5, 2015, a las 8:43, Konstantin Saveljev <
>> konstantin.saveljev at gmail.com> escribi?:
>>
>> > Hello,
>> >
>> > I'm having some trouble with cyclic dependency.
>> >
>> > My simplified version of hierarchy:
>> >
>> >     module A where
>> >
>> >     import MyState
>> >
>> >     data A a = A (StateT MyState IO a) deriving (...)
>> >
>> > Now there is a module MyState:
>> >
>> >     module MyState where
>> >
>> >     import SomeType
>> >
>> >     data MyState = MyState { st :: SomeType, ... }
>> >
>> > Finally the module that introduces cyclic dependency:
>> >
>> >     module SomeType where
>> >
>> >     import A
>> >
>> >     data SomeType = SomeType { f :: A (), ... }
>> >
>> > I have been suggested to move the types into one module and then import
>> it wherever needed. But the problem is that even if I put them into one
>> file those three data types still form a cyclic dependency and compiler
>> throws errors saying "i don't know about this"
>> >
>> > So if I have a single module:
>> >
>> >     data A a = A (StateT MyState IO a) deriving (...)
>> >     data MyState = MyState { st :: SomeType, ... }
>> >     data SomeType = SomeType { f :: A (), ... }
>> >
>> > With this setup the compiler is gonna tell us it doesn't know about
>> MyState. And no matter how we shuffle these types within a file we are
>> still getting some "unknowns"
>> >
>>
>> Did you try it? It should work fine -- haskell doesn't care about the
>> order of data declarations within a file.
>>
>> Tom
>>
>>
>> > How would one approach such a problem?
>> >
>> > Best regards,
>> > Konstantin
>> > _______________________________________________
>> > Beginners mailing list
>> > Beginners at haskell.org
>> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
>
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From konstantin.saveljev at gmail.com  Thu Mar  5 15:07:13 2015
From: konstantin.saveljev at gmail.com (Konstantin Saveljev)
Date: Thu, 5 Mar 2015 17:07:13 +0200
Subject: [Haskell-beginners] cyclic dependency problem
In-Reply-To: <CAKncEjr8-x8zeVA3pkW0=MOs_VTuep7EZsn53tYTNvLptp_1kA@mail.gmail.com>
References: <CAKncEjpiFcf2Ma_ePfWhCUMCES_jTPiMGGMgbzqLpOcqatUprQ@mail.gmail.com>
 <D2C35182-B034-44F6-893C-288067C7B1B7@gmail.com>
 <CAKncEjrZWtZMypQcxqXY3R=H+1T2D=RYnE4FQ4NkFp3pZ7vkxw@mail.gmail.com>
 <CAKncEjr8-x8zeVA3pkW0=MOs_VTuep7EZsn53tYTNvLptp_1kA@mail.gmail.com>
Message-ID: <CAKncEjrWdY3MyCpAGHfiOoaVWe_5r0mW=J0X_VUFBMQKgy+nTw@mail.gmail.com>

Guys, I'm so sorry. It actually turned out to be the problem with
makeLenses (from Control.Lens) which I also put into the same module. Now
that this stuff is in separate modules everything works and deriving is not
a problem. I'm so sorry for wasting you time and thank you very much for
your help.

Konstantin

On Thu, Mar 5, 2015 at 4:50 PM, Konstantin Saveljev <
konstantin.saveljev at gmail.com> wrote:

> Would this be a correct implementation?:
>
> {-# LANGUAGE MultiParamTypeClasses #-}     -- for some reason compiler
> says to use it ?
>
> import qualified Control.Monad.State.Lazy as Lazy
>
> instance MonadState MyState X where
>     get = X Lazy.get
>     put s = X $ Lazy.put s
>
>
> Konstantin
>
> On Thu, Mar 5, 2015 at 4:37 PM, Konstantin Saveljev <
> konstantin.saveljev at gmail.com> wrote:
>
>> I believe Francesco has a point here. I do have "deriving (MonadState
>> MyState)" which is probably causing the problem.
>>
>> Unfortunately I'm not capable of writing the instance myself. Maybe
>> someone could help me?
>>
>> My full declaration looks something like this:
>>
>>     newtype X a = X (StateT MyState (ExceptT String IO) a)
>>                     deriving (Functor, Applicative, Monad, MonadIO,
>> MonadError String, MonadState MyState)
>>
>> If I was to remove "MonadState MyState" from the "deriving" section, how
>> would one implement it?
>>
>>
>> Konstantin
>>
>> On Thu, Mar 5, 2015 at 4:25 PM, <amindfv at gmail.com> wrote:
>>
>>>
>>>
>>> El Mar 5, 2015, a las 8:43, Konstantin Saveljev <
>>> konstantin.saveljev at gmail.com> escribi?:
>>>
>>> > Hello,
>>> >
>>> > I'm having some trouble with cyclic dependency.
>>> >
>>> > My simplified version of hierarchy:
>>> >
>>> >     module A where
>>> >
>>> >     import MyState
>>> >
>>> >     data A a = A (StateT MyState IO a) deriving (...)
>>> >
>>> > Now there is a module MyState:
>>> >
>>> >     module MyState where
>>> >
>>> >     import SomeType
>>> >
>>> >     data MyState = MyState { st :: SomeType, ... }
>>> >
>>> > Finally the module that introduces cyclic dependency:
>>> >
>>> >     module SomeType where
>>> >
>>> >     import A
>>> >
>>> >     data SomeType = SomeType { f :: A (), ... }
>>> >
>>> > I have been suggested to move the types into one module and then
>>> import it wherever needed. But the problem is that even if I put them into
>>> one file those three data types still form a cyclic dependency and compiler
>>> throws errors saying "i don't know about this"
>>> >
>>> > So if I have a single module:
>>> >
>>> >     data A a = A (StateT MyState IO a) deriving (...)
>>> >     data MyState = MyState { st :: SomeType, ... }
>>> >     data SomeType = SomeType { f :: A (), ... }
>>> >
>>> > With this setup the compiler is gonna tell us it doesn't know about
>>> MyState. And no matter how we shuffle these types within a file we are
>>> still getting some "unknowns"
>>> >
>>>
>>> Did you try it? It should work fine -- haskell doesn't care about the
>>> order of data declarations within a file.
>>>
>>> Tom
>>>
>>>
>>> > How would one approach such a problem?
>>> >
>>> > Best regards,
>>> > Konstantin
>>> > _______________________________________________
>>> > Beginners mailing list
>>> > Beginners at haskell.org
>>> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>
>>
>
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From objitsu at gmail.com  Thu Mar  5 15:22:27 2015
From: objitsu at gmail.com (emacstheviking)
Date: Thu, 5 Mar 2015 15:22:27 +0000
Subject: [Haskell-beginners] Using LTS et. al, a small thank you.
Message-ID: <CAEiEuUJ1R5e=r5G6dt1pANk5rX-zEkPH_Tfiog=vG-=r4GNDrw@mail.gmail.com>

First, find yourself a Pharrel Williams youtube video of Happy!

;)

It might seem crazy what I'm about to say
Functional is here, you can take a break
No more OO confusion that takes up too much space
Punch the air like you don't care it's cool as I say...

Because I code Haskell
Code along if you feel like a project without a boss
Because I code Haskell
Code along if you know that OO is just a bunch of dross
Because I code Haskell
Code along if you know what productivity is to you
Because I code Haskell
Clap along if you that's what you wanna do

I am sure I could write more but that floated in out of nowhere and stuck
as the song came on the radio!! LMAO


In all honesty, I have now got OpenGL, http-client and a bunch of stuff all
playing nicely. Thanks very much everybody for your help yesterday, it's
noce to be hacking Haskell once more.

I'm happy..........
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From defigueiredo at ucdavis.edu  Thu Mar  5 18:30:09 2015
From: defigueiredo at ucdavis.edu (Dimitri DeFigueiredo)
Date: Thu, 05 Mar 2015 15:30:09 -0300
Subject: [Haskell-beginners] Using LTS et. al, a small thank you.
In-Reply-To: <CAEiEuUJ1R5e=r5G6dt1pANk5rX-zEkPH_Tfiog=vG-=r4GNDrw@mail.gmail.com>
References: <CAEiEuUJ1R5e=r5G6dt1pANk5rX-zEkPH_Tfiog=vG-=r4GNDrw@mail.gmail.com>
Message-ID: <54F8A0B1.3000703@ucdavis.edu>

This is the friendliest mailing list I have ever subscribed to.

The people in this list are so nice that even if Haskell were not so 
wonderfully elegant, I would like to learn it just to be able to chat 
and work with them.

:-)

Dimitri


On 05/03/15 12:22, emacstheviking wrote:
> First, find yourself a Pharrel Williams youtube video of Happy!
>
> ;)
>
> It might seem crazy what I'm about to say
> Functional is here, you can take a break
> No more OO confusion that takes up too much space
> Punch the air like you don't care it's cool as I say...
>
> Because I code Haskell
> Code along if you feel like a project without a boss
> Because I code Haskell
> Code along if you know that OO is just a bunch of dross
> Because I code Haskell
> Code along if you know what productivity is to you
> Because I code Haskell
> Clap along if you that's what you wanna do
>
> I am sure I could write more but that floated in out of nowhere and 
> stuck as the song came on the radio!! LMAO
>
>
> In all honesty, I have now got OpenGL, http-client and a bunch of 
> stuff all playing nicely. Thanks very much everybody for your help 
> yesterday, it's noce to be hacking Haskell once more.
>
> I'm happy..........
>
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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From maxwell.j.schneider at gmail.com  Fri Mar  6 01:39:13 2015
From: maxwell.j.schneider at gmail.com (Max Schneider)
Date: Thu, 5 Mar 2015 20:39:13 -0500
Subject: [Haskell-beginners] <$> and <*> pronunciation
Message-ID: <CAJvnjx2-CgvDTJXe=pot1btXL1444B0xajd1=H_koJnKqDNX-A@mail.gmail.com>

(I really hope 'no stupid questions' holds here)
I'm not even going to try framing this into a weighty or practical
question; how do you pronounce the <$> and <*> Applicative functions?
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From fa-ml at ariis.it  Fri Mar  6 01:44:44 2015
From: fa-ml at ariis.it (Francesco Ariis)
Date: Fri, 6 Mar 2015 02:44:44 +0100
Subject: [Haskell-beginners] <$> and <*> pronunciation
In-Reply-To: <CAJvnjx2-CgvDTJXe=pot1btXL1444B0xajd1=H_koJnKqDNX-A@mail.gmail.com>
References: <CAJvnjx2-CgvDTJXe=pot1btXL1444B0xajd1=H_koJnKqDNX-A@mail.gmail.com>
Message-ID: <20150306014444.GA366@x60s.casa>

On Thu, Mar 05, 2015 at 08:39:13PM -0500, Max Schneider wrote:
> [H]ow do you pronounce the <$> and <*> Applicative functions?

Hey Max, <$> is 'mapped to', <*> is 'applied' or 'applied over',

f <$> a <*> b -- reads "f mapped to a applied over b",
              -- but I guess it's more idiomatic and
              -- clear to say you are 'lifting'
              -- function f

From bob at redivi.com  Fri Mar  6 02:32:00 2015
From: bob at redivi.com (Bob Ippolito)
Date: Thu, 5 Mar 2015 18:32:00 -0800
Subject: [Haskell-beginners]  <$> and <*> pronunciation
In-Reply-To: <CAJvnjx2-CgvDTJXe=pot1btXL1444B0xajd1=H_koJnKqDNX-A@mail.gmail.com>
References: <CAJvnjx2-CgvDTJXe=pot1btXL1444B0xajd1=H_koJnKqDNX-A@mail.gmail.com>
Message-ID: <CACwMPm959mtcd6S19nsuJ=r1J6cBDXVoKA7E96rdje7t9ZdsYA@mail.gmail.com>

Not a stupid question at all!

<$> is fmap (literally it is just an operator version of Functor's fmap)
<*> is ap (although ap may still be specialized to Monad rather than
Applicative, it is the same operation)

On Thu, Mar 5, 2015 at 5:39 PM, Max Schneider <maxwell.j.schneider at gmail.com
<javascript:_e(%7B%7D,'cvml','maxwell.j.schneider at gmail.com');>> wrote:

> (I really hope 'no stupid questions' holds here)
> I'm not even going to try framing this into a weighty or practical
> question; how do you pronounce the <$> and <*> Applicative functions?
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> <javascript:_e(%7B%7D,'cvml','Beginners at haskell.org');>
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From animeshsaxena at icloud.com  Fri Mar  6 05:59:27 2015
From: animeshsaxena at icloud.com (Animesh Saxena)
Date: Fri, 06 Mar 2015 05:59:27 +0000 (GMT)
Subject: [Haskell-beginners] Function Composition/Map
Message-ID: <d1ec5a1b-4c04-4929-915f-4bfea89844bc@me.com>

I have a simple function?

fmove::Char->(Char,Char)->Char
? ? ?fmove s (x,y)
? ? ?| s == x = y
? ? ?| otherwise = s

If I say fmove 'a' ('a','b') it will move to 'b'
If I say fmove 'a' ('d','c') it will stay at 'a'

Now I am trying to use this for graph, coz one move can result in multiple result nodes. For example in graphs you can have move from a to b and a to c. So I was trying something like this,

:t (map fmove s) where s is the set of possible start nodes "abcd"

(map fmove s) :: [(Char, Char) -> Char]

Now I want to be able to pass the graph to this function and get all possible list of endings. For example my graph can be like?[('a','b'),('b','c'),('c','a'),('c','d'),('e','a')]. You can see that both 'a' and 'd' are possible from node 'c'.?

I can see that above type signature is wrong for passing the whole graph coz it accepts?[(Char, Char) -> Char]. I initially started with foldl but problem is recursion on top of foldl. The actual objective of the problem is to figure out if a node is reachable from a start node or not. Hope the problem is clear, can anyone suggest how to convert the map function to accept the graph??

For each of the possible start nodes (accumulating at each step) it should apply it to the same graph.?

-Animesh


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From animeshsaxena at icloud.com  Fri Mar  6 06:33:00 2015
From: animeshsaxena at icloud.com (Animesh Saxena)
Date: Fri, 06 Mar 2015 06:33:00 +0000 (GMT)
Subject: [Haskell-beginners] Function Composition/Map
In-Reply-To: <d1ec5a1b-4c04-4929-915f-4bfea89844bc@me.com>
Message-ID: <4e94998d-9f46-46e4-9a21-9993a90f08fd@me.com>

btw here's my ugly solution

type Node = Char
type Arc ?= (Node, Node)

solve [] arcs =	[]
solve (x:xs) arcs = (filter (/= ' ') $ map (fmove x) arcs) ++ (solve (filter (/= ' ') $ map (fmove x) arcs) arcs)

fmove::Char->(Char,Char)->Char
fmove s (x,y)
? ? | s == x = y
? ? | otherwise = ' '

isGraph s e arcs = (length $ filter (==e) (take 20 ?$ solve [s] arcs)) > 0

This last check for isGraph should not use this evil number....."20"...coz of infinite list...Hopefully should be able to figure it out..

-Animesh

On Mar 05, 2015, at 09:59 PM, Animesh Saxena <animeshsaxena at icloud.com> wrote:

I have a simple function?

fmove::Char->(Char,Char)->Char
? ? ?fmove s (x,y)
? ? ?| s == x = y
? ? ?| otherwise = s

If I say fmove 'a' ('a','b') it will move to 'b'
If I say fmove 'a' ('d','c') it will stay at 'a'

Now I am trying to use this for graph, coz one move can result in multiple result nodes. For example in graphs you can have move from a to b and a to c. So I was trying something like this,

:t (map fmove s) where s is the set of possible start nodes "abcd"

(map fmove s) :: [(Char, Char) -> Char]

Now I want to be able to pass the graph to this function and get all possible list of endings. For example my graph can be like?[('a','b'),('b','c'),('c','a'),('c','d'),('e','a')]. You can see that both 'a' and 'd' are possible from node 'c'.?

I can see that above type signature is wrong for passing the whole graph coz it accepts?[(Char, Char) -> Char]. I initially started with foldl but problem is recursion on top of foldl. The actual objective of the problem is to figure out if a node is reachable from a start node or not. Hope the problem is clear, can anyone suggest how to convert the map function to accept the graph??

For each of the possible start nodes (accumulating at each step) it should apply it to the same graph.?

-Animesh


_______________________________________________
Beginners mailing list
Beginners at haskell.org 
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners 
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From thbach at students.uni-mainz.de  Fri Mar  6 09:59:35 2015
From: thbach at students.uni-mainz.de (Thomas Bach)
Date: Fri, 6 Mar 2015 10:59:35 +0100
Subject: [Haskell-beginners] Function Composition/Map
In-Reply-To: <d1ec5a1b-4c04-4929-915f-4bfea89844bc@me.com> (Animesh Saxena's
 message of "Fri, 6 Mar 2015 05:59:27 +0000")
References: <d1ec5a1b-4c04-4929-915f-4bfea89844bc@me.com>
Message-ID: <87mw3qjvjs.fsf@ilxwinb01.fritz.box>

Animesh Saxena <animeshsaxena at icloud.com> writes:

> I have a simple function 
>
> fmove::Char->(Char,Char)->Char
> fmove s (x,y)
> | s == x = y
> | otherwise = s
>
> If I say fmove 'a' ('a','b') it will move to 'b'
> If I say fmove 'a' ('d','c') it will stay at 'a'

Note that `fmove 'a' ('c','a')` won't move you to 'c' and in general
`fmove x (y,z)` will move you to the node ' ' if x /= y. As for the
latter I'd suggest to use Maybe to have the possibility of a miss
encoded at type level:

import Data.Maybe

type Node = Char
type Edge = (Char,Char)            
type Graph = [Edge]

fmove:: Node -> Edge -> Maybe Node
fmove s (x,y)
  | s == x = Just y
  | otherwise = Nothing

>
> Now I am trying to use this for graph, coz one move can result in
> multiple result nodes.

Using mapMaybe from Data.Maybe:

oneHopInGraph :: Graph -> Node -> [Node]
oneHopInGraph g n = mapMaybe (fmove n) g

> For example in graphs you can have move from a to b and a to c. So I
> was trying something like this,
>
> :t (map fmove s) where s is the set of possible start nodes "abcd"
>
> (map fmove s) :: [(Char, Char) -> Char]

Note that this is equivalent to `((map fmove) s)` ? function application
binds most in Haskell.

So if you want to map from several start nodes this would be

oneHopFromSeveral :: [Node] -> [Edge] -> [[Node]]
oneHopFromSeveral ns g = map (oneHopInGraph g) ns

> I can see that above type signature is wrong for passing the whole
> graph coz it accepts [(Char, Char) -> Char]. I initially started with
> foldl but problem is recursion on top of foldl.

My suggestion would be to take it easy in the beginning (I take it that
you are a beginner): try to write the recursions yourself and really try
to wrap your head around types. After writing the recursion by hand
start thinking about your code and its types really hard. From time to
time folds and maps will jump on you. And these Heureka moments will
happen more and more frequently.

> The actual objective of the problem is to figure out if a node is
> reachable from a start node or not. Hope the problem is clear, can
> anyone suggest how to convert the map function to accept the graph?

So lets see what we have here the type of the resulting function should
be

allPossibleHops :: Node -> Graph -> [Node]

right? So what is the condition when we can stop? When there aren't any
possible hops:

allPossibleHops n g
  | oneHopInGraph g n == [] = [] 

in the other case the result are the next possible hops plus the result
of all possible hops of these hops:

  | otherwise = nextHops ++ concatMap (flip allPossibleHops g) nextHops
  where nextHops = oneHopInGraph g n

Testing:

?> allPossibleHops 'a' [('a','b'),('b','c'),('c','d'),('e','a')]
"bcd"

You have to make sure that there aren't any cycles for this solution to
work!

I hope this helps.

Regards

        Thomas.

From objitsu at gmail.com  Fri Mar  6 11:16:39 2015
From: objitsu at gmail.com (emacstheviking)
Date: Fri, 6 Mar 2015 11:16:39 +0000
Subject: [Haskell-beginners] Using LTS et. al, a small thank you.
In-Reply-To: <54F8A0B1.3000703@ucdavis.edu>
References: <CAEiEuUJ1R5e=r5G6dt1pANk5rX-zEkPH_Tfiog=vG-=r4GNDrw@mail.gmail.com>
 <54F8A0B1.3000703@ucdavis.edu>
Message-ID: <CAEiEuUJBgLqt+9KKP6qewPGP70iX9pVr8Fgs1SXMeDosc2mbZA@mail.gmail.com>

:)

You have a point. I have been on some really uncomfortable lists over the
years!!

I like the precision of the answers. Haskell hurts your head. (But in a
good way). That's a T-shirt slogan right there!

Sean.


On 5 March 2015 at 18:30, Dimitri DeFigueiredo <defigueiredo at ucdavis.edu>
wrote:

>  This is the friendliest mailing list I have ever subscribed to.
>
> The people in this list are so nice that even if Haskell were not so
> wonderfully elegant, I would like to learn it just to be able to chat and
> work with them.
>
> :-)
>
> Dimitri
>
>
>
> On 05/03/15 12:22, emacstheviking wrote:
>
> First, find yourself a Pharrel Williams youtube video of Happy!
>
>  ;)
>
>  It might seem crazy what I'm about to say
> Functional is here, you can take a break
> No more OO confusion that takes up too much space
> Punch the air like you don't care it's cool as I say...
>
>  Because I code Haskell
> Code along if you feel like a project without a boss
> Because I code Haskell
> Code along if you know that OO is just a bunch of dross
> Because I code Haskell
> Code along if you know what productivity is to you
> Because I code Haskell
> Clap along if you that's what you wanna do
>
>  I am sure I could write more but that floated in out of nowhere and
> stuck as the song came on the radio!! LMAO
>
>
>  In all honesty, I have now got OpenGL, http-client and a bunch of stuff
> all playing nicely. Thanks very much everybody for your help yesterday,
> it's noce to be hacking Haskell once more.
>
>  I'm happy..........
>
>
>
>
> _______________________________________________
> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From animeshsaxena at icloud.com  Sat Mar  7 04:42:21 2015
From: animeshsaxena at icloud.com (Animesh Saxena)
Date: Sat, 07 Mar 2015 04:42:21 +0000 (GMT)
Subject: [Haskell-beginners] State Monad stack example
Message-ID: <c497d8ee-4f8b-4511-8187-72cbbc43af3b@me.com>

I am trying to relate the state monad to a stack example and somehow found it easy to get recursively confused!

instance Monad (State s) where
? ? return x = State $ \s -> (x,s)
? ? (State h) >>= f = State $ \s -> let (a, newState) = h s
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? (State g) = f a
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?in g newState

Considering the stack computation

stackManip stack = let?
? ? ? ? ? ? ? ((), newStack1) = push 3 stack
? ? ? ? ? ? ? (a, newStack2) = pop newStack1
? ? ? ? ? ? ? ?in pop newStack2

in do notation this would become?
do ?
? ?push 3 ?
? ?a <- pop ?
? ?pop

If I consider the first computation push 3 >>= pop and try to translate it to the definition there are problems....
Copy paste again, I have?
? ? (State h) >>= f = State $ \s -> let (a, newState) = h s
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? (State g) = f a
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?in g newState

f is the push function to which we are shoving the old state. I can't exactly get around to what exactly is the state computation h? Ok assuming it's something which gives me a new state, but then thats push which is function f.?
Then push is applied to a which is assuming 3 in this case. This gives me a new state, which I would say newStack1 from the stockManip above.

Then somehow I apply g to newState?? All the more confusion. Back to the question what exactly is state computation and how is it different from f? It seems to be the same function?


-Animesh




? ? ? ? ? ? ? ? ? ? ? ? ? ?
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Sat Mar  7 05:09:00 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Sat, 7 Mar 2015 10:39:00 +0530
Subject: [Haskell-beginners] State Monad stack example
In-Reply-To: <c497d8ee-4f8b-4511-8187-72cbbc43af3b@me.com>
References: <c497d8ee-4f8b-4511-8187-72cbbc43af3b@me.com>
Message-ID: <CAJbEW8MCYuX_41BPq+bMhqoO7KhLk5aV126JxWmVUmeBhVUaoQ@mail.gmail.com>

I won't comment on what state exactly is, but you can read up on that and
gain some intuition here:
https://en.wikibooks.org/wiki/Haskell/Understanding_monads/State
It's helpful to implement it using a pen and paper, and consider how the
state flows and gets transformed.

According to the below example,

  stackManip stack =
    let ((), newStack1) = push 3 stack
        (a, newStack2)  = pop newStack1
    in pop newStack2

We get,

  push :: a -> Stack a -> ((), Stack a)    -- Assuming 'Stack a' is a
defined datatype
  pop  :: Stack a -> (a, Stack a)          -- Representing a stack with
elements of type 'a'

Thus,

    push 3 >>= pop
~~  (Stack a -> ((), Stack a)) >>= (Stack a -> (a, Stack a))
{ Replacing by types }

Bind (>>=) has the type, (for "State s")

  (>>=) :: State s a -> (a -> State s b) -> State s b

This is a type mismatch. The conversion to do syntax is at fault here.
First, you must write the computation using bind (>>=), and then convert to
do-notation.

On 7 March 2015 at 10:12, Animesh Saxena <animeshsaxena at icloud.com> wrote:

> I am trying to relate the state monad to a stack example and somehow found
> it easy to get recursively confused!
>
> instance Monad (State s) where
>     return x = State $ \s -> (x,s)
>     (State h) >>= f = State $ \s -> let (a, newState) = h s
>                                                     (State g) = f a
>                                                      in g newState
>
> Considering the stack computation
>
> stackManip stack = let
>               ((), newStack1) = push 3 stack
>               (a, newStack2) = pop newStack1
>                in pop newStack2
>
> in do notation this would become
> do
>    push 3
>    a <- pop
>    pop
>
> If I consider the first computation push 3 >>= pop and try to translate it
> to the definition there are problems....
> Copy paste again, I have
>     (State h) >>= f = State $ \s -> let (a, newState) = h s
>                                                     (State g) = f a
>                                                      in g newState
>
> f is the push function to which we are shoving the old state. I can't
> exactly get around to what exactly is the state computation h? Ok assuming
> it's something which gives me a new state, but then thats push which is
> function f.
> Then push is applied to a which is assuming 3 in this case. This gives me
> a new state, which I would say newStack1 from the stockManip above.
>
> Then somehow I apply g to newState?? All the more confusion. Back to the
> question what exactly is state computation and how is it different from f?
> It seems to be the same function?
>
>
> -Animesh
>
>
>
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>


-- 
Regards

Sumit Sahrawat
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From edwards.benj at gmail.com  Sat Mar  7 09:45:00 2015
From: edwards.benj at gmail.com (Benjamin Edwards)
Date: Sat, 07 Mar 2015 09:45:00 +0000
Subject: [Haskell-beginners] State Monad stack example
References: <c497d8ee-4f8b-4511-8187-72cbbc43af3b@me.com>
 <CAJbEW8MCYuX_41BPq+bMhqoO7KhLk5aV126JxWmVUmeBhVUaoQ@mail.gmail.com>
Message-ID: <CAN6k4nhfPpzRE0g-X_dN7mFBqpmyi_Zg9xO=7=L5AL1-ccUL5A@mail.gmail.com>

I gave an expansion of the state monad for a different computation on
Reddit some time ago. Perhaps it will be useful to you:

http://www.reddit.com/r/haskell/comments/25fnrj/nicta_course_help_with_state_exercise/

Best,
Ben

On Sat, 7 Mar 2015 5:09 am Sumit Sahrawat, Maths & Computing, IIT (BHU) <
sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

> I won't comment on what state exactly is, but you can read up on that and
> gain some intuition here:
> https://en.wikibooks.org/wiki/Haskell/Understanding_monads/State
> It's helpful to implement it using a pen and paper, and consider how the
> state flows and gets transformed.
>
> According to the below example,
>
>   stackManip stack =
>     let ((), newStack1) = push 3 stack
>         (a, newStack2)  = pop newStack1
>     in pop newStack2
>
> We get,
>
>   push :: a -> Stack a -> ((), Stack a)    -- Assuming 'Stack a' is a
> defined datatype
>   pop  :: Stack a -> (a, Stack a)          -- Representing a stack with
> elements of type 'a'
>
> Thus,
>
>     push 3 >>= pop
> ~~  (Stack a -> ((), Stack a)) >>= (Stack a -> (a, Stack a))
> { Replacing by types }
>
> Bind (>>=) has the type, (for "State s")
>
>   (>>=) :: State s a -> (a -> State s b) -> State s b
>
> This is a type mismatch. The conversion to do syntax is at fault here.
> First, you must write the computation using bind (>>=), and then convert
> to do-notation.
>
> On 7 March 2015 at 10:12, Animesh Saxena <animeshsaxena at icloud.com> wrote:
>
>> I am trying to relate the state monad to a stack example and somehow
>> found it easy to get recursively confused!
>>
>> instance Monad (State s) where
>>     return x = State $ \s -> (x,s)
>>     (State h) >>= f = State $ \s -> let (a, newState) = h s
>>                                                     (State g) = f a
>>                                                      in g newState
>>
>> Considering the stack computation
>>
>> stackManip stack = let
>>               ((), newStack1) = push 3 stack
>>               (a, newStack2) = pop newStack1
>>                in pop newStack2
>>
>> in do notation this would become
>> do
>>    push 3
>>    a <- pop
>>    pop
>>
>> If I consider the first computation push 3 >>= pop and try to translate
>> it to the definition there are problems....
>> Copy paste again, I have
>>     (State h) >>= f = State $ \s -> let (a, newState) = h s
>>                                                     (State g) = f a
>>                                                      in g newState
>>
>> f is the push function to which we are shoving the old state. I can't
>> exactly get around to what exactly is the state computation h? Ok assuming
>> it's something which gives me a new state, but then thats push which is
>> function f.
>> Then push is applied to a which is assuming 3 in this case. This gives me
>> a new state, which I would say newStack1 from the stockManip above.
>>
>> Then somehow I apply g to newState?? All the more confusion. Back to the
>> question what exactly is state computation and how is it different from f?
>> It seems to be the same function?
>>
>>
>> -Animesh
>>
>>
>>
>>
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
>
> --
> Regards
>
> Sumit Sahrawat
>  _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From animeshsaxena at icloud.com  Sat Mar  7 17:34:23 2015
From: animeshsaxena at icloud.com (Animesh Saxena)
Date: Sat, 07 Mar 2015 17:34:23 +0000 (GMT)
Subject: [Haskell-beginners] State Monad stack example
Message-ID: <ae951bf2-c56a-4316-8721-67460a772813@me.com>

Thanks for the links, I am able to get my head around this a bit.?

Here's my simple question
1. In the stack example what is the state. As per my understanding the state is the new stack.
If above statement is correct than by definition of >>=

? (>>=) :: State s a -> (a -> State s b) -> State s b

push 3 stack?
This returns ((), newstack1)
Now in this case the state or context is newstack1 which has to be passed to the next function if i apply >>=.
push 3 stack >>= pop newstack1
This might make sense coz I am shoving (or binding) the state to the pop function. State in this case is the stack which is being passed around or shoved around. Problem is pop doesn't need "a" argument like the definition of (>>=) indicates. It can very well produce a new state or return ?a new state.?

So if state = stack then this makes sense to me, where am i wrong...??

push 3 stack >>= pop newstack1

On Mar 06, 2015, at 09:09 PM, "Sumit Sahrawat, Maths & Computing, IIT (BHU)" <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

I won't comment on what state exactly is, but you can read up on that and gain some intuition here: https://en.wikibooks.org/wiki/Haskell/Understanding_monads/State
It's helpful to implement it using a pen and paper, and consider how the state flows and gets transformed.

According to the below example,

? stackManip stack =
? ? let?((), newStack1) = push 3 stack
? ? ? ? (a, newStack2) ?= pop newStack1
? ? in pop newStack2

We get,

? push :: a -> Stack a -> ((), Stack a) ? ?-- Assuming 'Stack a' is a defined datatype
? pop ?:: Stack a -> (a, Stack a) ? ? ? ? ?-- Representing a stack with elements of type 'a'

Thus,

? ? push 3 >>= pop
~~ ?(Stack a -> ((), Stack a)) >>= (Stack a -> (a, Stack a)) ? ? ? ? ? ? ? { Replacing by types }

Bind (>>=) has the type, (for "State s")

? (>>=) :: State s a -> (a -> State s b) -> State s b

This is a type mismatch. The conversion to do syntax is at fault here.
First, you must write the computation using bind (>>=), and then convert to do-notation.

On 7 March 2015 at 10:12, Animesh Saxena <animeshsaxena at icloud.com> wrote:
I am trying to relate the state monad to a stack example and somehow found it easy to get recursively confused!

instance Monad (State s) where
? ? return x = State $ \s -> (x,s)
? ? (State h) >>= f = State $ \s -> let (a, newState) = h s
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? (State g) = f a
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?in g newState

Considering the stack computation

stackManip stack = let?
? ? ? ? ? ? ? ((), newStack1) = push 3 stack
? ? ? ? ? ? ? (a, newStack2) = pop newStack1
? ? ? ? ? ? ? ?in pop newStack2

in do notation this would become?
do ?
? ?push 3 ?
? ?a <- pop ?
? ?pop

If I consider the first computation push 3 >>= pop and try to translate it to the definition there are problems....
Copy paste again, I have?
? ? (State h) >>= f = State $ \s -> let (a, newState) = h s
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? (State g) = f a
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?in g newState

f is the push function to which we are shoving the old state. I can't exactly get around to what exactly is the state computation h? Ok assuming it's something which gives me a new state, but then thats push which is function f.?
Then push is applied to a which is assuming 3 in this case. This gives me a new state, which I would say newStack1 from the stockManip above.

Then somehow I apply g to newState?? All the more confusion. Back to the question what exactly is state computation and how is it different from f? It seems to be the same function?


-Animesh




? ? ? ? ? ? ? ? ? ? ? ? ? ?

_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners




-- 
Regards

Sumit Sahrawat
_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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From jeffbrown.the at gmail.com  Sat Mar  7 21:52:08 2015
From: jeffbrown.the at gmail.com (Jeffrey Brown)
Date: Sat, 7 Mar 2015 13:52:08 -0800
Subject: [Haskell-beginners] Laziness to efficiently get one element from a
	set
Message-ID: <CAEc4Ma3eKJhBb3aAVSbVcMKFhgPoRDZu4BFDMLrQLck03Yp+mg@mail.gmail.com>

Hi,

Suppose S is a big set, and I'd like to access just a single element from
it. The only natural way I've come up with to do that is this: head $
Data.Set.toList S. If I do that, am I correct that Haskell will not try to
convert all of S to a list; instead it will only convert one element, and
then return it, and leave the rest of the list unevaluated?

Thanks,
Jeff
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From thbach at students.uni-mainz.de  Sat Mar  7 23:49:43 2015
From: thbach at students.uni-mainz.de (Thomas Bach)
Date: Sun, 8 Mar 2015 00:49:43 +0100
Subject: [Haskell-beginners] Laziness to efficiently get one element
	from a set
In-Reply-To: <CAEc4Ma3eKJhBb3aAVSbVcMKFhgPoRDZu4BFDMLrQLck03Yp+mg@mail.gmail.com>
 (Jeffrey Brown's message of "Sat, 7 Mar 2015 13:52:08 -0800")
References: <CAEc4Ma3eKJhBb3aAVSbVcMKFhgPoRDZu4BFDMLrQLck03Yp+mg@mail.gmail.com>
Message-ID: <87385gwep4.fsf@ilxwinb01.fritz.box>

Jeffrey Brown <jeffbrown.the at gmail.com> writes:

> head $ Data.Set.toList S. If I do that, am I correct that Haskell will
> not try to convert all of S to a list; instead it will only convert
> one element, and then return it, and leave the rest of the list
> unevaluated?

This is how toList from Data.Set.Base is defined in containers-0.5.0:

{--------------------------------------------------------------------
  Lists
--------------------------------------------------------------------}
-- | /O(n)/. Convert the set to a list of elements. Subject to list fusion.
toList :: Set a -> [a]
toList = toAscList

-- | /O(n)/. Convert the set to an ascending list of elements. Subject to list fusion.
toAscList :: Set a -> [a]
toAscList = foldr (:) []

The buzzword you are looking for is list fusion:

http://stackoverflow.com/questions/10945429/haskell-list-fusion-where-is-it-needed

Regards

        Thomas Bach.

From jeffbrown.the at gmail.com  Sun Mar  8 00:11:29 2015
From: jeffbrown.the at gmail.com (Jeffrey Brown)
Date: Sat, 7 Mar 2015 16:11:29 -0800
Subject: [Haskell-beginners] Laziness to efficiently get one element
 from a set
In-Reply-To: <87385gwep4.fsf@ilxwinb01.fritz.box>
References: <CAEc4Ma3eKJhBb3aAVSbVcMKFhgPoRDZu4BFDMLrQLck03Yp+mg@mail.gmail.com>
 <87385gwep4.fsf@ilxwinb01.fritz.box>
Message-ID: <CAEc4Ma0hi3jhKRDdeqaDdVu1dTC-TdrjD3ebG8ZHwC_NVW5AmQ@mail.gmail.com>

Fantastic! Thanks, Thomas!

On Sat, Mar 7, 2015 at 3:49 PM, Thomas Bach <thbach at students.uni-mainz.de>
wrote:

> Jeffrey Brown <jeffbrown.the at gmail.com> writes:
>
> > head $ Data.Set.toList S. If I do that, am I correct that Haskell will
> > not try to convert all of S to a list; instead it will only convert
> > one element, and then return it, and leave the rest of the list
> > unevaluated?
>
> This is how toList from Data.Set.Base is defined in containers-0.5.0:
>
> {--------------------------------------------------------------------
>   Lists
> --------------------------------------------------------------------}
> -- | /O(n)/. Convert the set to a list of elements. Subject to list fusion.
> toList :: Set a -> [a]
> toList = toAscList
>
> -- | /O(n)/. Convert the set to an ascending list of elements. Subject to
> list fusion.
> toAscList :: Set a -> [a]
> toAscList = foldr (:) []
>
> The buzzword you are looking for is list fusion:
>
>
> http://stackoverflow.com/questions/10945429/haskell-list-fusion-where-is-it-needed
>
> Regards
>
>         Thomas Bach.
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From animeshsaxena at icloud.com  Sun Mar  8 03:55:09 2015
From: animeshsaxena at icloud.com (Animesh Saxena)
Date: Sun, 08 Mar 2015 11:55:09 +0800
Subject: [Haskell-beginners] State Monad stack example
In-Reply-To: <ae951bf2-c56a-4316-8721-67460a772813@me.com>
References: <ae951bf2-c56a-4316-8721-67460a772813@me.com>
Message-ID: <317E110F-207A-4B0B-9531-A7CCF7BCAC57@icloud.com>

Ok I got the confusion clear, I was referring to the wrong definitions of push and pop

pop = Stack $ (\x:xs) -> (x,xs)
push a = State $ \xs -> ((), xs)

Now the do operation makes sense coz now I can relate all this to simple monads?.

(>>=) :: Monad m => m a -> (a -> m b) -> m b
Where m = state s being shoved around!

-Animesh


> On Mar 8, 2015, at 1:34 AM, Animesh Saxena <animeshsaxena at icloud.com> wrote:
> 
> Thanks for the links, I am able to get my head around this a bit. 
> 
> Here's my simple question
> 1. In the stack example what is the state. As per my understanding the state is the new stack.
> If above statement is correct than by definition of >>=
> 
>>   (>>=) :: State s a -> (a -> State s b) -> State s b
> 
> 
> push 3 stack 
> This returns ((), newstack1)
> Now in this case the state or context is newstack1 which has to be passed to the next function if i apply >>=.
> push 3 stack >>= pop newstack1
> This might make sense coz I am shoving (or binding) the state to the pop function. State in this case is the stack which is being passed around or shoved around. Problem is pop doesn't need "a" argument like the definition of (>>=) indicates. It can very well produce a new state or return  a new state. 
> 
> So if state = stack then this makes sense to me, where am i wrong...??
> 
> push 3 stack >>= pop newstack1
> 
> On Mar 06, 2015, at 09:09 PM, "Sumit Sahrawat, Maths & Computing, IIT (BHU)" <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:
> 
>> I won't comment on what state exactly is, but you can read up on that and gain some intuition here: https://en.wikibooks.org/wiki/Haskell/Understanding_monads/State <https://en.wikibooks.org/wiki/Haskell/Understanding_monads/State>
>> It's helpful to implement it using a pen and paper, and consider how the state flows and gets transformed.
>> 
>> According to the below example,
>> 
>>   stackManip stack =
>>     let ((), newStack1) = push 3 stack
>>         (a, newStack2)  = pop newStack1
>>     in pop newStack2
>> 
>> We get,
>> 
>>   push :: a -> Stack a -> ((), Stack a)    -- Assuming 'Stack a' is a defined datatype
>>   pop  :: Stack a -> (a, Stack a)          -- Representing a stack with elements of type 'a'
>> 
>> Thus,
>> 
>>     push 3 >>= pop
>> ~~  (Stack a -> ((), Stack a)) >>= (Stack a -> (a, Stack a))               { Replacing by types }
>> 
>> Bind (>>=) has the type, (for "State s")
>> 
>>   (>>=) :: State s a -> (a -> State s b) -> State s b
>> 
>> This is a type mismatch. The conversion to do syntax is at fault here.
>> First, you must write the computation using bind (>>=), and then convert to do-notation.
>> 
>> On 7 March 2015 at 10:12, Animesh Saxena <animeshsaxena at icloud.com <mailto:animeshsaxena at icloud.com>> wrote:
>> I am trying to relate the state monad to a stack example and somehow found it easy to get recursively confused!
>> 
>> instance Monad (State s) where
>>     return x = State $ \s -> (x,s)
>>     (State h) >>= f = State $ \s -> let (a, newState) = h s
>>                                                     (State g) = f a
>>                                                      in g newState
>> 
>> Considering the stack computation
>> 
>> stackManip stack = let 
>>               ((), newStack1) = push 3 stack
>>               (a, newStack2) = pop newStack1
>>                in pop newStack2
>> 
>> in do notation this would become 
>> do  
>>    push 3  
>>    a <- pop  
>>    pop
>> 
>> If I consider the first computation push 3 >>= pop and try to translate it to the definition there are problems....
>> Copy paste again, I have 
>>     (State h) >>= f = State $ \s -> let (a, newState) = h s
>>                                                     (State g) = f a
>>                                                      in g newState
>> 
>> f is the push function to which we are shoving the old state. I can't exactly get around to what exactly is the state computation h? Ok assuming it's something which gives me a new state, but then thats push which is function f. 
>> Then push is applied to a which is assuming 3 in this case. This gives me a new state, which I would say newStack1 from the stockManip above.
>> 
>> Then somehow I apply g to newState?? All the more confusion. Back to the question what exactly is state computation and how is it different from f? It seems to be the same function?
>> 
>> 
>> -Animesh
>> 
>> 
>> 
>> 
>>                            
>> 
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org <mailto:Beginners at haskell.org>
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners <http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners>
>> 
>> 
>> 
>> 
>> -- 
>> Regards
>> 
>> Sumit Sahrawat
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org <mailto:Beginners at haskell.org>
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners <http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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From apfelmus at quantentunnel.de  Sun Mar  8 08:59:47 2015
From: apfelmus at quantentunnel.de (Heinrich Apfelmus)
Date: Sun, 08 Mar 2015 09:59:47 +0100
Subject: [Haskell-beginners] Laziness to efficiently get one element
	from a set
In-Reply-To: <87385gwep4.fsf@ilxwinb01.fritz.box>
References: <CAEc4Ma3eKJhBb3aAVSbVcMKFhgPoRDZu4BFDMLrQLck03Yp+mg@mail.gmail.com>
 <87385gwep4.fsf@ilxwinb01.fritz.box>
Message-ID: <mdh324$21b$1@ger.gmane.org>

Thomas Bach wrote:
> Jeffrey Brown <jeffbrown.the at gmail.com> writes:
> 
>> head $ Data.Set.toList S. If I do that, am I correct that Haskell will
>> not try to convert all of S to a list; instead it will only convert
>> one element, and then return it, and leave the rest of the list
>> unevaluated?
> 
> This is how toList from Data.Set.Base is defined in containers-0.5.0:
> 
> {--------------------------------------------------------------------
>   Lists
> --------------------------------------------------------------------}
> -- | /O(n)/. Convert the set to a list of elements. Subject to list fusion.
> toList :: Set a -> [a]
> toList = toAscList
> 
> -- | /O(n)/. Convert the set to an ascending list of elements. Subject to list fusion.
> toAscList :: Set a -> [a]
> toAscList = foldr (:) []
> 
> The buzzword you are looking for is list fusion:
> 
> http://stackoverflow.com/questions/10945429/haskell-list-fusion-where-is-it-needed

Actually, I don't think that list fusion is appropriate here. The 
`foldr` used in the definition of `toAscList` is from the `Foldable` 
type class, and implemented specifically for the `Set` data type. It's 
not the usual fold on lists.

Jeffrey, if you want to pick a single element from a `Set`, I would 
recommend the functions `findMin` or `findMax`. The former satisfies

     Data.Set.findMin = head . Data.Set.toList

so you will get the same element as in your original approach. This 
time, however, you can be sure that it takes O(log n) time, whereas in 
the approach using `head`, it depends on the internals of the 
implementation of `foldr` whether it will take time O(n) or O(log n) or 
something in between. (In particular, it depends on how lazy the 
implementation of `foldr` for `Set` is. See [1] for more on lazy 
evaluation in this / a similar context.)


   [1]: https://hackhands.com/modular-code-lazy-evaluation-haskell/


Best regards,
Heinrich Apfelmus

--
http://apfelmus.nfshost.com


From dot_wangyushi at yeah.net  Sun Mar  8 10:59:57 2015
From: dot_wangyushi at yeah.net (m00nlight)
Date: Sun, 8 Mar 2015 18:59:57 +0800 (CST)
Subject: [Haskell-beginners] Strange behavior of program
Message-ID: <50a09477.2d5.14bf90a9cfb.Coremail.dot_wangyushi@yeah.net>

    
Hi Haskellers,

I encounter an strange behavior of haskell program recently.  The following is my program

```haskell
main = do
    _ <- getLine
    arr1 <- getLine
    _ <- getLine
   arr2 <- getLine
   let a = map (read :: String -> Int) (words arr1)
        b = map (read :: String -> Int) (words arr2)

  putStrLn $ show $ (foldl (*) 1 a)
  putStrLn $ show $ a == [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192]
  putStrLn $ show $ (foldl (*) 1 [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192])
```

With the input test file as following:

```test.in
13
1 2 4 8 32 64 128 256 512 1024 2048 4906 8192
9
1 3 9 27 81 243 729 2187 6561
```

The output is as:
```output
0
True
185343439719667835347140608
```

In fact, from the program, we know that a is equal to list  [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192] ,
but the product of a and the literal list is different.

Can anyone tell me why?

Thanks       








--m00nlight


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From Spam at gettingsharper.de  Sun Mar  8 11:22:45 2015
From: Spam at gettingsharper.de (=?windows-1252?Q?Carsten_K=F6nig?=)
Date: Sun, 08 Mar 2015 12:22:45 +0100
Subject: [Haskell-beginners] Strange behavior of program
In-Reply-To: <50a09477.2d5.14bf90a9cfb.Coremail.dot_wangyushi@yeah.net>
References: <50a09477.2d5.14bf90a9cfb.Coremail.dot_wangyushi@yeah.net>
Message-ID: <54FC3105.20309@gettingsharper.de>

Try

> foldl (*) 1 [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192]

and then

> foldl (*) 1 [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192] :: Int

what do you see?

You might want to check with

> :t foldl (*) 1 [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192]

next

Am 08.03.2015 um 11:59 schrieb m00nlight:
> foldl (*) 1 [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192]


From arjanen.loic at gmail.com  Sun Mar  8 12:01:44 2015
From: arjanen.loic at gmail.com (arjanen.loic at gmail.com)
Date: Sun, 8 Mar 2015 12:01:44 +0000
Subject: [Haskell-beginners] =?utf-8?q?Strange_behavior_of_program?=
In-Reply-To: <50a09477.2d5.14bf90a9cfb.Coremail.dot_wangyushi@yeah.net>
References: <50a09477.2d5.14bf90a9cfb.Coremail.dot_wangyushi@yeah.net>
Message-ID: <54fc3c0b.e22e460a.1f3a.ffff928d@mx.google.com>

Hello,


Given that you specifically indicated in your code that you wanted Int?s instance of Read and that the result of the product is way over 2 ^ 32 (or 2 ^ 64 for that matter), I?d guess the difference comes from the fact that the product of a is done with type Int (that is, modulo 32 or 64 depending on your computer?s CPU architecture) and the product with the litteral list is done with type Integer because that the type Haskell defaults to in this case.






ARJANEN Lo?c Jean David
http://blog.luigiscorner.com
--- 
?Computer science is no more about computers than astronomy is about telescopes, biology is about microscopes, or chemistry is about beakers and test tubes. Science is not about tools. It is about how we use them, and what we find out when we do.?
Michael R. Fellows and Ian Parberry





De : m00nlight
Envoy? : ?dimanche? ?8? ?mars? ?2015 ?21?:?59
? : The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell





    
Hi Haskellers,

I encounter an strange behavior of haskell program recently.  The following is my program

```haskell
main = do
    _ <- getLine
    arr1 <- getLine
    _ <- getLine
   arr2 <- getLine
   let a = map (read :: String -> Int) (words arr1)
        b = map (read :: String -> Int) (words arr2)

  putStrLn $ show $ (foldl (*) 1 a)
  putStrLn $ show $ a == [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192]
  putStrLn $ show $ (foldl (*) 1 [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192])
```

With the input test file as following:

```test.in
13
1 2 4 8 32 64 128 256 512 1024 2048 4906 8192
9
1 3 9 27 81 243 729 2187 6561
```

The output is as:
```output
0
True
185343439719667835347140608
```

In fact, from the program, we know that a is equal to list  [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192] ,
but the product of a and the literal list is different.

Can anyone tell me why?

Thanks       














--m00nlight
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From dot_wangyushi at yeah.net  Sun Mar  8 14:08:39 2015
From: dot_wangyushi at yeah.net (m00nlight)
Date: Sun, 8 Mar 2015 22:08:39 +0800 (CST)
Subject: [Haskell-beginners] Strange behavior of program
In-Reply-To: <54fc3c0b.e22e460a.1f3a.ffff928d@mx.google.com>
References: <50a09477.2d5.14bf90a9cfb.Coremail.dot_wangyushi@yeah.net>
 <54fc3c0b.e22e460a.1f3a.ffff928d@mx.google.com>
Message-ID: <eda9ce5.34b.14bf9b76107.Coremail.dot_wangyushi@yeah.net>

Carsten and Arjanen,

Thanks for your explanation.  :)







--m00nlight



?2015?03?08 20?01?, "arjanen.loic"<arjanen.loic at gmail.com>??:


Hello,


Given that you specifically indicated in your code that you wanted Int?s instance of Read and that the result of the product is way over 2 ^ 32 (or 2 ^ 64 for that matter), I?d guess the difference comes from the fact that the product of a is done with type Int (that is, modulo 32 or 64 depending on your computer?s CPU architecture) and the product with the litteral list is done with type Integer because that the type Haskell defaults to in this case.



ARJANEN Lo?c Jean David
http://blog.luigiscorner.com
---
?Computer science is no more about computers than astronomy is about telescopes, biology is about microscopes, or chemistry is about beakers and test tubes. Science is not about tools. It is about how we use them, and what we find out when we do.?
Michael R. Fellows and Ian Parberry


De : m00nlight
Envoy? : ?dimanche? ?8? ?mars? ?2015 ?21?:?59
? : The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell


    
Hi Haskellers,

I encounter an strange behavior of haskell program recently.  The following is my program

```haskell
main = do
    _ <- getLine
    arr1 <- getLine
    _ <- getLine
   arr2 <- getLine
   let a = map (read :: String -> Int) (words arr1)
        b = map (read :: String -> Int) (words arr2)

  putStrLn $ show $ (foldl (*) 1 a)
  putStrLn $ show $ a == [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192]
  putStrLn $ show $ (foldl (*) 1 [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192])
```

With the input test file as following:

```test.in
13
1 2 4 8 32 64 128 256 512 1024 2048 4906 8192
9
1 3 9 27 81 243 729 2187 6561
```

The output is as:
```output
0
True
185343439719667835347140608
```

In fact, from the program, we know that a is equal to list  [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192] ,
but the product of a and the literal list is different.

Can anyone tell me why?

Thanks       








--m00nlight





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From Spam at gettingsharper.de  Sun Mar  8 14:26:24 2015
From: Spam at gettingsharper.de (=?UTF-8?B?Q2Fyc3RlbiBLw7ZuaWc=?=)
Date: Sun, 08 Mar 2015 15:26:24 +0100
Subject: [Haskell-beginners] Strange behavior of program
In-Reply-To: <eda9ce5.34b.14bf9b76107.Coremail.dot_wangyushi@yeah.net>
References: <50a09477.2d5.14bf90a9cfb.Coremail.dot_wangyushi@yeah.net>
 <54fc3c0b.e22e460a.1f3a.ffff928d@mx.google.com>
 <eda9ce5.34b.14bf9b76107.Coremail.dot_wangyushi@yeah.net>
Message-ID: <54FC5C10.3080105@gettingsharper.de>

No problem - Num can be a bit confusing at first - just make sure to
compare the right things ;)

Am 08.03.2015 um 15:08 schrieb m00nlight:
> Carsten and Arjanen,
>
> Thanks for your explanation.  :)
>
>
>
> --m00nlight
>
> ?2015?03?08 20?01?, "arjanen.loic"<arjanen.loic at gmail.com>??:
>
>
>     Hello,
>
>     Given that you specifically indicated in your code that you
>     wanted Int?s instance of Read and that the result of the product
>     is way over 2 ^ 32 (or 2 ^ 64 for that matter), I?d guess the
>     difference comes from the fact that the product of a is done with
>     type Int (that is, modulo 32 or 64 depending on your computer?s
>     CPU architecture) and the product with the litteral list is done
>     with type Integer because that the type Haskell defaults to in
>     this case.
>
>     ARJANEN Lo?c Jean David
>     http://blog.luigiscorner.com
>     ---
>     ?Computer science is no more about computers than astronomy is
>     about telescopes, biology is about microscopes, or chemistry is
>     about beakers and test tubes. Science is not about tools. It is
>     about how we use them, and what we find out when we do.?
>     Michael R. Fellows and Ian Parberry
>
>     *De :* m00nlight <mailto:dot_wangyushi at yeah.net>
>     *Envoy? :* ?dimanche? ?8? ?mars? ?2015 ?21?:?59
>     *? :* The Haskell-Beginners Mailing List - Discussion of primarily
>     beginner-level topics related to Haskell
>     <mailto:beginners at haskell.org>
>
>         
>     Hi Haskellers,
>
>     I encounter an strange behavior of haskell program recently.  The
>     following is my program
>
>     ```haskell
>     main = do
>         _ <- getLine
>         arr1 <- getLine
>         _ <- getLine
>        arr2 <- getLine
>        let a = map (read :: String -> Int) (words arr1)
>             b = map (read :: String -> Int) (words arr2)
>
>       putStrLn $ show $ (foldl (*) 1 a)
>       putStrLn $ show $ a ==
>     [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192]
>       putStrLn $ show $ (foldl (*) 1
>     [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192])
>     ```
>
>     With the input test file as following:
>
>     ```test.in
>     13
>     1 2 4 8 32 64 128 256 512 1024 2048 4906 8192
>     9
>     1 3 9 27 81 243 729 2187 6561
>     ```
>
>     The output is as:
>     ```output
>     0
>     True
>     185343439719667835347140608
>     ```
>
>     In fact, from the program, we know that a is equal to list 
>     [1,2,4,8,32,64,128,256,512,1024,2048,4906,8192] ,
>     but the product of a and the literal list is different.
>
>     Can anyone tell me why?
>
>     Thanks       
>
>
>
>
>     --m00nlight
>
>
>
>
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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From jeffbrown.the at gmail.com  Sun Mar  8 17:57:01 2015
From: jeffbrown.the at gmail.com (Jeffrey Brown)
Date: Sun, 8 Mar 2015 10:57:01 -0700
Subject: [Haskell-beginners] Laziness to efficiently get one element
 from a set
In-Reply-To: <mdh324$21b$1@ger.gmane.org>
References: <CAEc4Ma3eKJhBb3aAVSbVcMKFhgPoRDZu4BFDMLrQLck03Yp+mg@mail.gmail.com>
 <87385gwep4.fsf@ilxwinb01.fritz.box> <mdh324$21b$1@ger.gmane.org>
Message-ID: <CAEc4Ma2FU7trg=wtO3ab-WFAjepPTRb0EvLuMqy-Dp7iL=awmA@mail.gmail.com>

That is surprising. I would have expected an O(1) way to retrieve some
value at random.

On Sun, Mar 8, 2015 at 12:59 AM, Heinrich Apfelmus <
apfelmus at quantentunnel.de> wrote:

> Thomas Bach wrote:
>
>> Jeffrey Brown <jeffbrown.the at gmail.com> writes:
>>
>>  head $ Data.Set.toList S. If I do that, am I correct that Haskell will
>>> not try to convert all of S to a list; instead it will only convert
>>> one element, and then return it, and leave the rest of the list
>>> unevaluated?
>>>
>>
>> This is how toList from Data.Set.Base is defined in containers-0.5.0:
>>
>> {--------------------------------------------------------------------
>>   Lists
>> --------------------------------------------------------------------}
>> -- | /O(n)/. Convert the set to a list of elements. Subject to list
>> fusion.
>> toList :: Set a -> [a]
>> toList = toAscList
>>
>> -- | /O(n)/. Convert the set to an ascending list of elements. Subject to
>> list fusion.
>> toAscList :: Set a -> [a]
>> toAscList = foldr (:) []
>>
>> The buzzword you are looking for is list fusion:
>>
>> http://stackoverflow.com/questions/10945429/haskell-
>> list-fusion-where-is-it-needed
>>
>
> Actually, I don't think that list fusion is appropriate here. The `foldr`
> used in the definition of `toAscList` is from the `Foldable` type class,
> and implemented specifically for the `Set` data type. It's not the usual
> fold on lists.
>
> Jeffrey, if you want to pick a single element from a `Set`, I would
> recommend the functions `findMin` or `findMax`. The former satisfies
>
>     Data.Set.findMin = head . Data.Set.toList
>
> so you will get the same element as in your original approach. This time,
> however, you can be sure that it takes O(log n) time, whereas in the
> approach using `head`, it depends on the internals of the implementation of
> `foldr` whether it will take time O(n) or O(log n) or something in between.
> (In particular, it depends on how lazy the implementation of `foldr` for
> `Set` is. See [1] for more on lazy evaluation in this / a similar context.)
>
>
>   [1]: https://hackhands.com/modular-code-lazy-evaluation-haskell/
>
>
> Best regards,
> Heinrich Apfelmus
>
> --
> http://apfelmus.nfshost.com
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From bob at redivi.com  Sun Mar  8 18:19:07 2015
From: bob at redivi.com (Bob Ippolito)
Date: Sun, 8 Mar 2015 11:19:07 -0700
Subject: [Haskell-beginners] Laziness to efficiently get one element
 from a set
In-Reply-To: <CAEc4Ma2FU7trg=wtO3ab-WFAjepPTRb0EvLuMqy-Dp7iL=awmA@mail.gmail.com>
References: <CAEc4Ma3eKJhBb3aAVSbVcMKFhgPoRDZu4BFDMLrQLck03Yp+mg@mail.gmail.com>
 <87385gwep4.fsf@ilxwinb01.fritz.box> <mdh324$21b$1@ger.gmane.org>
 <CAEc4Ma2FU7trg=wtO3ab-WFAjepPTRb0EvLuMqy-Dp7iL=awmA@mail.gmail.com>
Message-ID: <CACwMPm8S0anNwspeJ_opRxQHuRX33-XtojGBuH0Fwht+Rn58eg@mail.gmail.com>

It would be possible to return the value at the root of the tree in O(1)
time, but there's no function for this and the constructor isn't exported
so you can't implement it yourself.

On Sun, Mar 8, 2015 at 10:57 AM, Jeffrey Brown <jeffbrown.the at gmail.com>
wrote:

> That is surprising. I would have expected an O(1) way to retrieve some
> value at random.
>
> On Sun, Mar 8, 2015 at 12:59 AM, Heinrich Apfelmus <
> apfelmus at quantentunnel.de> wrote:
>
>> Thomas Bach wrote:
>>
>>> Jeffrey Brown <jeffbrown.the at gmail.com> writes:
>>>
>>>  head $ Data.Set.toList S. If I do that, am I correct that Haskell will
>>>> not try to convert all of S to a list; instead it will only convert
>>>> one element, and then return it, and leave the rest of the list
>>>> unevaluated?
>>>>
>>>
>>> This is how toList from Data.Set.Base is defined in containers-0.5.0:
>>>
>>> {--------------------------------------------------------------------
>>>   Lists
>>> --------------------------------------------------------------------}
>>> -- | /O(n)/. Convert the set to a list of elements. Subject to list
>>> fusion.
>>> toList :: Set a -> [a]
>>> toList = toAscList
>>>
>>> -- | /O(n)/. Convert the set to an ascending list of elements. Subject
>>> to list fusion.
>>> toAscList :: Set a -> [a]
>>> toAscList = foldr (:) []
>>>
>>> The buzzword you are looking for is list fusion:
>>>
>>> http://stackoverflow.com/questions/10945429/haskell-
>>> list-fusion-where-is-it-needed
>>>
>>
>> Actually, I don't think that list fusion is appropriate here. The `foldr`
>> used in the definition of `toAscList` is from the `Foldable` type class,
>> and implemented specifically for the `Set` data type. It's not the usual
>> fold on lists.
>>
>> Jeffrey, if you want to pick a single element from a `Set`, I would
>> recommend the functions `findMin` or `findMax`. The former satisfies
>>
>>     Data.Set.findMin = head . Data.Set.toList
>>
>> so you will get the same element as in your original approach. This time,
>> however, you can be sure that it takes O(log n) time, whereas in the
>> approach using `head`, it depends on the internals of the implementation of
>> `foldr` whether it will take time O(n) or O(log n) or something in between.
>> (In particular, it depends on how lazy the implementation of `foldr` for
>> `Set` is. See [1] for more on lazy evaluation in this / a similar context.)
>>
>>
>>   [1]: https://hackhands.com/modular-code-lazy-evaluation-haskell/
>>
>>
>> Best regards,
>> Heinrich Apfelmus
>>
>> --
>> http://apfelmus.nfshost.com
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From kalawatia_abhinav at yahoo.com  Mon Mar  9 15:47:06 2015
From: kalawatia_abhinav at yahoo.com (Abhinav Kalawatia)
Date: Mon, 9 Mar 2015 15:47:06 +0000 (UTC)
Subject: [Haskell-beginners] Saving intermediate calculations
Message-ID: <1367874770.1593552.1425916026462.JavaMail.yahoo@mail.yahoo.com>

? Hello,
I?am facing a slight difficulty trying to?do something while saving intermediate calculations

week ?? actual_?sales(a) ??? forecast(f) ?????????????? ??????? forecast(f) ???????????????????????????? forecast(f)
?1 ?????????? 20 ??????????????????????? ?f1 = a1 ???????????????????????????? f1 = 20 ?????????????????????????????????? f1 = 20
?2 ?????????? 27 ??????????????????????? ?f2 = f1 + 0.5*(a1 - f1)??????? f2 = 20 +0.5(20-20) ????????????? ?f2 = 20
?3 ?????????? 25 ??????????????????????? ?f3 = f2 +? 0.5*(a2-f2)?????????f3 = 20+0.5*(27-20)???????????? ? f3=23.5
?4 ?????????? 22 


When I execute?a function to achieve this?in Haskell, I get the forecast for the fourth period. Can I save the values for intermediate period(1..3) also?


Please find my Haskell code below:
a = 0.5
ipt = [20,27,25,22]
avg :: [Double] -> Double
avg (x:xs) = (a*x) + (1-a)*(avg xs)
avg [] = 0
Could you please help me with this, thanking you in anticipation

Regards,Abhinav
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From charioteer7 at gmail.com  Mon Mar  9 20:07:23 2015
From: charioteer7 at gmail.com (Geoffrey Bays)
Date: Mon, 9 Mar 2015 16:07:23 -0400
Subject: [Haskell-beginners] GHC not buying what I am offering this afternoon
Message-ID: <CAD8ukx4Otih6nLgmvEUXPjt64-KvZifUe7tO6MvqF+Cm8GQ_4Q@mail.gmail.com>

My main function looks like this:

main :: [IO()]
main = do
    let stud1 = Student {name = "Geoff", average = -99.0, grades =
[66,77,88]}
    let stud2 = Student {name = "Doug", average = -99.0, grades =
[77,88,99]}
    let stud3 = Student {name = "Ron", average = -99.0, grades = [55,66,77]}
    let studList = [stud1,stud2]
    let newList = calcAvg studList
    [putStrLn $ show s | s <- newList]
    --putStrLn $ show (newList !! 0)
    --putStrLn $ show (newList !! 1)

With this final line, putStrLn $ show (newList !! 0), the type IO () in the
function declaration compiles fine.
But with [putStrLn $ show s | s <- newList] as the final line, [IO ()] in
the function declaration will not compile, I get this error:

    Couldn't match expected type `IO t0' with actual type `[IO ()]'

What does the declared type need to be for a final line of:
[putStrLn $ show s | s <- newList]  ???

Thanks,

Geoffrey
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From joel.s.williamson at gmail.com  Mon Mar  9 20:15:59 2015
From: joel.s.williamson at gmail.com (Joel Williamson)
Date: Mon, 09 Mar 2015 20:15:59 +0000
Subject: [Haskell-beginners] GHC not buying what I am offering this
	afternoon
References: <CAD8ukx4Otih6nLgmvEUXPjt64-KvZifUe7tO6MvqF+Cm8GQ_4Q@mail.gmail.com>
Message-ID: <CAJGxSeoXeSqcbkW4cuPkMj5r8wDxSMyKmYK_P+qaX0=mcnhaTw@mail.gmail.com>

main must have type IO a. Hoogle tells me that to convert [IO a] -> IO [a],
you should use the function sequence. Try applying that to your final line.

On Mon, 9 Mar 2015 16:07 Geoffrey Bays <charioteer7 at gmail.com> wrote:

> My main function looks like this:
>
> main :: [IO()]
> main = do
>     let stud1 = Student {name = "Geoff", average = -99.0, grades =
> [66,77,88]}
>     let stud2 = Student {name = "Doug", average = -99.0, grades =
> [77,88,99]}
>     let stud3 = Student {name = "Ron", average = -99.0, grades =
> [55,66,77]}
>     let studList = [stud1,stud2]
>     let newList = calcAvg studList
>     [putStrLn $ show s | s <- newList]
>     --putStrLn $ show (newList !! 0)
>     --putStrLn $ show (newList !! 1)
>
> With this final line, putStrLn $ show (newList !! 0), the type IO () in
> the function declaration compiles fine.
> But with [putStrLn $ show s | s <- newList] as the final line, [IO ()] in
> the function declaration will not compile, I get this error:
>
>     Couldn't match expected type `IO t0' with actual type `[IO ()]'
>
> What does the declared type need to be for a final line of:
> [putStrLn $ show s | s <- newList]  ???
>
> Thanks,
>
> Geoffrey
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From charioteer7 at gmail.com  Mon Mar  9 20:25:02 2015
From: charioteer7 at gmail.com (Geoffrey Bays)
Date: Mon, 9 Mar 2015 16:25:02 -0400
Subject: [Haskell-beginners] GHC not buying what I am offering this
	afternoon
In-Reply-To: <CAJGxSeoXeSqcbkW4cuPkMj5r8wDxSMyKmYK_P+qaX0=mcnhaTw@mail.gmail.com>
References: <CAD8ukx4Otih6nLgmvEUXPjt64-KvZifUe7tO6MvqF+Cm8GQ_4Q@mail.gmail.com>
 <CAJGxSeoXeSqcbkW4cuPkMj5r8wDxSMyKmYK_P+qaX0=mcnhaTw@mail.gmail.com>
Message-ID: <CAD8ukx6T01B+Zb0rGSVVywYvk03JN8N9Z-CoGxYNCHpxGGi+qQ@mail.gmail.com>

Thanks, Joel.

Putting the type IO [()] in the main declaration and this as the final line
of the main function does do the trick:

sequence [putStrLn $ show s | s <- newList]

But this is the kind of thing that makes Haskell types difficult for
beginners to work with...

Geoffrey

On Mon, Mar 9, 2015 at 4:15 PM, Joel Williamson <joel.s.williamson at gmail.com
> wrote:

> main must have type IO a. Hoogle tells me that to convert [IO a] -> IO
> [a], you should use the function sequence. Try applying that to your final
> line.
>
> On Mon, 9 Mar 2015 16:07 Geoffrey Bays <charioteer7 at gmail.com> wrote:
>
>> My main function looks like this:
>>
>> main :: [IO()]
>> main = do
>>     let stud1 = Student {name = "Geoff", average = -99.0, grades =
>> [66,77,88]}
>>     let stud2 = Student {name = "Doug", average = -99.0, grades =
>> [77,88,99]}
>>     let stud3 = Student {name = "Ron", average = -99.0, grades =
>> [55,66,77]}
>>     let studList = [stud1,stud2]
>>     let newList = calcAvg studList
>>     [putStrLn $ show s | s <- newList]
>>     --putStrLn $ show (newList !! 0)
>>     --putStrLn $ show (newList !! 1)
>>
>> With this final line, putStrLn $ show (newList !! 0), the type IO () in
>> the function declaration compiles fine.
>> But with [putStrLn $ show s | s <- newList] as the final line, [IO ()] in
>> the function declaration will not compile, I get this error:
>>
>>     Couldn't match expected type `IO t0' with actual type `[IO ()]'
>>
>> What does the declared type need to be for a final line of:
>> [putStrLn $ show s | s <- newList]  ???
>>
>> Thanks,
>>
>> Geoffrey
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From toad3k at gmail.com  Mon Mar  9 20:37:59 2015
From: toad3k at gmail.com (David McBride)
Date: Mon, 9 Mar 2015 16:37:59 -0400
Subject: [Haskell-beginners] GHC not buying what I am offering this
	afternoon
In-Reply-To: <CAD8ukx6T01B+Zb0rGSVVywYvk03JN8N9Z-CoGxYNCHpxGGi+qQ@mail.gmail.com>
References: <CAD8ukx4Otih6nLgmvEUXPjt64-KvZifUe7tO6MvqF+Cm8GQ_4Q@mail.gmail.com>
 <CAJGxSeoXeSqcbkW4cuPkMj5r8wDxSMyKmYK_P+qaX0=mcnhaTw@mail.gmail.com>
 <CAD8ukx6T01B+Zb0rGSVVywYvk03JN8N9Z-CoGxYNCHpxGGi+qQ@mail.gmail.com>
Message-ID: <CAN+Tr40ysmoEpKi+JagMqQZxey1DNzgE9Hcf21=oxwJNkRhF=g@mail.gmail.com>

There is a difference between IO [()] and IO () and [IO ()]

A type of [IO ()] is a list of actions, none of which have actually been
executed.
A type of IO [()] is a single action that has executed and returned a bunch
of nils.

sequence is one way to combine a list of actions into a single action that
returns a list of their results, but it might be better to try and separate
the pure and impure part of that line of code:

mapM putStrLn $ ([show s | s <- [1,2,3]] :: [String]) :: IO [()]

The type annotations are for explanation only.  Then use mapM_ if you do
not want to save these nils for some reason (there are performance
implications).


On Mon, Mar 9, 2015 at 4:25 PM, Geoffrey Bays <charioteer7 at gmail.com> wrote:

> Thanks, Joel.
>
> Putting the type IO [()] in the main declaration and this as the final
> line of the main function does do the trick:
>
> sequence [putStrLn $ show s | s <- newList]
>
> But this is the kind of thing that makes Haskell types difficult for
> beginners to work with...
>
> Geoffrey
>
> On Mon, Mar 9, 2015 at 4:15 PM, Joel Williamson <
> joel.s.williamson at gmail.com> wrote:
>
>> main must have type IO a. Hoogle tells me that to convert [IO a] -> IO
>> [a], you should use the function sequence. Try applying that to your final
>> line.
>>
>> On Mon, 9 Mar 2015 16:07 Geoffrey Bays <charioteer7 at gmail.com> wrote:
>>
>>> My main function looks like this:
>>>
>>> main :: [IO()]
>>> main = do
>>>     let stud1 = Student {name = "Geoff", average = -99.0, grades =
>>> [66,77,88]}
>>>     let stud2 = Student {name = "Doug", average = -99.0, grades =
>>> [77,88,99]}
>>>     let stud3 = Student {name = "Ron", average = -99.0, grades =
>>> [55,66,77]}
>>>     let studList = [stud1,stud2]
>>>     let newList = calcAvg studList
>>>     [putStrLn $ show s | s <- newList]
>>>     --putStrLn $ show (newList !! 0)
>>>     --putStrLn $ show (newList !! 1)
>>>
>>> With this final line, putStrLn $ show (newList !! 0), the type IO () in
>>> the function declaration compiles fine.
>>> But with [putStrLn $ show s | s <- newList] as the final line, [IO ()]
>>> in the function declaration will not compile, I get this error:
>>>
>>>     Couldn't match expected type `IO t0' with actual type `[IO ()]'
>>>
>>> What does the declared type need to be for a final line of:
>>> [putStrLn $ show s | s <- newList]  ???
>>>
>>> Thanks,
>>>
>>> Geoffrey
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From joel.s.williamson at gmail.com  Mon Mar  9 20:39:37 2015
From: joel.s.williamson at gmail.com (Joel Williamson)
Date: Mon, 9 Mar 2015 16:39:37 -0400
Subject: [Haskell-beginners] GHC not buying what I am offering this
	afternoon
In-Reply-To: <CAD8ukx6T01B+Zb0rGSVVywYvk03JN8N9Z-CoGxYNCHpxGGi+qQ@mail.gmail.com>
References: <CAD8ukx4Otih6nLgmvEUXPjt64-KvZifUe7tO6MvqF+Cm8GQ_4Q@mail.gmail.com>
 <CAJGxSeoXeSqcbkW4cuPkMj5r8wDxSMyKmYK_P+qaX0=mcnhaTw@mail.gmail.com>
 <CAD8ukx6T01B+Zb0rGSVVywYvk03JN8N9Z-CoGxYNCHpxGGi+qQ@mail.gmail.com>
Message-ID: <CAJGxSeoc2FkvtQPs2oDcxCYHbtm+cFyXaXjEcMssnFMvM_1X5w@mail.gmail.com>

sequence will get the types to match up, but a more elegant solution
would be to get every line into a single string, then print that.

putStrLn $ unlines $ map show newList

I agree that getting the types to line up can be a nuisance, and with
such small programs it doesn't bring much of an advantage. Ultimately,
if you want side-effects to be reflected in the type system, there
will be times you have to do a bit of extra work to satisfy the type
checker. Learning to write well-typed code is much easier in this
context, where everything has a fairly concrete type and there is lots
of documentation, than having to learn it later when you have weird
types coming from 3 different libraries and are facing a problem no
one else has had. I would recommend getting very comfortable with GHCi
and Hoogle.

If you haven't already, add a hoogle prompt to GHCi by pasting something like
:def hoogle \str -> return $ ":! hoogle --count=15 \"" ++ str ++ "\""
in your ghci.conf. This will allow you to easily search for functions
of a given type. Typing
:hoogle [IO a] -> IO [a]
returned all the information needed to answer your question.


On Mon, Mar 9, 2015 at 4:25 PM, Geoffrey Bays <charioteer7 at gmail.com> wrote:
> Thanks, Joel.
>
> Putting the type IO [()] in the main declaration and this as the final line
> of the main function does do the trick:
>
> sequence [putStrLn $ show s | s <- newList]
>
> But this is the kind of thing that makes Haskell types difficult for
> beginners to work with...
>
> Geoffrey
>
> On Mon, Mar 9, 2015 at 4:15 PM, Joel Williamson
> <joel.s.williamson at gmail.com> wrote:
>>
>> main must have type IO a. Hoogle tells me that to convert [IO a] -> IO
>> [a], you should use the function sequence. Try applying that to your final
>> line.
>>
>>
>> On Mon, 9 Mar 2015 16:07 Geoffrey Bays <charioteer7 at gmail.com> wrote:
>>>
>>> My main function looks like this:
>>>
>>> main :: [IO()]
>>> main = do
>>>     let stud1 = Student {name = "Geoff", average = -99.0, grades =
>>> [66,77,88]}
>>>     let stud2 = Student {name = "Doug", average = -99.0, grades =
>>> [77,88,99]}
>>>     let stud3 = Student {name = "Ron", average = -99.0, grades =
>>> [55,66,77]}
>>>     let studList = [stud1,stud2]
>>>     let newList = calcAvg studList
>>>     [putStrLn $ show s | s <- newList]
>>>     --putStrLn $ show (newList !! 0)
>>>     --putStrLn $ show (newList !! 1)
>>>
>>> With this final line, putStrLn $ show (newList !! 0), the type IO () in
>>> the function declaration compiles fine.
>>> But with [putStrLn $ show s | s <- newList] as the final line, [IO ()] in
>>> the function declaration will not compile, I get this error:
>>>
>>>     Couldn't match expected type `IO t0' with actual type `[IO ()]'
>>>
>>> What does the declared type need to be for a final line of:
>>> [putStrLn $ show s | s <- newList]  ???
>>>
>>> Thanks,
>>>
>>> Geoffrey
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>

From apfelmus at quantentunnel.de  Mon Mar  9 23:15:20 2015
From: apfelmus at quantentunnel.de (Heinrich Apfelmus)
Date: Tue, 10 Mar 2015 00:15:20 +0100
Subject: [Haskell-beginners] Avoiding stack space overflow
In-Reply-To: <CACwMPm_CzD3=WkSQAcRj92MLFy=oYzexqH=OsT=hn627=kQXFA@mail.gmail.com>
References: <20150126121926.GA1981@golay.schaathun.net>
 <CAN+Tr436BnCAQtBjfidAK3PNhviE742dTqhd5=7YruY_W4mdqg@mail.gmail.com>
 <20150126164517.GA27771@golay.schaathun.net>
 <CACwMPm_CzD3=WkSQAcRj92MLFy=oYzexqH=OsT=hn627=kQXFA@mail.gmail.com>
Message-ID: <mdl9i8$lb7$1@ger.gmane.org>

Bob Ippolito wrote:
> Here are my go-to resources for Haskell's evaluation:
> 
> http://chimera.labs.oreilly.com/books/1230000000929/ch02.html#sec_par-eval-whnf
> https://hackhands.com/lazy-evaluation-works-haskell/

I just wanted to mention that I have now expanded the latter resource. 
It is best reached from the following URL:

   https://hackhands.com/guide-lazy-evaluation-haskell/


Best regards,
Heinrich Apfelmus

--
http://apfelmus.nfshost.com


From sumit.sahrawat.apm13 at iitbhu.ac.in  Tue Mar 10 05:26:52 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Tue, 10 Mar 2015 10:56:52 +0530
Subject: [Haskell-beginners] GHC not buying what I am offering this
	afternoon
In-Reply-To: <CAJGxSeoc2FkvtQPs2oDcxCYHbtm+cFyXaXjEcMssnFMvM_1X5w@mail.gmail.com>
References: <CAD8ukx4Otih6nLgmvEUXPjt64-KvZifUe7tO6MvqF+Cm8GQ_4Q@mail.gmail.com>
 <CAJGxSeoXeSqcbkW4cuPkMj5r8wDxSMyKmYK_P+qaX0=mcnhaTw@mail.gmail.com>
 <CAD8ukx6T01B+Zb0rGSVVywYvk03JN8N9Z-CoGxYNCHpxGGi+qQ@mail.gmail.com>
 <CAJGxSeoc2FkvtQPs2oDcxCYHbtm+cFyXaXjEcMssnFMvM_1X5w@mail.gmail.com>
Message-ID: <CAJbEW8OnYim0uQ_YFpRMxxevTUwi0gidag8gkxiy2akdEcszBQ@mail.gmail.com>

Just like sequence, there is also a function sequence_ that you might want
to use.

    sequence_ :: Monad m => [m a] -> m ()    -- for m = IO, sequence_ ::
[IO a] -> IO ()
    sequence_ lst = do
      sequence lst
      return ()

    -- also, mapM and mapM_ are useful too
    mapM f  = sequence . map f      -- retains results
    mapM_ f = sequence_ . map f     -- ignores results

    -- Some use cases
    ghci> sequence [print 2, print 3]
      2
      3
      [(),()]      -- The results, each print is of type IO (), i.e does IO
and produces ()
    ghci> mapM print [2,3] -- same as above
    ghci> mapM_ print [2,3] -- same as above, but without results (or
result of type unit, i.e ())


On 10 March 2015 at 02:09, Joel Williamson <joel.s.williamson at gmail.com>
wrote:

> sequence will get the types to match up, but a more elegant solution
> would be to get every line into a single string, then print that.
>
> putStrLn $ unlines $ map show newList
>
> I agree that getting the types to line up can be a nuisance, and with
> such small programs it doesn't bring much of an advantage. Ultimately,
> if you want side-effects to be reflected in the type system, there
> will be times you have to do a bit of extra work to satisfy the type
> checker. Learning to write well-typed code is much easier in this
> context, where everything has a fairly concrete type and there is lots
> of documentation, than having to learn it later when you have weird
> types coming from 3 different libraries and are facing a problem no
> one else has had. I would recommend getting very comfortable with GHCi
> and Hoogle.
>
> If you haven't already, add a hoogle prompt to GHCi by pasting something
> like
> :def hoogle \str -> return $ ":! hoogle --count=15 \"" ++ str ++ "\""
> in your ghci.conf. This will allow you to easily search for functions
> of a given type. Typing
> :hoogle [IO a] -> IO [a]
> returned all the information needed to answer your question.
>
>
> On Mon, Mar 9, 2015 at 4:25 PM, Geoffrey Bays <charioteer7 at gmail.com>
> wrote:
> > Thanks, Joel.
> >
> > Putting the type IO [()] in the main declaration and this as the final
> line
> > of the main function does do the trick:
> >
> > sequence [putStrLn $ show s | s <- newList]
> >
> > But this is the kind of thing that makes Haskell types difficult for
> > beginners to work with...
> >
> > Geoffrey
> >
> > On Mon, Mar 9, 2015 at 4:15 PM, Joel Williamson
> > <joel.s.williamson at gmail.com> wrote:
> >>
> >> main must have type IO a. Hoogle tells me that to convert [IO a] -> IO
> >> [a], you should use the function sequence. Try applying that to your
> final
> >> line.
> >>
> >>
> >> On Mon, 9 Mar 2015 16:07 Geoffrey Bays <charioteer7 at gmail.com> wrote:
> >>>
> >>> My main function looks like this:
> >>>
> >>> main :: [IO()]
> >>> main = do
> >>>     let stud1 = Student {name = "Geoff", average = -99.0, grades =
> >>> [66,77,88]}
> >>>     let stud2 = Student {name = "Doug", average = -99.0, grades =
> >>> [77,88,99]}
> >>>     let stud3 = Student {name = "Ron", average = -99.0, grades =
> >>> [55,66,77]}
> >>>     let studList = [stud1,stud2]
> >>>     let newList = calcAvg studList
> >>>     [putStrLn $ show s | s <- newList]
> >>>     --putStrLn $ show (newList !! 0)
> >>>     --putStrLn $ show (newList !! 1)
> >>>
> >>> With this final line, putStrLn $ show (newList !! 0), the type IO () in
> >>> the function declaration compiles fine.
> >>> But with [putStrLn $ show s | s <- newList] as the final line, [IO ()]
> in
> >>> the function declaration will not compile, I get this error:
> >>>
> >>>     Couldn't match expected type `IO t0' with actual type `[IO ()]'
> >>>
> >>> What does the declared type need to be for a final line of:
> >>> [putStrLn $ show s | s <- newList]  ???
> >>>
> >>> Thanks,
> >>>
> >>> Geoffrey
> >>>
> >>> _______________________________________________
> >>> Beginners mailing list
> >>> Beginners at haskell.org
> >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >>
> >>
> >> _______________________________________________
> >> Beginners mailing list
> >> Beginners at haskell.org
> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >>
> >
> >
> > _______________________________________________
> > Beginners mailing list
> > Beginners at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>



-- 
Regards

Sumit Sahrawat
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From bob at redivi.com  Tue Mar 10 06:03:49 2015
From: bob at redivi.com (Bob Ippolito)
Date: Mon, 9 Mar 2015 23:03:49 -0700
Subject: [Haskell-beginners] Avoiding stack space overflow
In-Reply-To: <mdl9i8$lb7$1@ger.gmane.org>
References: <20150126121926.GA1981@golay.schaathun.net>
 <CAN+Tr436BnCAQtBjfidAK3PNhviE742dTqhd5=7YruY_W4mdqg@mail.gmail.com>
 <20150126164517.GA27771@golay.schaathun.net>
 <CACwMPm_CzD3=WkSQAcRj92MLFy=oYzexqH=OsT=hn627=kQXFA@mail.gmail.com>
 <mdl9i8$lb7$1@ger.gmane.org>
Message-ID: <CACwMPm_9Kfj+qpHtKF3C2GXONNcoaZhEiyTR4qW6k12tpNXmCw@mail.gmail.com>

On Mon, Mar 9, 2015 at 4:15 PM, Heinrich Apfelmus <apfelmus at quantentunnel.de
> wrote:

> Bob Ippolito wrote:
>
>> Here are my go-to resources for Haskell's evaluation:
>>
>> http://chimera.labs.oreilly.com/books/1230000000929/ch02.
>> html#sec_par-eval-whnf
>> https://hackhands.com/lazy-evaluation-works-haskell/
>>
>
> I just wanted to mention that I have now expanded the latter resource. It
> is best reached from the following URL:
>
>   https://hackhands.com/guide-lazy-evaluation-haskell/


Thank you, this is great! Perhaps you could also link to this from the top
and/or bottom of each individual article to make them all more discoverable
from existing links?

-bob
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From apfelmus at quantentunnel.de  Tue Mar 10 09:49:08 2015
From: apfelmus at quantentunnel.de (Heinrich Apfelmus)
Date: Tue, 10 Mar 2015 10:49:08 +0100
Subject: [Haskell-beginners] Avoiding stack space overflow
In-Reply-To: <CACwMPm_9Kfj+qpHtKF3C2GXONNcoaZhEiyTR4qW6k12tpNXmCw@mail.gmail.com>
References: <20150126121926.GA1981@golay.schaathun.net>
 <CAN+Tr436BnCAQtBjfidAK3PNhviE742dTqhd5=7YruY_W4mdqg@mail.gmail.com>
 <20150126164517.GA27771@golay.schaathun.net>
 <CACwMPm_CzD3=WkSQAcRj92MLFy=oYzexqH=OsT=hn627=kQXFA@mail.gmail.com>
 <mdl9i8$lb7$1@ger.gmane.org>
 <CACwMPm_9Kfj+qpHtKF3C2GXONNcoaZhEiyTR4qW6k12tpNXmCw@mail.gmail.com>
Message-ID: <mdmeml$8r6$1@ger.gmane.org>

Bob Ippolito wrote:
> On Mon, Mar 9, 2015 at 4:15 PM, Heinrich Apfelmus <apfelmus at quantentunnel.de
>> wrote:
> 
>> Bob Ippolito wrote:
>>
>>> Here are my go-to resources for Haskell's evaluation:
>>>
>>> http://chimera.labs.oreilly.com/books/1230000000929/ch02.
>>> html#sec_par-eval-whnf
>>> https://hackhands.com/lazy-evaluation-works-haskell/
>>>
>> I just wanted to mention that I have now expanded the latter resource. It
>> is best reached from the following URL:
>>
>>   https://hackhands.com/guide-lazy-evaluation-haskell/
> 
> 
> Thank you, this is great! Perhaps you could also link to this from the top
> and/or bottom of each individual article to make them all more discoverable
> from existing links?

Good idea! I have added a link at the top of each post. Does this work 
as intended?


Best regards,
Heinrich Apfelmus

--
http://apfelmus.nfshost.com


From animeshsaxena at icloud.com  Tue Mar 10 13:43:33 2015
From: animeshsaxena at icloud.com (Animesh Saxena)
Date: Tue, 10 Mar 2015 21:43:33 +0800
Subject: [Haskell-beginners] Avoiding stack space overflow
In-Reply-To: <mdmeml$8r6$1@ger.gmane.org>
References: <20150126121926.GA1981@golay.schaathun.net>
 <CAN+Tr436BnCAQtBjfidAK3PNhviE742dTqhd5=7YruY_W4mdqg@mail.gmail.com>
 <20150126164517.GA27771@golay.schaathun.net>
 <CACwMPm_CzD3=WkSQAcRj92MLFy=oYzexqH=OsT=hn627=kQXFA@mail.gmail.com>
 <mdl9i8$lb7$1@ger.gmane.org>
 <CACwMPm_9Kfj+qpHtKF3C2GXONNcoaZhEiyTR4qW6k12tpNXmCw@mail.gmail.com>
 <mdmeml$8r6$1@ger.gmane.org>
Message-ID: <C00120A1-6436-414B-921D-CB0DAD95E4CC@icloud.com>

This is really nice stuff. Thanks

Animesh 

Sent from my iPhone

> On 10-Mar-2015, at 5:49 pm, Heinrich Apfelmus <apfelmus at quantentunnel.de> wrote:
> 
> Bob Ippolito wrote:
>> On Mon, Mar 9, 2015 at 4:15 PM, Heinrich Apfelmus <apfelmus at quantentunnel.de
>>> wrote:
>>> Bob Ippolito wrote:
>>> 
>>>> Here are my go-to resources for Haskell's evaluation:
>>>> 
>>>> http://chimera.labs.oreilly.com/books/1230000000929/ch02.
>>>> html#sec_par-eval-whnf
>>>> https://hackhands.com/lazy-evaluation-works-haskell/
>>> I just wanted to mention that I have now expanded the latter resource. It
>>> is best reached from the following URL:
>>> 
>>>  https://hackhands.com/guide-lazy-evaluation-haskell/
>> Thank you, this is great! Perhaps you could also link to this from the top
>> and/or bottom of each individual article to make them all more discoverable
>> from existing links?
> 
> Good idea! I have added a link at the top of each post. Does this work as intended?
> 
> 
> Best regards,
> Heinrich Apfelmus
> 
> --
> http://apfelmus.nfshost.com
> 
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

From bob at redivi.com  Tue Mar 10 20:54:17 2015
From: bob at redivi.com (Bob Ippolito)
Date: Tue, 10 Mar 2015 13:54:17 -0700
Subject: [Haskell-beginners] Avoiding stack space overflow
In-Reply-To: <mdmeml$8r6$1@ger.gmane.org>
References: <20150126121926.GA1981@golay.schaathun.net>
 <CAN+Tr436BnCAQtBjfidAK3PNhviE742dTqhd5=7YruY_W4mdqg@mail.gmail.com>
 <20150126164517.GA27771@golay.schaathun.net>
 <CACwMPm_CzD3=WkSQAcRj92MLFy=oYzexqH=OsT=hn627=kQXFA@mail.gmail.com>
 <mdl9i8$lb7$1@ger.gmane.org>
 <CACwMPm_9Kfj+qpHtKF3C2GXONNcoaZhEiyTR4qW6k12tpNXmCw@mail.gmail.com>
 <mdmeml$8r6$1@ger.gmane.org>
Message-ID: <CACwMPm_Tg0WS=LoiEMEJvexDHvSJwynuYEC_tB89VkWDA0-Gvg@mail.gmail.com>

On Tue, Mar 10, 2015 at 2:49 AM, Heinrich Apfelmus <
apfelmus at quantentunnel.de> wrote:

> Bob Ippolito wrote:
>
>> On Mon, Mar 9, 2015 at 4:15 PM, Heinrich Apfelmus <
>> apfelmus at quantentunnel.de
>>
>>> wrote:
>>>
>>
>>  Bob Ippolito wrote:
>>>
>>>  Here are my go-to resources for Haskell's evaluation:
>>>>
>>>> http://chimera.labs.oreilly.com/books/1230000000929/ch02.
>>>> html#sec_par-eval-whnf
>>>> https://hackhands.com/lazy-evaluation-works-haskell/
>>>>
>>>>  I just wanted to mention that I have now expanded the latter resource.
>>> It
>>> is best reached from the following URL:
>>>
>>>   https://hackhands.com/guide-lazy-evaluation-haskell/
>>>
>>
>>
>> Thank you, this is great! Perhaps you could also link to this from the top
>> and/or bottom of each individual article to make them all more
>> discoverable
>> from existing links?
>>
>
> Good idea! I have added a link at the top of each post. Does this work as
> intended?


Absolutely, works perfectly!

-bob
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From hjgtuyl at chello.nl  Wed Mar 11 00:05:10 2015
From: hjgtuyl at chello.nl (Henk-Jan van Tuyl)
Date: Wed, 11 Mar 2015 01:05:10 +0100
Subject: [Haskell-beginners] Saving intermediate calculations
In-Reply-To: <1367874770.1593552.1425916026462.JavaMail.yahoo@mail.yahoo.com>
References: <1367874770.1593552.1425916026462.JavaMail.yahoo@mail.yahoo.com>
Message-ID: <op.xva3u0gypz0j5l@alquantor>

On Mon, 09 Mar 2015 16:47:06 +0100, Abhinav Kalawatia  
<kalawatia_abhinav at yahoo.com> wrote:

:
> week    actual_ sales(a)     forecast(f)                         
> forecast(f)                              forecast(f)
>  1            20                          f1 = a1  
>                              f1 = 20                                     
> f1 = 20
>  2            27                          f2 = f1 + 0.5*(a1 - f1)         
> f2 = 20 +0.5(20-20)                f2 = 20
>  3            25                          f3 = f2 +  0.5*(a2-f2)          
> f3 = 20+0.5*(27-20)               f3=23.5
>  4            22
>
>
> When I execute a function to achieve this in Haskell, I get the forecast  
> for the fourth period. Can I save the values for intermediate  
> period(1..3) also?
>
>
> Please find my Haskell code below:
> a = 0.5
> ipt = [20,27,25,22]
> avg :: [Double] -> Double
> avg (x:xs) = (a*x) + (1-a)*(avg xs)
> avg [] = 0
:

It looks to me, that mapAccumL[0] might do the trick (I haven't studied  
this in detail).

Regards,
Henk-Jan van Tuyl


[0]  
http://haddocks.fpcomplete.com/fp/7.8/20140916-162/base/Data-List.html#v:mapAccumL



-- 
Folding at home
What if you could share your unused computer power to help find a cure? In  
just 5 minutes you can join the world's biggest networked computer and get  
us closer sooner. Watch the video.
http://folding.stanford.edu/


http://Van.Tuyl.eu/
http://members.chello.nl/hjgtuyl/tourdemonad.html
Haskell programming
--

From hengruo.z at gmail.com  Wed Mar 11 13:21:23 2015
From: hengruo.z at gmail.com (Zhang Hengruo)
Date: Wed, 11 Mar 2015 21:21:23 +0800
Subject: [Haskell-beginners] What's the type of function "abs"?
Message-ID: <CAMeL8ffwki=xg1n48JmEbTuUjhpacw6p0FqfmiQKz3M5YkSsYA@mail.gmail.com>

I'm a newbie, knowing a little about Haskell, and hope my question isn't
very silly...
=====================================================================
My absolute value function looks like this:

abs2 :: Num a => a -> a
abs2 n = if n >= 0 then n else 0 - n

GHCi tells me that I should add Ord type class to its definition. Well,
it's true. It has used relational operators and Num isn't a subclass of Ord.
However, when I input ":t abs" in GHCi, the interpreter shows "abs :: Num a
=> a -> a". I read GHC/Num.lhs and find that abs is defined as "abs :: a ->
a" and has no concrete content. So I think the abs function is written in C
as a module to implement directly and the type of abs just follows its
class GHC.Num. Is it right? Or there are any other reasons?

Thanks,

Hengruo
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From raabe at froglogic.com  Wed Mar 11 13:28:05 2015
From: raabe at froglogic.com (Frerich Raabe)
Date: Wed, 11 Mar 2015 14:28:05 +0100
Subject: [Haskell-beginners]
 =?utf-8?q?What=27s_the_type_of_function_=22ab?= =?utf-8?b?cyI/?=
In-Reply-To: <CAMeL8ffwki=xg1n48JmEbTuUjhpacw6p0FqfmiQKz3M5YkSsYA@mail.gmail.com>
References: <CAMeL8ffwki=xg1n48JmEbTuUjhpacw6p0FqfmiQKz3M5YkSsYA@mail.gmail.com>
Message-ID: <71d09d762202621a09390f2b33f4f6cf@roundcube.froglogic.com>

Hi,

On 2015-03-11 14:21, Zhang Hengruo wrote:
> My absolute value function looks like this:
> 
> abs2 :: Num a => a -> a
> abs2 n = if n >= 0 then n else 0 - n
> 
> GHCi tells me that I should add Ord type class to its definition. Well, it's 
> true. It has used relational operators and Num isn't a subclass
> of Ord.
> However, when I input ":t abs" in GHCi, the interpreter shows "abs :: Num a 
> => a -> a". I read GHC/Num.lhs and find that abs is defined as
> "abs :: a -> a" and has no concrete content. So I think the abs function is 
> written in C as a module to implement directly and the type of abs
> just follows its class GHC.Num. Is it right? Or there are any other reasons?

'abs' is a function defined on the 'Num' class, i.e. any instance of 'Num' 
(such as 'Num Int') may define 'abs' as it pleases. See

   http://hackage.haskell.org/package/base-4.7.0.2/docs/src/GHC-Num.html#abs

for a few instantiations of the 'Num' class. In particular, concrete 
instances of the class know the actual type of 'a'. For instance, the 
definition of 'Num Int' could actually use '>=' because there's indeed an Ord 
instance for Int. In other cases, e.g. 'Num Word', 'abs' is just the identity 
function, i.e. 'abs x = x' since there are no non-positive Word values.

-- 
Frerich Raabe - raabe at froglogic.com
www.froglogic.com - Multi-Platform GUI Testing

From dani at dpwright.com  Wed Mar 11 13:30:45 2015
From: dani at dpwright.com (Daniel P. Wright)
Date: Wed, 11 Mar 2015 22:30:45 +0900
Subject: [Haskell-beginners] What's the type of function "abs"?
In-Reply-To: <CAMeL8ffwki=xg1n48JmEbTuUjhpacw6p0FqfmiQKz3M5YkSsYA@mail.gmail.com>
References: <CAMeL8ffwki=xg1n48JmEbTuUjhpacw6p0FqfmiQKz3M5YkSsYA@mail.gmail.com>
Message-ID: <924EF1DE-B9DA-45DE-89CE-2623022FF22C@dpwright.com>

One nice thing about haskell is that the source to most things is available on hackage. If you look at abs, here:

https://hackage.haskell.org/package/base-4.7.0.2/docs/src/GHC-Num.html#abs

You will see two things:

1) abs is actually part of the Num typeclass, so can be defined differently for the different numerical types
2) the definition for Int is similar to yours:
    abs n = if n `geInt` 0 then n else negate n

Notice `geInt` is just an int-specific (>=) operator. Because we're defining an instance we know the concrete type we're dealing with (the type is Int -> Int by this point, rather than a -> a), so we don't need to make use of Ord in this case.



11 Mar 2015 22:21?Zhang Hengruo <hengruo.z at gmail.com> ??????:

> I'm a newbie, knowing a little about Haskell, and hope my question isn't very silly...
> =====================================================================
> My absolute value function looks like this:
> 
> abs2 :: Num a => a -> a
> abs2 n = if n >= 0 then n else 0 - n
> 
> GHCi tells me that I should add Ord type class to its definition. Well, it's true. It has used relational operators and Num isn't a subclass of Ord.
> However, when I input ":t abs" in GHCi, the interpreter shows "abs :: Num a => a -> a". I read GHC/Num.lhs and find that abs is defined as "abs :: a -> a" and has no concrete content. So I think the abs function is written in C as a module to implement directly and the type of abs just follows its class GHC.Num. Is it right? Or there are any other reasons? 
> 
> Thanks,
> 
> Hengruo
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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From hengruo.z at gmail.com  Wed Mar 11 14:08:18 2015
From: hengruo.z at gmail.com (Zhang Hengruo)
Date: Wed, 11 Mar 2015 22:08:18 +0800
Subject: [Haskell-beginners] What's the type of function "abs"?
In-Reply-To: <924EF1DE-B9DA-45DE-89CE-2623022FF22C@dpwright.com>
References: <CAMeL8ffwki=xg1n48JmEbTuUjhpacw6p0FqfmiQKz3M5YkSsYA@mail.gmail.com>
 <924EF1DE-B9DA-45DE-89CE-2623022FF22C@dpwright.com>
Message-ID: <CAMeL8fdHdKQo1_+mN4GbLOkq7H7YNyt2R7-UAxS8ran7HmhPnw@mail.gmail.com>

Oh! Thank you very much! I took notice of instances of Num just now... You
two are so kind!

2015-03-11 21:30 GMT+08:00 Daniel P. Wright <dani at dpwright.com>:

> One nice thing about haskell is that the source to most things is
> available on hackage. If you look at abs, here:
>
> https://hackage.haskell.org/package/base-4.7.0.2/docs/src/GHC-Num.html#abs
>
> You will see two things:
>
> 1) abs is actually part of the Num typeclass, so can be defined
> differently for the different numerical types
> 2) the definition for Int is similar to yours:
>
>     abs n  = if n `geInt` 0 then n else negate n
>
>
> Notice `geInt` is just an int-specific (>=) operator. Because we're
> defining an instance we know the concrete type we're dealing with (the type
> is Int -> Int by this point, rather than a -> a), so we don't need to make
> use of Ord in this case.
>
>
>
> 11 Mar 2015 22:21?Zhang Hengruo <hengruo.z at gmail.com> ??????:
>
> I'm a newbie, knowing a little about Haskell, and hope my question isn't
> very silly...
> =====================================================================
> My absolute value function looks like this:
>
> abs2 :: Num a => a -> a
> abs2 n = if n >= 0 then n else 0 - n
>
> GHCi tells me that I should add Ord type class to its definition. Well,
> it's true. It has used relational operators and Num isn't a subclass of Ord.
> However, when I input ":t abs" in GHCi, the interpreter shows "abs :: Num
> a => a -> a". I read GHC/Num.lhs and find that abs is defined as "abs :: a
> -> a" and has no concrete content. So I think the abs function is written
> in C as a module to implement directly and the type of abs just follows its
> class GHC.Num. Is it right? Or there are any other reasons?
>
> Thanks,
>
> Hengruo
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Wed Mar 11 21:45:03 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Thu, 12 Mar 2015 03:15:03 +0530
Subject: [Haskell-beginners] Mathematical functions with multiple arguments
Message-ID: <CAJbEW8OZg8a6824U1j7gsHj2SXzrRCnd=vzTueX9ZLEoNo3VZw@mail.gmail.com>

Hi everybody,

I have a function of type

    plot :: ([Double] -> Double)    -- A function to plot
         -> [(Double, Double)]      -- Range for all arguments
         -> IO ()

I want to enforce the fact that ranges for all arguments should be provided.
Is there a way to make the type system enforce it?

-- 
Regards

Sumit Sahrawat
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Wed Mar 11 21:58:51 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Thu, 12 Mar 2015 03:28:51 +0530
Subject: [Haskell-beginners] [Haskell-cafe] Mathematical functions with
	multiple arguments
In-Reply-To: <CACnqJZZ=cZ1zdW24AXub53b4WzmHL1rm0ffHYWSrv9r5VRHqKw@mail.gmail.com>
References: <CAJbEW8OZg8a6824U1j7gsHj2SXzrRCnd=vzTueX9ZLEoNo3VZw@mail.gmail.com>
 <CACnqJZZ=cZ1zdW24AXub53b4WzmHL1rm0ffHYWSrv9r5VRHqKw@mail.gmail.com>
Message-ID: <CAJbEW8NpJ0e9j5ujd9cixMinh9dexXWiVOxLn=5p_OLZ3TBeEA@mail.gmail.com>

I was wondering whether it was possible using fixed-vector, ut dependent
types seem to be a new thing I can experiment with.

Thanks for the quick reply.

On 12 March 2015 at 03:26, Adam Bergmark <adam at bergmark.nl> wrote:

> If I understand you correctly you seem to want dependent types, this
> article uses the same example you need, promoting the length of a
> vector/list to the type level:
> https://www.fpcomplete.com/user/konn/prove-your-haskell-for-great-safety/dependent-types-in-haskell
>
> You'd end up with `plot :: (Vector n Double -> Double) -> Vector n
> (Double, Double) -> IO ()' where `n' is the length of the vector.
>
> HTH,
> Adam
>
>
> On Wed, Mar 11, 2015 at 10:45 PM, Sumit Sahrawat, Maths & Computing, IIT
> (BHU) <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:
>
>> Hi everybody,
>>
>> I have a function of type
>>
>>     plot :: ([Double] -> Double)    -- A function to plot
>>          -> [(Double, Double)]      -- Range for all arguments
>>          -> IO ()
>>
>> I want to enforce the fact that ranges for all arguments should be
>> provided.
>> Is there a way to make the type system enforce it?
>>
>> --
>> Regards
>>
>> Sumit Sahrawat
>>
>> _______________________________________________
>> Haskell-Cafe mailing list
>> Haskell-Cafe at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>>
>>
>


-- 
Regards

Sumit Sahrawat
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Wed Mar 11 23:16:42 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Thu, 12 Mar 2015 04:46:42 +0530
Subject: [Haskell-beginners] [Haskell-cafe] Mathematical functions with
	multiple arguments
In-Reply-To: <CAMgWh9uiKxPTOo1by1jpbdAWBiONaVxRDwrgvET7KQhtG9mfpQ@mail.gmail.com>
References: <CAJbEW8OZg8a6824U1j7gsHj2SXzrRCnd=vzTueX9ZLEoNo3VZw@mail.gmail.com>
 <CAMgWh9uiKxPTOo1by1jpbdAWBiONaVxRDwrgvET7KQhtG9mfpQ@mail.gmail.com>
Message-ID: <CAJbEW8N9G6XF-P-mPsznnZCABxKSiFqmRBEHm9WCaWMoyRncEA@mail.gmail.com>

The fixed-vector package uses a similar technique. The only trouble I'm
having is with converting Vec v (Double, Double) to [(Double, Double)] for
further use. I don't want to change all the code, but only the part where
the user provides me with arguments.

I'll keep looking into it. Thanks for the help.

On 12 March 2015 at 04:44, David Feuer <david.feuer at gmail.com> wrote:

> There are a lot of ways to do this sort of thing, and which one you choose
> will depend on exactly what you're trying to do. For example, you can write
> something vaguely like
>
> data Nat = Z | S Nat
> data SL (n :: Nat) a where
>   Nil :: SL Z
>   Cons :: a -> SL n a -> SL (S n) a
>
> plot :: forall (n::Nat) . (SL n Double -> Double) ->
>                                         SL n (Double, Double) -> IO ()
> On Mar 11, 2015 5:45 PM, "Sumit Sahrawat, Maths & Computing, IIT (BHU)" <
> sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:
>
>> Hi everybody,
>>
>> I have a function of type
>>
>>     plot :: ([Double] -> Double)    -- A function to plot
>>          -> [(Double, Double)]      -- Range for all arguments
>>          -> IO ()
>>
>> I want to enforce the fact that ranges for all arguments should be
>> provided.
>> Is there a way to make the type system enforce it?
>>
>> --
>> Regards
>>
>> Sumit Sahrawat
>>
>> _______________________________________________
>> Haskell-Cafe mailing list
>> Haskell-Cafe at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>>
>>


-- 
Regards

Sumit Sahrawat
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Thu Mar 12 00:19:25 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Thu, 12 Mar 2015 05:49:25 +0530
Subject: [Haskell-beginners] [Haskell-cafe] Mathematical functions with
	multiple arguments
In-Reply-To: <CAMgWh9sRUJ0oG=s3qTHTLeqt5ZuE_OvFe5+yUVL-nhri+Ndf=g@mail.gmail.com>
References: <CAJbEW8OZg8a6824U1j7gsHj2SXzrRCnd=vzTueX9ZLEoNo3VZw@mail.gmail.com>
 <CAMgWh9uiKxPTOo1by1jpbdAWBiONaVxRDwrgvET7KQhtG9mfpQ@mail.gmail.com>
 <CAJbEW8N9G6XF-P-mPsznnZCABxKSiFqmRBEHm9WCaWMoyRncEA@mail.gmail.com>
 <CAMgWh9sRUJ0oG=s3qTHTLeqt5ZuE_OvFe5+yUVL-nhri+Ndf=g@mail.gmail.com>
Message-ID: <CAJbEW8MBZFTM0OAqKp_Euh_ofSijB11cO3zjh+MPhzwARxK45A@mail.gmail.com>

It's done. fixed-vector provides many implementations for Vec. The one I
was using (Primitive) requires primitive constraint on data types. I
shifted to Unboxed ones, and now it's working.

This means that I am able to use toList with Vec v (Double, Double), which
was previously complaining of missing constraints.

Also, I'll slowly convert the whole thing. I wanted to just see if it works
initially.
Thanks, once again.

On 12 March 2015 at 04:58, David Feuer <david.feuer at gmail.com> wrote:

> I'd urge you to *try* to maintain that safety throughout, but you can try
> something like this if necessary (I'm not at my computer, so I can't test
> it):
>
> {-#LANGUAGE FlexibleContexts, FlexibleInstances #-}
> instance Foldable (SL Z) where
>   foldr _ n Nil = n
>
> instance Foldable (SL n) => Foldable (SL (S n)) where
>   foldr c n (Cons x xs) = c x (foldr c n xs)
>
> In fact, I would expect whatever package you're looking at to offer this.
> Then you can just use Data.Foldable.toList.
> On Mar 11, 2015 7:16 PM, "Sumit Sahrawat, Maths & Computing, IIT (BHU)" <
> sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:
>
>> The fixed-vector package uses a similar technique. The only trouble I'm
>> having is with converting Vec v (Double, Double) to [(Double, Double)] for
>> further use. I don't want to change all the code, but only the part where
>> the user provides me with arguments.
>>
>> I'll keep looking into it. Thanks for the help.
>>
>> On 12 March 2015 at 04:44, David Feuer <david.feuer at gmail.com> wrote:
>>
>>> There are a lot of ways to do this sort of thing, and which one you
>>> choose will depend on exactly what you're trying to do. For example, you
>>> can write something vaguely like
>>>
>>> data Nat = Z | S Nat
>>> data SL (n :: Nat) a where
>>>   Nil :: SL Z
>>>   Cons :: a -> SL n a -> SL (S n) a
>>>
>>> plot :: forall (n::Nat) . (SL n Double -> Double) ->
>>>                                         SL n (Double, Double) -> IO ()
>>> On Mar 11, 2015 5:45 PM, "Sumit Sahrawat, Maths & Computing, IIT (BHU)" <
>>> sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:
>>>
>>>> Hi everybody,
>>>>
>>>> I have a function of type
>>>>
>>>>     plot :: ([Double] -> Double)    -- A function to plot
>>>>          -> [(Double, Double)]      -- Range for all arguments
>>>>          -> IO ()
>>>>
>>>> I want to enforce the fact that ranges for all arguments should be
>>>> provided.
>>>> Is there a way to make the type system enforce it?
>>>>
>>>> --
>>>> Regards
>>>>
>>>> Sumit Sahrawat
>>>>
>>>> _______________________________________________
>>>> Haskell-Cafe mailing list
>>>> Haskell-Cafe at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>>>>
>>>>
>>
>>
>> --
>> Regards
>>
>> Sumit Sahrawat
>>
>


-- 
Regards

Sumit Sahrawat
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From k-bx at k-bx.com  Thu Mar 12 07:05:05 2015
From: k-bx at k-bx.com (Konstantine Rybnikov)
Date: Thu, 12 Mar 2015 09:05:05 +0200
Subject: [Haskell-beginners] [Haskell-cafe] Mathematical functions with
 multiple arguments
In-Reply-To: <CAJbEW8N9G6XF-P-mPsznnZCABxKSiFqmRBEHm9WCaWMoyRncEA@mail.gmail.com>
References: <CAJbEW8OZg8a6824U1j7gsHj2SXzrRCnd=vzTueX9ZLEoNo3VZw@mail.gmail.com>
 <CAMgWh9uiKxPTOo1by1jpbdAWBiONaVxRDwrgvET7KQhtG9mfpQ@mail.gmail.com>
 <CAJbEW8N9G6XF-P-mPsznnZCABxKSiFqmRBEHm9WCaWMoyRncEA@mail.gmail.com>
Message-ID: <CAAbahfSACap7cr7vAYO-OHqRY_F5HakK6anRpOf4gmrTQwiOPg@mail.gmail.com>

Is `Data.Vector.Fixed.toList` is what you're looking for?

On Thu, Mar 12, 2015 at 1:16 AM, Sumit Sahrawat, Maths & Computing, IIT
(BHU) <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

> The fixed-vector package uses a similar technique. The only trouble I'm
> having is with converting Vec v (Double, Double) to [(Double, Double)] for
> further use. I don't want to change all the code, but only the part where
> the user provides me with arguments.
>
> I'll keep looking into it. Thanks for the help.
>
> On 12 March 2015 at 04:44, David Feuer <david.feuer at gmail.com> wrote:
>
>> There are a lot of ways to do this sort of thing, and which one you
>> choose will depend on exactly what you're trying to do. For example, you
>> can write something vaguely like
>>
>> data Nat = Z | S Nat
>> data SL (n :: Nat) a where
>>   Nil :: SL Z
>>   Cons :: a -> SL n a -> SL (S n) a
>>
>> plot :: forall (n::Nat) . (SL n Double -> Double) ->
>>                                         SL n (Double, Double) -> IO ()
>> On Mar 11, 2015 5:45 PM, "Sumit Sahrawat, Maths & Computing, IIT (BHU)" <
>> sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:
>>
>>> Hi everybody,
>>>
>>> I have a function of type
>>>
>>>     plot :: ([Double] -> Double)    -- A function to plot
>>>          -> [(Double, Double)]      -- Range for all arguments
>>>          -> IO ()
>>>
>>> I want to enforce the fact that ranges for all arguments should be
>>> provided.
>>> Is there a way to make the type system enforce it?
>>>
>>> --
>>> Regards
>>>
>>> Sumit Sahrawat
>>>
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>>>
>>>
>
>
> --
> Regards
>
> Sumit Sahrawat
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Thu Mar 12 08:42:29 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Thu, 12 Mar 2015 14:12:29 +0530
Subject: [Haskell-beginners] [Haskell-cafe] Mathematical functions with
 multiple arguments
In-Reply-To: <CAAbahfSACap7cr7vAYO-OHqRY_F5HakK6anRpOf4gmrTQwiOPg@mail.gmail.com>
References: <CAJbEW8OZg8a6824U1j7gsHj2SXzrRCnd=vzTueX9ZLEoNo3VZw@mail.gmail.com>
 <CAMgWh9uiKxPTOo1by1jpbdAWBiONaVxRDwrgvET7KQhtG9mfpQ@mail.gmail.com>
 <CAJbEW8N9G6XF-P-mPsznnZCABxKSiFqmRBEHm9WCaWMoyRncEA@mail.gmail.com>
 <CAAbahfSACap7cr7vAYO-OHqRY_F5HakK6anRpOf4gmrTQwiOPg@mail.gmail.com>
Message-ID: <CAJbEW8MXuJXUPc=fT_mr0H9ak6Zhm5fzH7Z0i8LufPjACQ=Oxw@mail.gmail.com>

Yeah, Data.Vector.Fixed.toList worked. I had found it previously, but it
didn't work for Vec imported from Data.Fixed.Vector.Primitive (as (Double,
Double) doesn't have a Prim instance).
Then I had to switch to Data.Fixed.Vector.Unboxed, and it works now.

Also, those shingles look interesting too. Thanks everybody.

On 12 March 2015 at 12:35, Konstantine Rybnikov <k-bx at k-bx.com> wrote:

> Is `Data.Vector.Fixed.toList` is what you're looking for?
>
> On Thu, Mar 12, 2015 at 1:16 AM, Sumit Sahrawat, Maths & Computing, IIT
> (BHU) <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:
>
>> The fixed-vector package uses a similar technique. The only trouble I'm
>> having is with converting Vec v (Double, Double) to [(Double, Double)] for
>> further use. I don't want to change all the code, but only the part where
>> the user provides me with arguments.
>>
>> I'll keep looking into it. Thanks for the help.
>>
>> On 12 March 2015 at 04:44, David Feuer <david.feuer at gmail.com> wrote:
>>
>>> There are a lot of ways to do this sort of thing, and which one you
>>> choose will depend on exactly what you're trying to do. For example, you
>>> can write something vaguely like
>>>
>>> data Nat = Z | S Nat
>>> data SL (n :: Nat) a where
>>>   Nil :: SL Z
>>>   Cons :: a -> SL n a -> SL (S n) a
>>>
>>> plot :: forall (n::Nat) . (SL n Double -> Double) ->
>>>                                         SL n (Double, Double) -> IO ()
>>> On Mar 11, 2015 5:45 PM, "Sumit Sahrawat, Maths & Computing, IIT (BHU)" <
>>> sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:
>>>
>>>> Hi everybody,
>>>>
>>>> I have a function of type
>>>>
>>>>     plot :: ([Double] -> Double)    -- A function to plot
>>>>          -> [(Double, Double)]      -- Range for all arguments
>>>>          -> IO ()
>>>>
>>>> I want to enforce the fact that ranges for all arguments should be
>>>> provided.
>>>> Is there a way to make the type system enforce it?
>>>>
>>>> --
>>>> Regards
>>>>
>>>> Sumit Sahrawat
>>>>
>>>> _______________________________________________
>>>> Haskell-Cafe mailing list
>>>> Haskell-Cafe at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>>>>
>>>>
>>
>>
>> --
>> Regards
>>
>> Sumit Sahrawat
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>


-- 
Regards

Sumit Sahrawat
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From twashing at gmail.com  Thu Mar 12 23:02:12 2015
From: twashing at gmail.com (Timothy Washington)
Date: Thu, 12 Mar 2015 19:02:12 -0400
Subject: [Haskell-beginners] Dipping Toes Into Haskell
Message-ID: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>

To get started, I'm trying to implement a simple *tictactoe* game. And I
would like to be able to represent a Piece on the board, as either the
string "X" or "O". This is what I have so far.

module Main where

data Piece = X | O
type Row = [Piece]
type Board = [Row]

-- put an X or O in a position
move :: Board -> Piece -> Board
move board piece = board

-- check win vertically
-- check win horizontally
-- check win diagonally

main :: IO ()
main = putStrLn "Hello World"



*A)* Now, I'd like to be able to *load code interactively*, preferably
within emacs. However I don't have access to my types with *ghci* or *ghc-mod
(Interactive-Haskell)*. In either case, this call fails with the below
error.


let p = Piece X
<interactive>:20:9-13: Not in scope: data constructor `Piece'


*B)* And how do I make a *custom datatype* that's one of two strings
(enumeration of either "X" or "O"). Cabal builds and runs the abouve code,
so I know it can compile. But I'm confused as to where X or O is defined,
and how I would supply it as an input.

*C)* Finally, how do we update nested lists in Haskell. I want the move
function to take a Board, Piece, and Position, and return a Board. I see
some results from Hoogle <https://www.haskell.org/hoogle/?hoogle=update>.
Is this where Lenses <https://www.haskell.org/hoogle/?hoogle=lens> or
Zippers <https://www.haskell.org/hoogle/?hoogle=zipper> come into play?


Thanks

Tim Washington
Interruptsoftware.com <http://interruptsoftware.com>
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Thu Mar 12 23:50:16 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Fri, 13 Mar 2015 05:20:16 +0530
Subject: [Haskell-beginners] Dipping Toes Into Haskell
In-Reply-To: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
References: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
Message-ID: <CAJbEW8PXy=VsQpxE0H=G5V1sZC4Qn8xFDCyqvTienadiHfnz8Q@mail.gmail.com>

data Piece = X | O

means that X and O are constants, having data type 'Piece'.
In other words, you directly use X and O instead of Piece X and Piece O.

Piece is a type, and X and O are constructors (or simply the two possible
values).

A good similar example is the Bool type,

    data Bool = False | True
    -- Two possible values, i.e. False and True

For more info, take a look here:
http://en.wikibooks.org/wiki/Haskell/Type_declarations

Also, to make life easier, you might want to use:

    data Piece = X | O
      deriving Show -- Make this type showable

With this, you will be able to use 'print' with elements of this datatype.

For a tutorial on setting up emacs, take a look here:
https://github.com/serras/emacs-haskell-tutorial/blob/master/tutorial.md

For updating nested lists, you have to create a function that takes two
numbers (the positions) and iterates over the whole structure, just
updating the required position.
While this may seem like an overkill, it gets optimized by ghc.

I don't have any experience with lenses, but they should be usable here.
Understanding them will require a good understanding of the type system.

On 13 March 2015 at 04:32, Timothy Washington <twashing at gmail.com> wrote:

> To get started, I'm trying to implement a simple *tictactoe* game. And I
> would like to be able to represent a Piece on the board, as either the
> string "X" or "O". This is what I have so far.
>
> module Main where
>
> data Piece = X | O
> type Row = [Piece]
> type Board = [Row]
>
> -- put an X or O in a position
> move :: Board -> Piece -> Board
> move board piece = board
>
> -- check win vertically
> -- check win horizontally
> -- check win diagonally
>
> main :: IO ()
> main = putStrLn "Hello World"
>
>
>
> *A)* Now, I'd like to be able to *load code interactively*, preferably
> within emacs. However I don't have access to my types with *ghci* or *ghc-mod
> (Interactive-Haskell)*. In either case, this call fails with the below
> error.
>
>
> let p = Piece X
> <interactive>:20:9-13: Not in scope: data constructor `Piece'
>
>
> *B)* And how do I make a *custom datatype* that's one of two strings
> (enumeration of either "X" or "O"). Cabal builds and runs the abouve code,
> so I know it can compile. But I'm confused as to where X or O is defined,
> and how I would supply it as an input.
>
> *C)* Finally, how do we update nested lists in Haskell. I want the move
> function to take a Board, Piece, and Position, and return a Board. I see
> some results from Hoogle <https://www.haskell.org/hoogle/?hoogle=update>.
> Is this where Lenses <https://www.haskell.org/hoogle/?hoogle=lens> or
> Zippers <https://www.haskell.org/hoogle/?hoogle=zipper> come into play?
>
>
> Thanks
>
> Tim Washington
> Interruptsoftware.com <http://interruptsoftware.com>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>


-- 
Regards

Sumit Sahrawat
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From ky3 at atamo.com  Thu Mar 12 23:53:55 2015
From: ky3 at atamo.com (Kim-Ee Yeoh)
Date: Fri, 13 Mar 2015 06:53:55 +0700
Subject: [Haskell-beginners] Dipping Toes Into Haskell
In-Reply-To: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
References: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
Message-ID: <CAPY+ZdQT4k8V5Z9uRZvQVjoDSLULvZ0_3YpKBkKRnfquWC_ZYw@mail.gmail.com>

Hi Tim,

Congrats on your choice of tic-tac-toe. I also believe it's a good toy to
get your feet wet with haskell.

1. Your concerns about writing a "custom datatype" are best addressed by
reading up on it. If you search for "haskell data type declarations",
you'll find good material explaining the difference between "data",
"newtype", and "type" declarations. You'll also understand the meaning of
the error message and what "data constructor" refers to.

2. Functions defined on lists are here:

https://hackage.haskell.org/package/base/docs/Data-List.html

The update function you're looking for is (!!).




-- Kim-Ee

On Fri, Mar 13, 2015 at 6:02 AM, Timothy Washington <twashing at gmail.com>
wrote:

> To get started, I'm trying to implement a simple *tictactoe* game. And I
> would like to be able to represent a Piece on the board, as either the
> string "X" or "O". This is what I have so far.
>
> module Main where
>
> data Piece = X | O
> type Row = [Piece]
> type Board = [Row]
>
> -- put an X or O in a position
> move :: Board -> Piece -> Board
> move board piece = board
>
> -- check win vertically
> -- check win horizontally
> -- check win diagonally
>
> main :: IO ()
> main = putStrLn "Hello World"
>
>
>
> *A)* Now, I'd like to be able to *load code interactively*, preferably
> within emacs. However I don't have access to my types with *ghci* or *ghc-mod
> (Interactive-Haskell)*. In either case, this call fails with the below
> error.
>
>
> let p = Piece X
> <interactive>:20:9-13: Not in scope: data constructor `Piece'
>
>
> *B)* And how do I make a *custom datatype* that's one of two strings
> (enumeration of either "X" or "O"). Cabal builds and runs the abouve code,
> so I know it can compile. But I'm confused as to where X or O is defined,
> and how I would supply it as an input.
>
> *C)* Finally, how do we update nested lists in Haskell. I want the move
> function to take a Board, Piece, and Position, and return a Board. I see
> some results from Hoogle <https://www.haskell.org/hoogle/?hoogle=update>.
> Is this where Lenses <https://www.haskell.org/hoogle/?hoogle=lens> or
> Zippers <https://www.haskell.org/hoogle/?hoogle=zipper> come into play?
>
>
> Thanks
>
> Tim Washington
> Interruptsoftware.com <http://interruptsoftware.com>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From mwm at mired.org  Fri Mar 13 00:03:51 2015
From: mwm at mired.org (Mike Meyer)
Date: Thu, 12 Mar 2015 19:03:51 -0500
Subject: [Haskell-beginners] Dipping Toes Into Haskell
In-Reply-To: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
References: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
Message-ID: <CAD=7U2DFK_T9Z+iWs9S1QtQyZz+yRiNGYeboEcV0pj3GvmWZZQ@mail.gmail.com>

On Thu, Mar 12, 2015 at 6:02 PM, Timothy Washington <twashing at gmail.com>
wrote:

> To get started, I'm trying to implement a simple *tictactoe* game. And I
> would like to be able to represent a Piece on the board, as either the
> string "X" or "O". This is what I have so far.
>
> module Main where
>
> data Piece = X | O
> type Row = [Piece]
> type Board = [Row]
>
> -- put an X or O in a position
> move :: Board -> Piece -> Board
> move board piece = board
>
> -- check win vertically
> -- check win horizontally
> -- check win diagonally
>
> main :: IO ()
> main = putStrLn "Hello World"
>
>
>
> *A)* Now, I'd like to be able to *load code interactively*, preferably
> within emacs. However I don't have access to my types with *ghci* or *ghc-mod
> (Interactive-Haskell)*. In either case, this call fails with the below
> error.
>
>
> let p = Piece X
> <interactive>:20:9-13: Not in scope: data constructor `Piece'
>
>
Piece is the data TYPE. It has two constructors, X, and O. If you load your
module into ghci, you can do ":t X" to get the type of X, which is Piece.
You want to do "let p = X" here.

Check your mode docs while editing the haskell file. There should be  a
command to load the current buffer into a ghci running in an interactive
emacs buffer.


> *B)* And how do I make a *custom datatype* that's one of two strings
> (enumeration of either "X" or "O"). Cabal builds and runs the abouve code,
> so I know it can compile. But I'm confused as to where X or O is defined,
> and how I would supply it as an input.
>


As noted, you can derive "Show" so that Piece types print as "X" and "O".
Similarly, you can derive "Read":

data Piece = X | O deriving (Show, Read)

so that you can then read "X" and get back an X when the context calls for
a Piece value:

*Main> read "X" :: Piece

X

However, this isn't used a lot, as it tends to be slow and not very
flexible for more complex types. For instance, it will let you read in
lists of Pieces and lists of lists, but you'l lhave to use the list syntax,
 not something that looks like an actual board when printed.
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From twashing at gmail.com  Fri Mar 13 00:14:30 2015
From: twashing at gmail.com (Timothy Washington)
Date: Thu, 12 Mar 2015 20:14:30 -0400
Subject: [Haskell-beginners] Dipping Toes Into Haskell
In-Reply-To: <CAJbEW8PXy=VsQpxE0H=G5V1sZC4Qn8xFDCyqvTienadiHfnz8Q@mail.gmail.com>
References: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
 <CAJbEW8PXy=VsQpxE0H=G5V1sZC4Qn8xFDCyqvTienadiHfnz8Q@mail.gmail.com>
Message-ID: <CAADtM-a=5pV8pU_Yb8McEpscfdU_kJCMw9DVgiUvAMS1aYvtWg@mail.gmail.com>

Good explanation. Thank-you.


Tim Washington
Interruptsoftware.com <http://interruptsoftware.com>


On Thu, Mar 12, 2015 at 7:50 PM, Sumit Sahrawat, Maths & Computing, IIT
(BHU) <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

> data Piece = X | O
>
> means that X and O are constants, having data type 'Piece'.
> In other words, you directly use X and O instead of Piece X and Piece O.
>
> Piece is a type, and X and O are constructors (or simply the two possible
> values).
>
> A good similar example is the Bool type,
>
>     data Bool = False | True
>     -- Two possible values, i.e. False and True
>
> For more info, take a look here:
> http://en.wikibooks.org/wiki/Haskell/Type_declarations
>
> Also, to make life easier, you might want to use:
>
>     data Piece = X | O
>       deriving Show -- Make this type showable
>
> With this, you will be able to use 'print' with elements of this datatype.
>
> For a tutorial on setting up emacs, take a look here:
> https://github.com/serras/emacs-haskell-tutorial/blob/master/tutorial.md
>
> For updating nested lists, you have to create a function that takes two
> numbers (the positions) and iterates over the whole structure, just
> updating the required position.
> While this may seem like an overkill, it gets optimized by ghc.
>
> I don't have any experience with lenses, but they should be usable here.
> Understanding them will require a good understanding of the type system.
>
> On 13 March 2015 at 04:32, Timothy Washington <twashing at gmail.com> wrote:
>
>> To get started, I'm trying to implement a simple *tictactoe* game. And I
>> would like to be able to represent a Piece on the board, as either the
>> string "X" or "O". This is what I have so far.
>>
>> module Main where
>>
>> data Piece = X | O
>> type Row = [Piece]
>> type Board = [Row]
>>
>> -- put an X or O in a position
>> move :: Board -> Piece -> Board
>> move board piece = board
>>
>> -- check win vertically
>> -- check win horizontally
>> -- check win diagonally
>>
>> main :: IO ()
>> main = putStrLn "Hello World"
>>
>>
>>
>> *A)* Now, I'd like to be able to *load code interactively*, preferably
>> within emacs. However I don't have access to my types with *ghci* or *ghc-mod
>> (Interactive-Haskell)*. In either case, this call fails with the below
>> error.
>>
>>
>> let p = Piece X
>> <interactive>:20:9-13: Not in scope: data constructor `Piece'
>>
>>
>> *B)* And how do I make a *custom datatype* that's one of two strings
>> (enumeration of either "X" or "O"). Cabal builds and runs the abouve code,
>> so I know it can compile. But I'm confused as to where X or O is defined,
>> and how I would supply it as an input.
>>
>> *C)* Finally, how do we update nested lists in Haskell. I want the move
>> function to take a Board, Piece, and Position, and return a Board. I see
>> some results from Hoogle <https://www.haskell.org/hoogle/?hoogle=update>.
>> Is this where Lenses <https://www.haskell.org/hoogle/?hoogle=lens> or
>> Zippers <https://www.haskell.org/hoogle/?hoogle=zipper> come into play?
>>
>>
>> Thanks
>>
>> Tim Washington
>> Interruptsoftware.com <http://interruptsoftware.com>
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
>
> --
> Regards
>
> Sumit Sahrawat
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From twashing at gmail.com  Fri Mar 13 00:15:14 2015
From: twashing at gmail.com (Timothy Washington)
Date: Thu, 12 Mar 2015 20:15:14 -0400
Subject: [Haskell-beginners] Dipping Toes Into Haskell
In-Reply-To: <CAPY+ZdQT4k8V5Z9uRZvQVjoDSLULvZ0_3YpKBkKRnfquWC_ZYw@mail.gmail.com>
References: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
 <CAPY+ZdQT4k8V5Z9uRZvQVjoDSLULvZ0_3YpKBkKRnfquWC_ZYw@mail.gmail.com>
Message-ID: <CAADtM-aZd8BKF-26dh3rEPoW+ruJH1AWwUPAY2KCEQ6whuiYEQ@mail.gmail.com>

Thank-you :) And thanks for the Data.List docs. That's a good place to
start.


Tim Washington
Interruptsoftware.com <http://interruptsoftware.com>


On Thu, Mar 12, 2015 at 7:53 PM, Kim-Ee Yeoh <ky3 at atamo.com> wrote:

> Hi Tim,
>
> Congrats on your choice of tic-tac-toe. I also believe it's a good toy to
> get your feet wet with haskell.
>
> 1. Your concerns about writing a "custom datatype" are best addressed by
> reading up on it. If you search for "haskell data type declarations",
> you'll find good material explaining the difference between "data",
> "newtype", and "type" declarations. You'll also understand the meaning of
> the error message and what "data constructor" refers to.
>
> 2. Functions defined on lists are here:
>
> https://hackage.haskell.org/package/base/docs/Data-List.html
>
> The update function you're looking for is (!!).
>
>
>
>
> -- Kim-Ee
>
> On Fri, Mar 13, 2015 at 6:02 AM, Timothy Washington <twashing at gmail.com>
> wrote:
>
>> To get started, I'm trying to implement a simple *tictactoe* game. And I
>> would like to be able to represent a Piece on the board, as either the
>> string "X" or "O". This is what I have so far.
>>
>> module Main where
>>
>> data Piece = X | O
>> type Row = [Piece]
>> type Board = [Row]
>>
>> -- put an X or O in a position
>> move :: Board -> Piece -> Board
>> move board piece = board
>>
>> -- check win vertically
>> -- check win horizontally
>> -- check win diagonally
>>
>> main :: IO ()
>> main = putStrLn "Hello World"
>>
>>
>>
>> *A)* Now, I'd like to be able to *load code interactively*, preferably
>> within emacs. However I don't have access to my types with *ghci* or *ghc-mod
>> (Interactive-Haskell)*. In either case, this call fails with the below
>> error.
>>
>>
>> let p = Piece X
>> <interactive>:20:9-13: Not in scope: data constructor `Piece'
>>
>>
>> *B)* And how do I make a *custom datatype* that's one of two strings
>> (enumeration of either "X" or "O"). Cabal builds and runs the abouve code,
>> so I know it can compile. But I'm confused as to where X or O is defined,
>> and how I would supply it as an input.
>>
>> *C)* Finally, how do we update nested lists in Haskell. I want the move
>> function to take a Board, Piece, and Position, and return a Board. I see
>> some results from Hoogle <https://www.haskell.org/hoogle/?hoogle=update>.
>> Is this where Lenses <https://www.haskell.org/hoogle/?hoogle=lens> or
>> Zippers <https://www.haskell.org/hoogle/?hoogle=zipper> come into play?
>>
>>
>> Thanks
>>
>> Tim Washington
>> Interruptsoftware.com <http://interruptsoftware.com>
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From joel.neely at gmail.com  Fri Mar 13 11:53:05 2015
From: joel.neely at gmail.com (Joel Neely)
Date: Fri, 13 Mar 2015 06:53:05 -0500
Subject: [Haskell-beginners] Problems installing lambdabot
Message-ID: <CAEEzXAgbYXnTF6vHyAUTF3pmnU2a8BCwiYhp5MJrM1fddG7JOA@mail.gmail.com>

I am using a recent install of Haskell platform (GHC/GHCi version 7.8.3) in
OS X 10.9.5, and have been working through Real World Haskell and last
year's CIS194 lectures and exercises.

I attempted to follow the instructions for installing lambdabot (as given
by https://wiki.haskell.org/Lambdabot ) via cabal install lambdabot.

After 24 1/2 minutes, the process ended with a little over a dozen error
messages apparently tracing to two occurrences of ExitFailure 1:

MonadRandom-0.3.0.1 failed during the building phase. The exception was:

ExitFailure 1

regex-pcre-0.94.4 failed during the building phase. The exception was:

ExitFailure 1

?The remaining messages were from components dependent on one or the other
of those two.

The logged failure for regex-pcre-0.94.4 ?included the following:

Preprocessing library regex-pcre-0.94.4...
Wrap.hsc:148:10: fatal error: 'pcre.h' file not found
#include <pcre.h>
         ^
1 error generated.
compiling dist/build/Text/Regex/PCRE/Wrap_hsc_make.c failed (exit code 1)


?and for MonadRandom-0.3.0.1 included:

Preprocessing library MonadRandom-0.3.0.1...
[1 of 2] Compiling Control.Monad.Random.Class (
Control/Monad/Random/Class.hs, dist/build/Control/Monad/Random/Class.o )
[2 of 2] Compiling Control.Monad.Random ( Control/Monad/Random.hs,
dist/build/Control/Monad/Random.o )
[1 of 2] Compiling Control.Monad.Random.Class (
Control/Monad/Random/Class.hs, dist/build/Control/Monad/Random/Class.p_o )
[2 of 2] Compiling Control.Monad.Random ( Control/Monad/Random.hs,
dist/build/Control/Monad/Random.p_o )
/Users/joel/.ghc/x86_64-darwin-7.8.3/package.conf.d/package.cache:
openBinaryFile: does not exist (No such file or directory)

?
?I am at a loss for corrective actions. I would assume that I need to
locate the pcre.h header file and put it in the "right place"?, but was
unable to find the location of Wrap_hsc_make.c. And I don't know how to
address the non-existence of openBinaryFile in package.cache.

Thanks in advance for any advice on correcting the above problems!

-jn-


-- 
Beauty of style and harmony and grace and good rhythm depend on simplicity.
- Plato
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From allbery.b at gmail.com  Fri Mar 13 12:01:38 2015
From: allbery.b at gmail.com (Brandon Allbery)
Date: Fri, 13 Mar 2015 08:01:38 -0400
Subject: [Haskell-beginners] Problems installing lambdabot
In-Reply-To: <CAEEzXAgbYXnTF6vHyAUTF3pmnU2a8BCwiYhp5MJrM1fddG7JOA@mail.gmail.com>
References: <CAEEzXAgbYXnTF6vHyAUTF3pmnU2a8BCwiYhp5MJrM1fddG7JOA@mail.gmail.com>
Message-ID: <CAKFCL4UXW_0bHkV=cvyK+wrvAYTAfeWWLqc7MdYEDNF7Wa4idg@mail.gmail.com>

On Fri, Mar 13, 2015 at 7:53 AM, Joel Neely <joel.neely at gmail.com> wrote:

> The logged failure for regex-pcre-0.94.4 ?included the following:
>
> Preprocessing library regex-pcre-0.94.4...
> Wrap.hsc:148:10: fatal error: 'pcre.h' file not found
> #include <pcre.h>
>          ^
> 1 error generated.
> compiling dist/build/Text/Regex/PCRE/Wrap_hsc_make.c failed (exit code 1)
>
>
Both of these are the same problem: cabal cannot operate your system
package manager to install the developer headers / libraries for the
system's pcre package (if it even has one; on a Mac you may need to get a
pcre package from somewhere else).

The other one looks like an internal problem with either ghc or cabal; I
can't help offhand.

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Sat Mar 14 18:11:45 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Sat, 14 Mar 2015 23:41:45 +0530
Subject: [Haskell-beginners] [Haskell-cafe] Mathematical functions with
 multiple arguments
In-Reply-To: <CAJbEW8MXuJXUPc=fT_mr0H9ak6Zhm5fzH7Z0i8LufPjACQ=Oxw@mail.gmail.com>
References: <CAJbEW8OZg8a6824U1j7gsHj2SXzrRCnd=vzTueX9ZLEoNo3VZw@mail.gmail.com>
 <CAMgWh9uiKxPTOo1by1jpbdAWBiONaVxRDwrgvET7KQhtG9mfpQ@mail.gmail.com>
 <CAJbEW8N9G6XF-P-mPsznnZCABxKSiFqmRBEHm9WCaWMoyRncEA@mail.gmail.com>
 <CAAbahfSACap7cr7vAYO-OHqRY_F5HakK6anRpOf4gmrTQwiOPg@mail.gmail.com>
 <CAJbEW8MXuJXUPc=fT_mr0H9ak6Zhm5fzH7Z0i8LufPjACQ=Oxw@mail.gmail.com>
Message-ID: <CAJbEW8Ob3prd=4nseEHafWEAs73JPpN3J2it+x-HMCEyOqotcA@mail.gmail.com>

Hi everybody, I have landed on a small problem.

I have a list of length n, and a function ([Double] -> Double) that
requires a list of length n.
I want to use them with a my plotting function that takes a function
(Vector n Double -> Double) and a (Vector n Double) where both n represent
same length.

I convert the known function to a new function

    newFunc = f . toList :: Vec pn Double -> Double
    -- Where pn is the peano encoding of n

I then convert the list to a vector using fromList, but then I cannot pass
them both to my plotting function.

In other words, how can I convince the type system that the length of
provided vector is the same as that required by the function.

On 12 March 2015 at 14:12, Sumit Sahrawat, Maths & Computing, IIT (BHU) <
sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

> Yeah, Data.Vector.Fixed.toList worked. I had found it previously, but it
> didn't work for Vec imported from Data.Fixed.Vector.Primitive (as (Double,
> Double) doesn't have a Prim instance).
> Then I had to switch to Data.Fixed.Vector.Unboxed, and it works now.
>
> Also, those shingles look interesting too. Thanks everybody.
>
> On 12 March 2015 at 12:35, Konstantine Rybnikov <k-bx at k-bx.com> wrote:
>
>> Is `Data.Vector.Fixed.toList` is what you're looking for?
>>
>> On Thu, Mar 12, 2015 at 1:16 AM, Sumit Sahrawat, Maths & Computing, IIT
>> (BHU) <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:
>>
>>> The fixed-vector package uses a similar technique. The only trouble I'm
>>> having is with converting Vec v (Double, Double) to [(Double, Double)] for
>>> further use. I don't want to change all the code, but only the part where
>>> the user provides me with arguments.
>>>
>>> I'll keep looking into it. Thanks for the help.
>>>
>>> On 12 March 2015 at 04:44, David Feuer <david.feuer at gmail.com> wrote:
>>>
>>>> There are a lot of ways to do this sort of thing, and which one you
>>>> choose will depend on exactly what you're trying to do. For example, you
>>>> can write something vaguely like
>>>>
>>>> data Nat = Z | S Nat
>>>> data SL (n :: Nat) a where
>>>>   Nil :: SL Z
>>>>   Cons :: a -> SL n a -> SL (S n) a
>>>>
>>>> plot :: forall (n::Nat) . (SL n Double -> Double) ->
>>>>                                         SL n (Double, Double) -> IO ()
>>>> On Mar 11, 2015 5:45 PM, "Sumit Sahrawat, Maths & Computing, IIT (BHU)"
>>>> <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:
>>>>
>>>>> Hi everybody,
>>>>>
>>>>> I have a function of type
>>>>>
>>>>>     plot :: ([Double] -> Double)    -- A function to plot
>>>>>          -> [(Double, Double)]      -- Range for all arguments
>>>>>          -> IO ()
>>>>>
>>>>> I want to enforce the fact that ranges for all arguments should be
>>>>> provided.
>>>>> Is there a way to make the type system enforce it?
>>>>>
>>>>> --
>>>>> Regards
>>>>>
>>>>> Sumit Sahrawat
>>>>>
>>>>> _______________________________________________
>>>>> Haskell-Cafe mailing list
>>>>> Haskell-Cafe at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>>>>>
>>>>>
>>>
>>>
>>> --
>>> Regards
>>>
>>> Sumit Sahrawat
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>
>
>
> --
> Regards
>
> Sumit Sahrawat
>



-- 
Regards

Sumit Sahrawat
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From simon at vonos.net  Sun Mar 15 16:17:17 2015
From: simon at vonos.net (Simon Kitching)
Date: Sun, 15 Mar 2015 17:17:17 +0100
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
	languages?
Message-ID: <5505B08D.3030305@vonos.net>

[Note: learning haskell, not familiar with other functional languages]

I have a question about handling impure operations in functional or 
semi-functional languages other than Haskell (eg OCaml, F#, Scala, ..).

Haskell uses the IO monad to (a) cleanly separate pure and impure code, 
and (b) prevent function-calls from being reordered when they have 
side-effects that must happen in the right order. How do other languages 
handle this?

AIUI, whether a Haskell function is 'pure' or not can be seen from its 
method signature : if it returns IO then it is impure. All standard 
library functions that have side-effects (read/write/etc) return IO, and 
any function that uses an IO value must return type IO. Do other 
functional languages (1) use a similar approach, or (2) not provide a 
way to be certain that a function is 'pure' (side-effect-free)?

And AIUI the Haskell compiler/runtime can postpone evaluation of any 
function (laziness), or reorder function calls whever it thinks this 
good. However using the monad design pattern (deliberately) prevents 
this (each operation passes its value as a parameter to the subsequent 
operation, thus forcing an order of evaluation). Do other languages (a) 
not support compiler/runtime reordering of functions, or (b) have some 
other way of preventing functions with side-effects from being reordered?

Thanks..
Simon

From marcin.jan.mrotek at gmail.com  Sun Mar 15 16:55:12 2015
From: marcin.jan.mrotek at gmail.com (Marcin Mrotek)
Date: Sun, 15 Mar 2015 17:55:12 +0100
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
	languages?
In-Reply-To: <5505B08D.3030305@vonos.net>
References: <5505B08D.3030305@vonos.net>
Message-ID: <CAJcfPzmwUquH23A6TMXKF1W-xh20Z5o3GvR19ms6pvg54mmzhw@mail.gmail.com>

Hello,

F* uses a somewhat similar approach: https://fstar-lang.org/tutorial/
(section 2, Types and Effects)

Best regards,
Marcin Mrotek

From sumit.sahrawat.apm13 at iitbhu.ac.in  Sun Mar 15 17:15:03 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Sun, 15 Mar 2015 22:45:03 +0530
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
	languages?
In-Reply-To: <CAJcfPzmwUquH23A6TMXKF1W-xh20Z5o3GvR19ms6pvg54mmzhw@mail.gmail.com>
References: <5505B08D.3030305@vonos.net>
 <CAJcfPzmwUquH23A6TMXKF1W-xh20Z5o3GvR19ms6pvg54mmzhw@mail.gmail.com>
Message-ID: <CAJbEW8O6d54hmO=4JiNW4oJA95K6Mt2p5afscvchiCX+dYnbiA@mail.gmail.com>

Hello Simon,

If you changed your perspective, you would realize that all functions in
haskell are pure.
A function is pure if it returns the same output if given the same input.
Every monadic function (including functions returning IO) is also pure.
For example,

    putStrLn :: String -> IO ()
    -- A function that takes a string, and returns an impure computation
    -- which, when executed will print the given String.

For any string, putStrLn applied to that same string always describes the
same impure computation, thus the function is actually pure.
I am not familiar with any other functional language, but there are not
many purely functional ones out there [1].
I guess the impure ones get around this issue by giving in to impurity, but
I'm not sure.

I'll be interested in hearing more about the other languages too.

[1] :
http://en.wikipedia.org/wiki/List_of_programming_languages_by_type#Pure

On 15 March 2015 at 22:25, Marcin Mrotek <marcin.jan.mrotek at gmail.com>
wrote:

> Hello,
>
> F* uses a somewhat similar approach: https://fstar-lang.org/tutorial/
> (section 2, Types and Effects)
>
> Best regards,
> Marcin Mrotek
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>



-- 
Regards

Sumit Sahrawat
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From amindfv at gmail.com  Sun Mar 15 17:49:04 2015
From: amindfv at gmail.com (amindfv at gmail.com)
Date: Sun, 15 Mar 2015 13:49:04 -0400
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
	languages?
In-Reply-To: <CAJbEW8O6d54hmO=4JiNW4oJA95K6Mt2p5afscvchiCX+dYnbiA@mail.gmail.com>
References: <5505B08D.3030305@vonos.net>
 <CAJcfPzmwUquH23A6TMXKF1W-xh20Z5o3GvR19ms6pvg54mmzhw@mail.gmail.com>
 <CAJbEW8O6d54hmO=4JiNW4oJA95K6Mt2p5afscvchiCX+dYnbiA@mail.gmail.com>
Message-ID: <BC7EA4F8-C829-4F11-81EB-564FB3E38DFD@gmail.com>

From that perspective isn't every language pure? Haskell's still got "randomIO" and "print <=< readMVar"

Tom


El Mar 15, 2015, a las 13:15, "Sumit Sahrawat, Maths & Computing, IIT (BHU)" <sumit.sahrawat.apm13 at iitbhu.ac.in> escribi?:

> Hello Simon,
> 
> If you changed your perspective, you would realize that all functions in haskell are pure.
> A function is pure if it returns the same output if given the same input. Every monadic function (including functions returning IO) is also pure.
> For example,
> 
>     putStrLn :: String -> IO ()
>     -- A function that takes a string, and returns an impure computation
>     -- which, when executed will print the given String.
> 
> For any string, putStrLn applied to that same string always describes the same impure computation, thus the function is actually pure.
> I am not familiar with any other functional language, but there are not many purely functional ones out there [1].
> I guess the impure ones get around this issue by giving in to impurity, but I'm not sure.
> 
> I'll be interested in hearing more about the other languages too.
> 
> [1] : http://en.wikipedia.org/wiki/List_of_programming_languages_by_type#Pure
> 
> On 15 March 2015 at 22:25, Marcin Mrotek <marcin.jan.mrotek at gmail.com> wrote:
>> Hello,
>> 
>> F* uses a somewhat similar approach: https://fstar-lang.org/tutorial/
>> (section 2, Types and Effects)
>> 
>> Best regards,
>> Marcin Mrotek
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> 
> 
> 
> -- 
> Regards
> 
> Sumit Sahrawat
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Sun Mar 15 17:57:58 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Sun, 15 Mar 2015 23:27:58 +0530
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
	languages?
In-Reply-To: <BC7EA4F8-C829-4F11-81EB-564FB3E38DFD@gmail.com>
References: <5505B08D.3030305@vonos.net>
 <CAJcfPzmwUquH23A6TMXKF1W-xh20Z5o3GvR19ms6pvg54mmzhw@mail.gmail.com>
 <CAJbEW8O6d54hmO=4JiNW4oJA95K6Mt2p5afscvchiCX+dYnbiA@mail.gmail.com>
 <BC7EA4F8-C829-4F11-81EB-564FB3E38DFD@gmail.com>
Message-ID: <CAJbEW8N9bx1YogQJfaDc3o2yQ++ohN=qaNJxAn0NWdQ25YgRxA@mail.gmail.com>

Haskell is impure only when you use the unsafe* functions (atleast that's
how I understand purity).

My understanding is that a language is impure if it allows one to write
arbitrary IO within a function and still give it a proper (mathematical)
function type. In other words impurity arises only if you can unwrap the IO
monad (which is what the unsafe functions do).

The two examples you give above are pure under such a perspective, but I
might be wrong.

On 15 March 2015 at 23:19, <amindfv at gmail.com> wrote:

> From that perspective isn't every language pure? Haskell's still got
> "randomIO" and "print <=< readMVar"
>
> Tom
>
>
> El Mar 15, 2015, a las 13:15, "Sumit Sahrawat, Maths & Computing, IIT
> (BHU)" <sumit.sahrawat.apm13 at iitbhu.ac.in> escribi?:
>
> Hello Simon,
>
> If you changed your perspective, you would realize that all functions in
> haskell are pure.
> A function is pure if it returns the same output if given the same input.
> Every monadic function (including functions returning IO) is also pure.
> For example,
>
>     putStrLn :: String -> IO ()
>     -- A function that takes a string, and returns an impure computation
>     -- which, when executed will print the given String.
>
> For any string, putStrLn applied to that same string always describes the
> same impure computation, thus the function is actually pure.
> I am not familiar with any other functional language, but there are not
> many purely functional ones out there [1].
> I guess the impure ones get around this issue by giving in to impurity,
> but I'm not sure.
>
> I'll be interested in hearing more about the other languages too.
>
> [1] :
> http://en.wikipedia.org/wiki/List_of_programming_languages_by_type#Pure
>
> On 15 March 2015 at 22:25, Marcin Mrotek <marcin.jan.mrotek at gmail.com>
> wrote:
>
>> Hello,
>>
>> F* uses a somewhat similar approach: https://fstar-lang.org/tutorial/
>> (section 2, Types and Effects)
>>
>> Best regards,
>> Marcin Mrotek
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
>
>
> --
> Regards
>
> Sumit Sahrawat
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>


-- 
Regards

Sumit Sahrawat
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From tonymorris at gmail.com  Sun Mar 15 21:17:33 2015
From: tonymorris at gmail.com (Tony Morris)
Date: Mon, 16 Mar 2015 07:17:33 +1000
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
 languages?
In-Reply-To: <5505B08D.3030305@vonos.net>
References: <5505B08D.3030305@vonos.net>
Message-ID: <5505F6ED.5040806@gmail.com>

Scalaz implements IO and ST as a library for Scala.

http://eed3si9n.com/learning-scalaz/IO+Monad.html

On 16/03/15 02:17, Simon Kitching wrote:
> [Note: learning haskell, not familiar with other functional languages]
>
> I have a question about handling impure operations in functional or 
> semi-functional languages other than Haskell (eg OCaml, F#, Scala, ..).
>
> Haskell uses the IO monad to (a) cleanly separate pure and impure 
> code, and (b) prevent function-calls from being reordered when they 
> have side-effects that must happen in the right order. How do other 
> languages handle this?
>
> AIUI, whether a Haskell function is 'pure' or not can be seen from its 
> method signature : if it returns IO then it is impure. All standard 
> library functions that have side-effects (read/write/etc) return IO, 
> and any function that uses an IO value must return type IO. Do other 
> functional languages (1) use a similar approach, or (2) not provide a 
> way to be certain that a function is 'pure' (side-effect-free)?
>
> And AIUI the Haskell compiler/runtime can postpone evaluation of any 
> function (laziness), or reorder function calls whever it thinks this 
> good. However using the monad design pattern (deliberately) prevents 
> this (each operation passes its value as a parameter to the subsequent 
> operation, thus forcing an order of evaluation). Do other languages 
> (a) not support compiler/runtime reordering of functions, or (b) have 
> some other way of preventing functions with side-effects from being 
> reordered?
>
> Thanks..
> Simon
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners


From karl at karlv.net  Sun Mar 15 21:44:02 2015
From: karl at karlv.net (Karl Voelker)
Date: Sun, 15 Mar 2015 14:44:02 -0700
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
 languages?
In-Reply-To: <5505B08D.3030305@vonos.net>
References: <5505B08D.3030305@vonos.net>
Message-ID: <1426455842.3291431.240699409.6E305DDB@webmail.messagingengine.com>

On Sun, Mar 15, 2015, at 09:17 AM, Simon Kitching wrote:
> And AIUI the Haskell compiler/runtime can postpone evaluation of any 
> function (laziness), or reorder function calls whever it thinks this 
> good.

It's not that it "can" but that it must. Haskell's evaluation strategy
is a part of the language specification.

Consider this expression:

let f = 1 : f in take 5 f

You can paste this into ghci and feel confident that it's going to
terminate. You're not at the whim of the runtime.

I think you may find these articles interesting:

https://wiki.haskell.org/Lazy_evaluation
https://wiki.haskell.org/Non-strict_semantics

-Karl

From apfelmus at quantentunnel.de  Mon Mar 16 14:04:08 2015
From: apfelmus at quantentunnel.de (Heinrich Apfelmus)
Date: Mon, 16 Mar 2015 15:04:08 +0100
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
	languages?
In-Reply-To: <CAJbEW8N9bx1YogQJfaDc3o2yQ++ohN=qaNJxAn0NWdQ25YgRxA@mail.gmail.com>
References: <5505B08D.3030305@vonos.net>
 <CAJcfPzmwUquH23A6TMXKF1W-xh20Z5o3GvR19ms6pvg54mmzhw@mail.gmail.com>
 <CAJbEW8O6d54hmO=4JiNW4oJA95K6Mt2p5afscvchiCX+dYnbiA@mail.gmail.com>
 <BC7EA4F8-C829-4F11-81EB-564FB3E38DFD@gmail.com>
 <CAJbEW8N9bx1YogQJfaDc3o2yQ++ohN=qaNJxAn0NWdQ25YgRxA@mail.gmail.com>
Message-ID: <me6nso$9cf$1@ger.gmane.org>

Sumit Sahrawat, Maths & Computing, IIT (BHU) wrote:
> On 15 March 2015 at 23:19, <amindfv at gmail.com> wrote:
> 
>> From that perspective isn't every language pure? Haskell's still got
>> "randomIO" and "print <=< readMVar"
> 
> Haskell is impure only when you use the unsafe* functions (atleast that's
> how I understand purity).
> 
> My understanding is that a language is impure if it allows one to write
> arbitrary IO within a function and still give it a proper (mathematical)
> function type. In other words impurity arises only if you can unwrap the IO
> monad (which is what the unsafe functions do).
> 
> The two examples you give above are pure under such a perspective, but I
> might be wrong.

You're right, indeed. A language is pure if supplying a value `x` of 
type `A` to the a function

     A -> B

will always returns the same result, no matter how often or in which 
order this function is called. This is true for both

     randomIO             :: Random a => IO a
     (readMVar >=> print) :: Show a   => MVar a -> IO ()

because they return an IO action. This action will always be the same 
given the same arguments.

The language would be impure if these functions had the types

     randomIO             :: Random a => a
     (readMVar >=> print) :: Show a   => MVar a -> ()


Best regards,
Heinrich Apfelmus

--
http://apfelmus.nfshost.com


From apfelmus at quantentunnel.de  Mon Mar 16 14:11:15 2015
From: apfelmus at quantentunnel.de (Heinrich Apfelmus)
Date: Mon, 16 Mar 2015 15:11:15 +0100
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
	languages?
In-Reply-To: <1426455842.3291431.240699409.6E305DDB@webmail.messagingengine.com>
References: <5505B08D.3030305@vonos.net>
 <1426455842.3291431.240699409.6E305DDB@webmail.messagingengine.com>
Message-ID: <me6oa3$gvm$1@ger.gmane.org>

Karl Voelker wrote:
> On Sun, Mar 15, 2015, at 09:17 AM, Simon Kitching wrote:
>> And AIUI the Haskell compiler/runtime can postpone evaluation of any 
>> function (laziness), or reorder function calls whever it thinks this 
>> good.
> 
> It's not that it "can" but that it must. Haskell's evaluation strategy
> is a part of the language specification.
> 
> Consider this expression:
> 
> let f = 1 : f in take 5 f
> 
> You can paste this into ghci and feel confident that it's going to
> terminate. You're not at the whim of the runtime.
> 
> I think you may find these articles interesting:
> 
> https://wiki.haskell.org/Lazy_evaluation
> https://wiki.haskell.org/Non-strict_semantics

Strictly speaking, the Haskell language standard only specifies that 
Haskell has non-strict semantics, not that it needs to be evaluated 
using lazy evaluation.

Of course, the two are related. I have tried to expand on the details here:

   https://hackhands.com/non-strict-semantics-haskell


Best regards,
Heinrich Apfelmus

--
http://apfelmus.nfshost.com


From mrz.vtl at gmail.com  Mon Mar 16 14:21:33 2015
From: mrz.vtl at gmail.com (Maurizio Vitale)
Date: Mon, 16 Mar 2015 10:21:33 -0400
Subject: [Haskell-beginners] testing IO code
Message-ID: <CAAeLbQJMB0VkcESZoawYkT0pZrEyOCL1WCwHXt0qppQjWcuBdw@mail.gmail.com>

suppose I have a restricted IO monad, RIO that only exposes readFile.
and then I have a monad SIO that will eventually provide a virtual file
system from a map path->content, also with a readFile function returning
SIO(String).

What is the way to write a function parseFile that can operate in both
monads so that I can use SIO for testing? should I define a third monad
CompileMonad that has instances for both RIO and SIO and then having
parseFile :: CompileMonad ast?

Thanks,

  Maurizio
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Mon Mar 16 14:24:41 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Mon, 16 Mar 2015 19:54:41 +0530
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
	languages?
In-Reply-To: <me6oa3$gvm$1@ger.gmane.org>
References: <5505B08D.3030305@vonos.net>
 <1426455842.3291431.240699409.6E305DDB@webmail.messagingengine.com>
 <me6oa3$gvm$1@ger.gmane.org>
Message-ID: <CAJbEW8MFCdJuXArwei9oY5a6ZxAgVaKduy+tV=9XdczA6_2f1g@mail.gmail.com>

Hi Heinrich,

Thanks for confirming my concepts. It took me a long time pondering over
what it meant to be purely functional.
Great articles btw (+1).

On 16 March 2015 at 19:41, Heinrich Apfelmus <apfelmus at quantentunnel.de>
wrote:

> Karl Voelker wrote:
>
>> On Sun, Mar 15, 2015, at 09:17 AM, Simon Kitching wrote:
>>
>>> And AIUI the Haskell compiler/runtime can postpone evaluation of any
>>> function (laziness), or reorder function calls whever it thinks this good.
>>>
>>
>> It's not that it "can" but that it must. Haskell's evaluation strategy
>> is a part of the language specification.
>>
>> Consider this expression:
>>
>> let f = 1 : f in take 5 f
>>
>> You can paste this into ghci and feel confident that it's going to
>> terminate. You're not at the whim of the runtime.
>>
>> I think you may find these articles interesting:
>>
>> https://wiki.haskell.org/Lazy_evaluation
>> https://wiki.haskell.org/Non-strict_semantics
>>
>
> Strictly speaking, the Haskell language standard only specifies that
> Haskell has non-strict semantics, not that it needs to be evaluated using
> lazy evaluation.
>
> Of course, the two are related. I have tried to expand on the details here:
>
>   https://hackhands.com/non-strict-semantics-haskell
>
>
> Best regards,
> Heinrich Apfelmus
>
> --
> http://apfelmus.nfshost.com
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>



-- 
Regards

Sumit Sahrawat
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From raabe at froglogic.com  Mon Mar 16 14:34:41 2015
From: raabe at froglogic.com (Frerich Raabe)
Date: Mon, 16 Mar 2015 15:34:41 +0100
Subject: [Haskell-beginners]
 =?utf-8?q?Equivalent_of_IO_Monad_in_other_fun?=
 =?utf-8?q?ctional_languages=3F?=
In-Reply-To: <CAJbEW8N9bx1YogQJfaDc3o2yQ++ohN=qaNJxAn0NWdQ25YgRxA@mail.gmail.com>
References: <5505B08D.3030305@vonos.net>
 <CAJcfPzmwUquH23A6TMXKF1W-xh20Z5o3GvR19ms6pvg54mmzhw@mail.gmail.com>
 <CAJbEW8O6d54hmO=4JiNW4oJA95K6Mt2p5afscvchiCX+dYnbiA@mail.gmail.com>
 <BC7EA4F8-C829-4F11-81EB-564FB3E38DFD@gmail.com>
 <CAJbEW8N9bx1YogQJfaDc3o2yQ++ohN=qaNJxAn0NWdQ25YgRxA@mail.gmail.com>
Message-ID: <791472dcf2df420d2c3dcc8ab1a9ecb0@roundcube.froglogic.com>

Hi,

I also used to share this "Every function in Haskell is pure, as long as it 
does not use any of the unsafe* functions" view but by now I'm wondering 
whether that's really a useful definition. It may be true that

   putStrLn :: String -> IO ()

is pure in the sense that it only yields a 'recipe' for printing the given 
string instead of actually printing it. However, I think it's important to 
realize that putStrLn *could* actually yield a poisoned recipe, something 
which not *only* prints a string but which also plays a sound at 4AM. And the 
types won't let you know whether this is the case.

Hence, even though putStrLn may be pure in the classical 'same input yields 
same output' sense, I wondr whether it's more useful to consider putStrLn to 
be impure on the grounds that a value of type 'IO a' means "anything can 
happen, and whatever happens doesn't necessarily only depend on the 
arguments".

On 2015-03-15 18:57, Sumit Sahrawat, Maths & Computing, IIT (BHU) wrote:
> Haskell is impure only when you use the unsafe* functions (atleast that's 
> how I understand purity).
> 
> My understanding is that a language is impure if it allows one to write 
> arbitrary IO within a function and still give it a proper
> (mathematical) function type. In other words impurity arises only if you can 
> unwrap the IO monad (which is what the unsafe functions do).
> 
> The two examples you give above are pure under such a perspective, but I 
> might be wrong.
> 
> On 15 March 2015 at 23:19, <amindfv at gmail.com> wrote:
> 
>> From that perspective isn't every language pure? Haskell's still got 
>> "randomIO" and "print <=< readMVar"
>> 
>> Tom
>> 
>> El Mar 15, 2015, a las 13:15, "Sumit Sahrawat, Maths & Computing, IIT 
>> (BHU)" <sumit.sahrawat.apm13 at iitbhu.ac.in> escribi?:
>> 
>> Hello Simon,
>> 
>> If you changed your perspective, you would realize that all functions in 
>> haskell are pure.
>> A function is pure if it returns the same output if given the same input. 
>> Every monadic function (including functions returning IO) is also
>> pure.
>> For example,
>> 
>> putStrLn :: String -> IO ()
>> -- A function that takes a string, and returns an impure computation
>> -- which, when executed will print the given String.
>> 
>> For any string, putStrLn applied to that same string always describes the 
>> same impure computation, thus the function is actually pure.
>> I am not familiar with any other functional language, but there are not 
>> many purely functional ones out there [1].
>> I guess the impure ones get around this issue by giving in to impurity, but 
>> I'm not sure.
>> 
>> I'll be interested in hearing more about the other languages too.
>> 
>> [1] : 
>> http://en.wikipedia.org/wiki/List_of_programming_languages_by_type#Pure [1]
>> 
>> On 15 March 2015 at 22:25, Marcin Mrotek <marcin.jan.mrotek at gmail.com> 
>> wrote:
>> Hello,
>> 
>> F* uses a somewhat similar approach: https://fstar-lang.org/tutorial/ [2]
>> (section 2, Types and Effects)
>> 
>> Best regards,
>> Marcin Mrotek
>> 
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners [3]
>> 
>> --
>> 
>> Regards
>> 
>> Sumit Sahrawat
> 
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners [3]
> 
> --
> 
> Regards
> 
> Sumit Sahrawat
> 
> Links:
> ------
> [1] http://en.wikipedia.org/wiki/List_of_programming_languages_by_type#Pure
> [2] https://fstar-lang.org/tutorial/
> [3] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> 
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

-- 
Frerich Raabe - raabe at froglogic.com
www.froglogic.com - Multi-Platform GUI Testing

From simon at vonos.net  Mon Mar 16 14:35:33 2015
From: simon at vonos.net (Simon Kitching)
Date: Mon, 16 Mar 2015 15:35:33 +0100
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
 languages?
In-Reply-To: <me6nso$9cf$1@ger.gmane.org>
References: <5505B08D.3030305@vonos.net>
 <CAJcfPzmwUquH23A6TMXKF1W-xh20Z5o3GvR19ms6pvg54mmzhw@mail.gmail.com>
 <CAJbEW8O6d54hmO=4JiNW4oJA95K6Mt2p5afscvchiCX+dYnbiA@mail.gmail.com>
 <BC7EA4F8-C829-4F11-81EB-564FB3E38DFD@gmail.com>
 <CAJbEW8N9bx1YogQJfaDc3o2yQ++ohN=qaNJxAn0NWdQ25YgRxA@mail.gmail.com>
 <me6nso$9cf$1@ger.gmane.org>
Message-ID: <5506EA35.8080303@vonos.net>

On 03/16/2015 03:04 PM, Heinrich Apfelmus wrote:
> Sumit Sahrawat, Maths & Computing, IIT (BHU) wrote:
>> On 15 March 2015 at 23:19, <amindfv at gmail.com> wrote:
>>
>>> From that perspective isn't every language pure? Haskell's still got
>>> "randomIO" and "print <=< readMVar"
>>
>> Haskell is impure only when you use the unsafe* functions (atleast 
>> that's
>> how I understand purity).
>>
>> My understanding is that a language is impure if it allows one to write
>> arbitrary IO within a function and still give it a proper (mathematical)
>> function type. In other words impurity arises only if you can unwrap 
>> the IO
>> monad (which is what the unsafe functions do).
>>
>> The two examples you give above are pure under such a perspective, but I
>> might be wrong.
>
> You're right, indeed. A language is pure if supplying a value `x` of 
> type `A` to the a function
>
>     A -> B
>
> will always returns the same result, no matter how often or in which 
> order this function is called. This is true for both
>
>     randomIO             :: Random a => IO a
>     (readMVar >=> print) :: Show a   => MVar a -> IO ()
>
> because they return an IO action. This action will always be the same 
> given the same arguments.
>
> The language would be impure if these functions had the types
>
>     randomIO             :: Random a => a
>     (readMVar >=> print) :: Show a   => MVar a -> ()

Maybe I should have phrased my original question differently. Let me try 
again..

In an imperative language, you can write something like the following 
pseudocode:

    String transform(String name) {
        print "enter greeting:"
        greeting = read-line-from-stdin
        return greeting + " " + name
    }

 From the "outside", there is no way to tell whether the function 
"transform" performs IO, mutates global state, mutates
its parameter, or anything else. And this is just _normal_ for 
imperative-programming; if you want to write a unit-test for
a method, you need to somehow _know_ what external state it might depend 
on/affect. And compilers cannot reorder
or omit functions because any function might have side-effects.

In Haskell, the function signature clearly indicates if this kind of 
thing is happening - any function like the above will have
a return type of IO, thus clearly separating code with side-effects from 
code without side-effects.

I was wondering what other functional or partly-functional languages 
(OCaml, F#, Scheme etc) do - do they also give the
user of a function a way to tell whether the function has IO 
side-effects (reading/writing) or does the user just need to
"consult the documentation" or "read the source code"?

Thanks,
Simon

From simon at vonos.net  Mon Mar 16 14:45:20 2015
From: simon at vonos.net (Simon Kitching)
Date: Mon, 16 Mar 2015 15:45:20 +0100
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
 languages?
In-Reply-To: <me6nso$9cf$1@ger.gmane.org>
References: <5505B08D.3030305@vonos.net>
 <CAJcfPzmwUquH23A6TMXKF1W-xh20Z5o3GvR19ms6pvg54mmzhw@mail.gmail.com>
 <CAJbEW8O6d54hmO=4JiNW4oJA95K6Mt2p5afscvchiCX+dYnbiA@mail.gmail.com>
 <BC7EA4F8-C829-4F11-81EB-564FB3E38DFD@gmail.com>
 <CAJbEW8N9bx1YogQJfaDc3o2yQ++ohN=qaNJxAn0NWdQ25YgRxA@mail.gmail.com>
 <me6nso$9cf$1@ger.gmane.org>
Message-ID: <5506EC80.4080602@vonos.net>

On 03/16/2015 03:04 PM, Heinrich Apfelmus wrote:
> Sumit Sahrawat, Maths & Computing, IIT (BHU) wrote:
>> On 15 March 2015 at 23:19, <amindfv at gmail.com> wrote:
>>
>>> From that perspective isn't every language pure? Haskell's still got
>>> "randomIO" and "print <=< readMVar"
>>
>> Haskell is impure only when you use the unsafe* functions (atleast 
>> that's
>> how I understand purity).
>>
>> My understanding is that a language is impure if it allows one to write
>> arbitrary IO within a function and still give it a proper (mathematical)
>> function type. In other words impurity arises only if you can unwrap 
>> the IO
>> monad (which is what the unsafe functions do).
>>
>> The two examples you give above are pure under such a perspective, but I
>> might be wrong.
>
> You're right, indeed. A language is pure if supplying a value `x` of 
> type `A` to the a function
>
>     A -> B
>
> will always returns the same result, no matter how often or in which 
> order this function is called. This is true for both
>
>     randomIO             :: Random a => IO a
>     (readMVar >=> print) :: Show a   => MVar a -> IO ()
>
> because they return an IO action. This action will always be the same 
> given the same arguments.
>
> The language would be impure if these functions had the types
>
>     randomIO             :: Random a => a
>     (readMVar >=> print) :: Show a   => MVar a -> ()

Doesn't "pure" correspond to "can write a unit test for"? When a 
function's return value only ever depends on its inputs, then I can
write a set of test-cases for various inputs, and assert that the 
return-value has the expected content.

A function that reads from a file, stdin, etc. cannot be tested in the 
same way; I cannot _assert_ that the returned value has specific content.

  Sumit stated earlier that IO is "a pure program that can be executed", 
which seems similar to your description above (which I admit I
don't yet 100% understand). How can I "assert" anything about this 
"program", or make a Haskell-based application 'safer' in any way by
leveraging this kind of 'purity'?

Thanks,
Simon

From sumit.sahrawat.apm13 at iitbhu.ac.in  Mon Mar 16 14:40:35 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Mon, 16 Mar 2015 20:10:35 +0530
Subject: [Haskell-beginners] testing IO code
In-Reply-To: <CAAeLbQJMB0VkcESZoawYkT0pZrEyOCL1WCwHXt0qppQjWcuBdw@mail.gmail.com>
References: <CAAeLbQJMB0VkcESZoawYkT0pZrEyOCL1WCwHXt0qppQjWcuBdw@mail.gmail.com>
Message-ID: <CAJbEW8MjJ1mM5emEsWjgfKxs0xWUU98f_xRGmuf9erjaVDsDHg@mail.gmail.com>

On 16 March 2015 at 19:51, Maurizio Vitale <mrz.vtl at gmail.com> wrote:

> suppose I have a restricted IO monad, RIO that only exposes readFile.
> and then I have a monad SIO that will eventually provide a virtual file
> system from a map path->content, also with a readFile function returning
> SIO(String).
>
> What is the way to write a function parseFile that can operate in both
> monads so that I can use SIO for testing? should I define a third monad
> CompileMonad that has instances for both RIO and SIO and then having
> parseFile :: CompileMonad ast?
>

You might be able to do something like,

    class MonadIO m => ProvidesReadFile m where
        readFile :: FilePath -> m String

    instance ProvidesReadFile RIO where
        readFile = readFileRIO    -- the RIO specific readFile

    instance ProvidesReadFile SIO where
        readFile = readFileSIO    -- the SIO specific readFile

    parseFile :: ProvidesReadFile m => FilePath -> m ast
    parseFile = do
        f <- readFile
        let ast = parse f    -- the pure parser
        return ast           -- works for both monads


> Thanks,
>
>   Maurizio
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
This is more suitable for the haskell-cafe. I am posting it there so that
more people might comment on it.
HTH.

-- 
Regards

Sumit Sahrawat
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Mon Mar 16 14:58:27 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Mon, 16 Mar 2015 20:28:27 +0530
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
	languages?
In-Reply-To: <5506EC80.4080602@vonos.net>
References: <5505B08D.3030305@vonos.net>
 <CAJcfPzmwUquH23A6TMXKF1W-xh20Z5o3GvR19ms6pvg54mmzhw@mail.gmail.com>
 <CAJbEW8O6d54hmO=4JiNW4oJA95K6Mt2p5afscvchiCX+dYnbiA@mail.gmail.com>
 <BC7EA4F8-C829-4F11-81EB-564FB3E38DFD@gmail.com>
 <CAJbEW8N9bx1YogQJfaDc3o2yQ++ohN=qaNJxAn0NWdQ25YgRxA@mail.gmail.com>
 <me6nso$9cf$1@ger.gmane.org> <5506EC80.4080602@vonos.net>
Message-ID: <CAJbEW8MoCG1YR4f7YwJFM4LjPgRSDuGQG_PrBAcJFuEOBchaUQ@mail.gmail.com>

Sorry for having strayed towards a different path. I'll try to answer only
your question this time :)

An example from [1], for SML (bottom of the page) shows a function, with
the following type signature in haskell

    copyTextFile :: String -> String

One cannot write such a function in haskell that does the same thing
because the example uses IO within a function with a pure looking type.

I'm sure people experienced with other functional languages will have good
examples, so I'll leave it to them.

To answer your second question, I'm still not sure :).
I was just relaying how I understood purity, so that somebody might also
correct me if I'm wrong.

Summarizing the discussion above, a function is pure if it gives the same
output for the same input. Now a function returning 'IO a' always returns
the same IO instructions (these instructions might be impure), but the
function (by the virtue of always returning the same output for the same
inputs) is ultimately pure. Frerich has also raised an important point that
an IO computation might actually depend on some random file, which breaks
this pattern.

[1]: http://www.cs.cornell.edu/courses/cs312/2006fa/recitations/rec09.html

On 16 March 2015 at 20:15, Simon Kitching <simon at vonos.net> wrote:

> On 03/16/2015 03:04 PM, Heinrich Apfelmus wrote:
>
>> Sumit Sahrawat, Maths & Computing, IIT (BHU) wrote:
>>
>>> On 15 March 2015 at 23:19, <amindfv at gmail.com> wrote:
>>>
>>>  From that perspective isn't every language pure? Haskell's still got
>>>> "randomIO" and "print <=< readMVar"
>>>>
>>>
>>> Haskell is impure only when you use the unsafe* functions (atleast that's
>>> how I understand purity).
>>>
>>> My understanding is that a language is impure if it allows one to write
>>> arbitrary IO within a function and still give it a proper (mathematical)
>>> function type. In other words impurity arises only if you can unwrap the
>>> IO
>>> monad (which is what the unsafe functions do).
>>>
>>> The two examples you give above are pure under such a perspective, but I
>>> might be wrong.
>>>
>>
>> You're right, indeed. A language is pure if supplying a value `x` of type
>> `A` to the a function
>>
>>     A -> B
>>
>> will always returns the same result, no matter how often or in which
>> order this function is called. This is true for both
>>
>>     randomIO             :: Random a => IO a
>>     (readMVar >=> print) :: Show a   => MVar a -> IO ()
>>
>> because they return an IO action. This action will always be the same
>> given the same arguments.
>>
>> The language would be impure if these functions had the types
>>
>>     randomIO             :: Random a => a
>>     (readMVar >=> print) :: Show a   => MVar a -> ()
>>
>
> Doesn't "pure" correspond to "can write a unit test for"? When a
> function's return value only ever depends on its inputs, then I can
> write a set of test-cases for various inputs, and assert that the
> return-value has the expected content.
>
> A function that reads from a file, stdin, etc. cannot be tested in the
> same way; I cannot _assert_ that the returned value has specific content.
>
>  Sumit stated earlier that IO is "a pure program that can be executed",
> which seems similar to your description above (which I admit I
> don't yet 100% understand). How can I "assert" anything about this
> "program", or make a Haskell-based application 'safer' in any way by
> leveraging this kind of 'purity'?
>
> Thanks,
> Simon
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>



-- 
Regards

Sumit Sahrawat
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Mon Mar 16 15:01:06 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Mon, 16 Mar 2015 20:31:06 +0530
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
	languages?
In-Reply-To: <CAJbEW8MoCG1YR4f7YwJFM4LjPgRSDuGQG_PrBAcJFuEOBchaUQ@mail.gmail.com>
References: <5505B08D.3030305@vonos.net>
 <CAJcfPzmwUquH23A6TMXKF1W-xh20Z5o3GvR19ms6pvg54mmzhw@mail.gmail.com>
 <CAJbEW8O6d54hmO=4JiNW4oJA95K6Mt2p5afscvchiCX+dYnbiA@mail.gmail.com>
 <BC7EA4F8-C829-4F11-81EB-564FB3E38DFD@gmail.com>
 <CAJbEW8N9bx1YogQJfaDc3o2yQ++ohN=qaNJxAn0NWdQ25YgRxA@mail.gmail.com>
 <me6nso$9cf$1@ger.gmane.org> <5506EC80.4080602@vonos.net>
 <CAJbEW8MoCG1YR4f7YwJFM4LjPgRSDuGQG_PrBAcJFuEOBchaUQ@mail.gmail.com>
Message-ID: <CAJbEW8Ma4WEL-=hCve5dWB8e-QYV8rsXtVZQ2E5dnz_ogbt5ng@mail.gmail.com>

Frerich, the function would still be pure, even if it returns a poisoned
recipe as you say. Thus, a little thought leads me to believe that the
pattern would still not break if no unsafe* functions are used.


On 16 March 2015 at 20:28, Sumit Sahrawat, Maths & Computing, IIT (BHU) <
sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

> Sorry for having strayed towards a different path. I'll try to answer only
> your question this time :)
>
> An example from [1], for SML (bottom of the page) shows a function, with
> the following type signature in haskell
>
>     copyTextFile :: String -> String
>
> One cannot write such a function in haskell that does the same thing
> because the example uses IO within a function with a pure looking type.
>
> I'm sure people experienced with other functional languages will have good
> examples, so I'll leave it to them.
>
> To answer your second question, I'm still not sure :).
> I was just relaying how I understood purity, so that somebody might also
> correct me if I'm wrong.
>
> Summarizing the discussion above, a function is pure if it gives the same
> output for the same input. Now a function returning 'IO a' always returns
> the same IO instructions (these instructions might be impure), but the
> function (by the virtue of always returning the same output for the same
> inputs) is ultimately pure. Frerich has also raised an important point that
> an IO computation might actually depend on some random file, which breaks
> this pattern.
>
> [1]: http://www.cs.cornell.edu/courses/cs312/2006fa/recitations/rec09.html
>
> On 16 March 2015 at 20:15, Simon Kitching <simon at vonos.net> wrote:
>
>> On 03/16/2015 03:04 PM, Heinrich Apfelmus wrote:
>>
>>> Sumit Sahrawat, Maths & Computing, IIT (BHU) wrote:
>>>
>>>> On 15 March 2015 at 23:19, <amindfv at gmail.com> wrote:
>>>>
>>>>  From that perspective isn't every language pure? Haskell's still got
>>>>> "randomIO" and "print <=< readMVar"
>>>>>
>>>>
>>>> Haskell is impure only when you use the unsafe* functions (atleast
>>>> that's
>>>> how I understand purity).
>>>>
>>>> My understanding is that a language is impure if it allows one to write
>>>> arbitrary IO within a function and still give it a proper (mathematical)
>>>> function type. In other words impurity arises only if you can unwrap
>>>> the IO
>>>> monad (which is what the unsafe functions do).
>>>>
>>>> The two examples you give above are pure under such a perspective, but I
>>>> might be wrong.
>>>>
>>>
>>> You're right, indeed. A language is pure if supplying a value `x` of
>>> type `A` to the a function
>>>
>>>     A -> B
>>>
>>> will always returns the same result, no matter how often or in which
>>> order this function is called. This is true for both
>>>
>>>     randomIO             :: Random a => IO a
>>>     (readMVar >=> print) :: Show a   => MVar a -> IO ()
>>>
>>> because they return an IO action. This action will always be the same
>>> given the same arguments.
>>>
>>> The language would be impure if these functions had the types
>>>
>>>     randomIO             :: Random a => a
>>>     (readMVar >=> print) :: Show a   => MVar a -> ()
>>>
>>
>> Doesn't "pure" correspond to "can write a unit test for"? When a
>> function's return value only ever depends on its inputs, then I can
>> write a set of test-cases for various inputs, and assert that the
>> return-value has the expected content.
>>
>> A function that reads from a file, stdin, etc. cannot be tested in the
>> same way; I cannot _assert_ that the returned value has specific content.
>>
>>  Sumit stated earlier that IO is "a pure program that can be executed",
>> which seems similar to your description above (which I admit I
>> don't yet 100% understand). How can I "assert" anything about this
>> "program", or make a Haskell-based application 'safer' in any way by
>> leveraging this kind of 'purity'?
>>
>> Thanks,
>> Simon
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
>
>
> --
> Regards
>
> Sumit Sahrawat
>



-- 
Regards

Sumit Sahrawat
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From mrz.vtl at gmail.com  Mon Mar 16 16:06:01 2015
From: mrz.vtl at gmail.com (Maurizio Vitale)
Date: Mon, 16 Mar 2015 09:06:01 -0700
Subject: [Haskell-beginners] testing IO code
In-Reply-To: <CAJbEW8MjJ1mM5emEsWjgfKxs0xWUU98f_xRGmuf9erjaVDsDHg@mail.gmail.com>
References: <CAAeLbQJMB0VkcESZoawYkT0pZrEyOCL1WCwHXt0qppQjWcuBdw@mail.gmail.com>
 <CAJbEW8MjJ1mM5emEsWjgfKxs0xWUU98f_xRGmuf9erjaVDsDHg@mail.gmail.com>
Message-ID: <CAAeLbQL5v5B5ke+UZTptgUW+bm35ECd4fPNRDNNO6kkJhwAbXg@mail.gmail.com>

Thanks!
This is what I had in mind, except that I'm new too Haskell so I tried
English instead of code for expressing it :-)
I'll also check the IOSpec package Paul suggested,

On Mon, Mar 16, 2015 at 7:40 AM, Sumit Sahrawat, Maths & Computing, IIT
(BHU) <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

> On 16 March 2015 at 19:51, Maurizio Vitale <mrz.vtl at gmail.com> wrote:
>
>> suppose I have a restricted IO monad, RIO that only exposes readFile.
>> and then I have a monad SIO that will eventually provide a virtual file
>> system from a map path->content, also with a readFile function returning
>> SIO(String).
>>
>> What is the way to write a function parseFile that can operate in both
>> monads so that I can use SIO for testing? should I define a third monad
>> CompileMonad that has instances for both RIO and SIO and then having
>> parseFile :: CompileMonad ast?
>>
>
> You might be able to do something like,
>
>     class MonadIO m => ProvidesReadFile m where
>         readFile :: FilePath -> m String
>
>     instance ProvidesReadFile RIO where
>         readFile = readFileRIO    -- the RIO specific readFile
>
>     instance ProvidesReadFile SIO where
>         readFile = readFileSIO    -- the SIO specific readFile
>
>     parseFile :: ProvidesReadFile m => FilePath -> m ast
>     parseFile = do
>         f <- readFile
>         let ast = parse f    -- the pure parser
>         return ast           -- works for both monads
>
>
>> Thanks,
>>
>>   Maurizio
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
> This is more suitable for the haskell-cafe. I am posting it there so that
> more people might comment on it.
> HTH.
>
> --
> Regards
>
> Sumit Sahrawat
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From kc1956 at gmail.com  Mon Mar 16 20:53:13 2015
From: kc1956 at gmail.com (KC)
Date: Mon, 16 Mar 2015 13:53:13 -0700
Subject: [Haskell-beginners] There has been discussion on matrix operations
 and cache thrashing;
 does management think using Haskell would lead to cash thrashing?
Message-ID: <CAMLKXykmywx24MX6-bWkSs04piOYLKidobKdkgmVuALaFG+eUg@mail.gmail.com>

There has been discussion on matrix operations and cache thrashing; does
management think using Haskell would lead to cash thrashing?

What could change management's mind?

--
--

Sent from an expensive device which will be obsolete in a few months! :D

Casey
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From hjgtuyl at chello.nl  Mon Mar 16 22:55:56 2015
From: hjgtuyl at chello.nl (Henk-Jan van Tuyl)
Date: Mon, 16 Mar 2015 23:55:56 +0100
Subject: [Haskell-beginners] Saving intermediate calculations
In-Reply-To: <110089426.4891942.1426174459028.JavaMail.yahoo@mail.yahoo.com>
References: <op.xva3u0gypz0j5l@alquantor>
 <110089426.4891942.1426174459028.JavaMail.yahoo@mail.yahoo.com>
Message-ID: <op.xvl4nmo3pz0j5l@alquantor>

On Thu, 12 Mar 2015 16:34:19 +0100, Abhinav Kalawatia
<kalawatia_abhinav at yahoo.com> wrote:

> Hi Henk,
> I hope you are well :)
> Thanks for your help with the solution for saving the intermediate  
> calculations. Henk, could you please help me understand the following  
> code snippet?
> ewma1 a (x:xs) = reverse $ foldl' f [x] xs
>     where f m@(x':xs') n = ((a * n) + ((1 - a) * x')):m

You can do mathematical substitution:

ewma1 a (x:xs) = reverse $ foldl' f [x] xs
         where f m@(x':xs') n = ((a * n) + ((1 - a) * x')):m

+ (definition of foldl, which performs the same calculation as foldl',  
though not strict)

foldl f z []     =  z
foldl f z (x:xs) =  foldl f (f z x) xs

=> (substitute definition of foldl in ewma1)

ewma1 a (x1:x2:xs) = reverse $ foldl' f (f [x1] x2) xs
         where f m@(x':xs') n = ((a * n) + ((1 - a) * x')):m

=> (substitute f)

ewma1 a (x1:x2:xs) = reverse $ foldl' f (((a * x2) + ((1 - a) * x1)):[x1])  
xs
         where f m@(x':xs') n = ((a * n) + ((1 - a) * x')):m

Repeat the last two steps until the list is processed.

There is a tool to display this automatically: Hat, but I could not get it  
to run properly on my Windows PC.
The homepage is at:
   http://projects.haskell.org/hat/

Regards,
Henk-Jan

-- 
Folding at home
What if you could share your unused computer power to help find a cure? In
just 5 minutes you can join the world's biggest networked computer and get
us closer sooner. Watch the video.
http://folding.stanford.edu/

From apfelmus at quantentunnel.de  Tue Mar 17 09:29:53 2015
From: apfelmus at quantentunnel.de (Heinrich Apfelmus)
Date: Tue, 17 Mar 2015 10:29:53 +0100
Subject: [Haskell-beginners] Equivalent of IO Monad in other functional
	languages?
In-Reply-To: <791472dcf2df420d2c3dcc8ab1a9ecb0@roundcube.froglogic.com>
References: <5505B08D.3030305@vonos.net>
 <CAJcfPzmwUquH23A6TMXKF1W-xh20Z5o3GvR19ms6pvg54mmzhw@mail.gmail.com>
 <CAJbEW8O6d54hmO=4JiNW4oJA95K6Mt2p5afscvchiCX+dYnbiA@mail.gmail.com>
 <BC7EA4F8-C829-4F11-81EB-564FB3E38DFD@gmail.com>
 <CAJbEW8N9bx1YogQJfaDc3o2yQ++ohN=qaNJxAn0NWdQ25YgRxA@mail.gmail.com>
 <791472dcf2df420d2c3dcc8ab1a9ecb0@roundcube.froglogic.com>
Message-ID: <me8s6i$ank$1@ger.gmane.org>

Frerich Raabe wrote:
> 
> I also used to share this "Every function in Haskell is pure, as long as 
> it does not use any of the unsafe* functions" view but by now I'm 
> wondering whether that's really a useful definition. It may be true that
> 
>   putStrLn :: String -> IO ()
> 
> is pure in the sense that it only yields a 'recipe' for printing the 
> given string instead of actually printing it. However, I think it's 
> important to realize that putStrLn *could* actually yield a poisoned 
> recipe, something which not *only* prints a string but which also plays 
> a sound at 4AM. And the types won't let you know whether this is the case.
> 
> Hence, even though putStrLn may be pure in the classical 'same input 
> yields same output' sense, I wondr whether it's more useful to consider 
> putStrLn to be impure on the grounds that a value of type 'IO a' means 
> "anything can happen, and whatever happens doesn't necessarily only 
> depend on the arguments".

Well, purity of functions is still a useful concept, because it provides 
a useful invariant in the types. For instance, the well-known function

     map :: (a -> b) -> [a] -> [b]

may assume that the first argument always returns equal results given 
equal arguments and hence can be applied in any order and as often as 
desired. On the other hand,

     mapM :: (a -> IO b) -> [a] -> IO [b]

may not assume that and must be careful about the sequence of operations.


Of course, an action  IO a  means that "anything can happen", but I 
don't think it's useful to call that "impure". It's a good thing that 
the nastiness of the `IO` type constructor is separated cleanly from the 
properties of the function arrow `->`.

That leaves the question of how to reason about `IO` programs. This is 
explored in detail in Simon Peyton-Jones' tutorial

"Tackling the awkward squad: monadic input/output, concurrency, 
exceptions, and foreign-language calls in Haskell"

http://research.microsoft.com/en-us/um/people/simonpj/papers/marktoberdorf/


You may also like "The Operational Monad Tutorial", which doesn't cover 
IO, but tries to explain how other monads can be understood and 
implemented in terms of operational semantics:

   http://apfelmus.nfshost.com/articles/operational-monad.html


Best regards,
Heinrich Apfelmus

--
http://apfelmus.nfshost.com


From objitsu at gmail.com  Tue Mar 17 11:52:46 2015
From: objitsu at gmail.com (emacstheviking)
Date: Tue, 17 Mar 2015 11:52:46 +0000
Subject: [Haskell-beginners] There has been discussion on matrix
 operations and cache thrashing;
 does management think using Haskell would lead to cash thrashing?
In-Reply-To: <CAMLKXykmywx24MX6-bWkSs04piOYLKidobKdkgmVuALaFG+eUg@mail.gmail.com>
References: <CAMLKXykmywx24MX6-bWkSs04piOYLKidobKdkgmVuALaFG+eUg@mail.gmail.com>
Message-ID: <CAEiEuULt7AgXbn52SwyLEHvvOE6wZjn7uryNNrzprYR2=4VzbQ@mail.gmail.com>

If "consultancy" was involved then cash thrashing is always an issue ;)


On 16 March 2015 at 20:53, KC <kc1956 at gmail.com> wrote:

> There has been discussion on matrix operations and cache thrashing; does
> management think using Haskell would lead to cash thrashing?
>
> What could change management's mind?
>
> --
> --
>
> Sent from an expensive device which will be obsolete in a few months! :D
>
> Casey
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From kelongcong at gmail.com  Tue Mar 17 17:26:39 2015
From: kelongcong at gmail.com (Kelong Cong)
Date: Tue, 17 Mar 2015 17:26:39 +0000
Subject: [Haskell-beginners] Is it possible to check list length at compile
	time?
Message-ID: <CAAuZ_wVBTen=TQ03XtzvH7LOSvoduVALE6b7nKwUu9ceWBto6w@mail.gmail.com>

Hello,

I have a function that takes a list as its parameter. The list must always
have the same length, for example 16. Instead of producing a "Nothing" or
bottom out when an invalid list is supplied, are there ways to verify this
at compile time? If not, are there alternative data types that has this
kind of functionality?

Thanks,
Kelong
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From toad3k at gmail.com  Tue Mar 17 17:53:35 2015
From: toad3k at gmail.com (David McBride)
Date: Tue, 17 Mar 2015 13:53:35 -0400
Subject: [Haskell-beginners] Is it possible to check list length at
 compile time?
In-Reply-To: <CAAuZ_wVBTen=TQ03XtzvH7LOSvoduVALE6b7nKwUu9ceWBto6w@mail.gmail.com>
References: <CAAuZ_wVBTen=TQ03XtzvH7LOSvoduVALE6b7nKwUu9ceWBto6w@mail.gmail.com>
Message-ID: <CAN+Tr410yTDUfqX6PwB08POp8HgrKVhrwqKrDTqk58ZvcS8qVA@mail.gmail.com>

It is possible to go part of the way.  See for example, the NonEmpty type
in the semigroups package.  It is a list that always has at least one
element by restricting the operations you can do to the type without
turning it into a normal list.  You could easily adapt that solution to a
sixteen element list, but it might not be a reasonable amount of trouble to
go through.

On Tue, Mar 17, 2015 at 1:26 PM, Kelong Cong <kelongcong at gmail.com> wrote:

> Hello,
>
> I have a function that takes a list as its parameter. The list must always
> have the same length, for example 16. Instead of producing a "Nothing" or
> bottom out when an invalid list is supplied, are there ways to verify this
> at compile time? If not, are there alternative data types that has this
> kind of functionality?
>
> Thanks,
> Kelong
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From fa-ml at ariis.it  Tue Mar 17 18:05:04 2015
From: fa-ml at ariis.it (Francesco Ariis)
Date: Tue, 17 Mar 2015 19:05:04 +0100
Subject: [Haskell-beginners] Is it possible to check list length at
 compile time?
In-Reply-To: <CAAuZ_wVBTen=TQ03XtzvH7LOSvoduVALE6b7nKwUu9ceWBto6w@mail.gmail.com>
References: <CAAuZ_wVBTen=TQ03XtzvH7LOSvoduVALE6b7nKwUu9ceWBto6w@mail.gmail.com>
Message-ID: <20150317180504.GA6830@x60s.casa>

On Tue, Mar 17, 2015 at 05:26:39PM +0000, Kelong Cong wrote:
> I have a function that takes a list as its parameter. The list must always
> have the same length, for example 16. Instead of producing a "Nothing" or
> bottom out when an invalid list is supplied, are there ways to verify this
> at compile time? If not, are there alternative data types that has this
> kind of functionality?

Wouldn't a gigantic 16-sized tuple solve the matter (or any datatype
with 16 accessors)?

What functions are you using on the list?

From k-bx at k-bx.com  Tue Mar 17 18:35:58 2015
From: k-bx at k-bx.com (Konstantine Rybnikov)
Date: Tue, 17 Mar 2015 20:35:58 +0200
Subject: [Haskell-beginners] Is it possible to check list length at
 compile time?
In-Reply-To: <CAAuZ_wVBTen=TQ03XtzvH7LOSvoduVALE6b7nKwUu9ceWBto6w@mail.gmail.com>
References: <CAAuZ_wVBTen=TQ03XtzvH7LOSvoduVALE6b7nKwUu9ceWBto6w@mail.gmail.com>
Message-ID: <CAAbahfSoq6JEYiew+nqkFgmS_kOsZe4+O6sJUrft4j4KMMYD8w@mail.gmail.com>

Hi Kelong!

Check out
https://hackage.haskell.org/package/sized-vector-0.0.2.0/docs/Data-Vector-Sized.html
for a vector which has its size stored in type itself.

Cheers.

On Tue, Mar 17, 2015 at 7:26 PM, Kelong Cong <kelongcong at gmail.com> wrote:

> Hello,
>
> I have a function that takes a list as its parameter. The list must always
> have the same length, for example 16. Instead of producing a "Nothing" or
> bottom out when an invalid list is supplied, are there ways to verify this
> at compile time? If not, are there alternative data types that has this
> kind of functionality?
>
> Thanks,
> Kelong
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From alt.mcarter at gmail.com  Tue Mar 17 21:55:29 2015
From: alt.mcarter at gmail.com (Mark Carter)
Date: Tue, 17 Mar 2015 21:55:29 +0000
Subject: [Haskell-beginners] Have I just broken Haskell?
Message-ID: <5508A2D1.1090604@gmail.com>

Persuant to my previous question, I have made some progress, but there 
is something odd going on.

Suppose we want to implement the following algorithm, which is a little 
simpler than the original problem:

The user types the name of a file as input. Haskell then puts the text 
"it works" to that file. I tried this:

import System.IO
let openFileWrite name = openFile name WriteMode
h :: IO Handle
h <- fmap openFileWrite getLine -- you need to type in a file name

This looks wrong, because h presumably needs to be a fixed Handle. Being 
an IO Handle seems to imply it is mutable. Haskell doesn't complain, though.

We also have the problem of how we are going to use hPutStr to put a 
string to the file. We can't do
     hPutStr h "it works"
because h doesn't have the right type. I tried getting around it by 
creating a function
     let put  s h = hPutStr h s
     fmap (put "it works") h
But that doesn't work, it says
    *** Exception: wow: openFile: resource busy (file is locked)

I'm guessing what I've done is create a stream OUT OF the file handle, 
rather than FOR THE file handle.

Urgh! Any ideas what the fix is?

From bob at redivi.com  Tue Mar 17 22:16:25 2015
From: bob at redivi.com (Bob Ippolito)
Date: Tue, 17 Mar 2015 15:16:25 -0700
Subject: [Haskell-beginners] Have I just broken Haskell?
In-Reply-To: <5508A2D1.1090604@gmail.com>
References: <5508A2D1.1090604@gmail.com>
Message-ID: <CACwMPm9NEjKXS61eoxmFuVbxm+ZYKeVL0PTms3z1f_oGh_OMqw@mail.gmail.com>

On Tue, Mar 17, 2015 at 2:55 PM, Mark Carter <alt.mcarter at gmail.com> wrote:

> Persuant to my previous question, I have made some progress, but there is
> something odd going on.
>
> Suppose we want to implement the following algorithm, which is a little
> simpler than the original problem:
>
> The user types the name of a file as input. Haskell then puts the text "it
> works" to that file. I tried this:
>
> import System.IO
> let openFileWrite name = openFile name WriteMode
> h :: IO Handle
> h <- fmap openFileWrite getLine -- you need to type in a file name
>
> This looks wrong, because h presumably needs to be a fixed Handle. Being
> an IO Handle seems to imply it is mutable. Haskell doesn't complain, though.
>

This is wrong because the type of `h` should be Handle, not IO Handle.
`fmap` is not the correct function to use here, you use `fmap` to lift a
pure function into some Functor. What you need to do is sequence these two
IO actions, like this:

name <- getLine
h <- openFileWrite name

Which is equivalent to this:

h <- getLine >>= openFileWrite

or this (`=<<` is just a flipped `>>=` operator):

h <- openFileWrite =<< getLine

-bob
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From kelongcong at gmail.com  Tue Mar 17 22:50:19 2015
From: kelongcong at gmail.com (Kelong Cong)
Date: Tue, 17 Mar 2015 22:50:19 +0000
Subject: [Haskell-beginners] Is it possible to check list length at
 compile time?
In-Reply-To: <CAAbahfSoq6JEYiew+nqkFgmS_kOsZe4+O6sJUrft4j4KMMYD8w@mail.gmail.com>
References: <CAAuZ_wVBTen=TQ03XtzvH7LOSvoduVALE6b7nKwUu9ceWBto6w@mail.gmail.com>
 <CAAbahfSoq6JEYiew+nqkFgmS_kOsZe4+O6sJUrft4j4KMMYD8w@mail.gmail.com>
Message-ID: <CAAuZ_wWbCR2PH8x9roiJKR9-MeubOpOiKqHNUt=Y27dWe_kwLA@mail.gmail.com>

Thanks for the suggestion guys, I think the sized vector is just what I
needed!

I know about tuples but I can't use the usual list functions like map,
splitAt etc, also there is no need to have more than one data type.

On 17 March 2015 at 18:35, Konstantine Rybnikov <k-bx at k-bx.com> wrote:

> Hi Kelong!
>
> Check out
> https://hackage.haskell.org/package/sized-vector-0.0.2.0/docs/Data-Vector-Sized.html
> for a vector which has its size stored in type itself.
>
> Cheers.
>
> On Tue, Mar 17, 2015 at 7:26 PM, Kelong Cong <kelongcong at gmail.com> wrote:
>
>> Hello,
>>
>> I have a function that takes a list as its parameter. The list must
>> always have the same length, for example 16. Instead of producing a
>> "Nothing" or bottom out when an invalid list is supplied, are there ways to
>> verify this at compile time? If not, are there alternative data types that
>> has this kind of functionality?
>>
>> Thanks,
>> Kelong
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From marcin.jan.mrotek at gmail.com  Tue Mar 17 23:00:48 2015
From: marcin.jan.mrotek at gmail.com (Marcin Mrotek)
Date: Wed, 18 Mar 2015 00:00:48 +0100
Subject: [Haskell-beginners] Is it possible to check list length at
 compile time?
In-Reply-To: <CAAuZ_wWbCR2PH8x9roiJKR9-MeubOpOiKqHNUt=Y27dWe_kwLA@mail.gmail.com>
References: <CAAuZ_wVBTen=TQ03XtzvH7LOSvoduVALE6b7nKwUu9ceWBto6w@mail.gmail.com>
 <CAAbahfSoq6JEYiew+nqkFgmS_kOsZe4+O6sJUrft4j4KMMYD8w@mail.gmail.com>
 <CAAuZ_wWbCR2PH8x9roiJKR9-MeubOpOiKqHNUt=Y27dWe_kwLA@mail.gmail.com>
Message-ID: <CAJcfPz=y+4yu1KiG7zHcp-034W12w0C5KEWDVjFJT3AgHrT+uA@mail.gmail.com>

I think you're looking for dependend types or refinement types. It's
sort of possible in Haskell, but look into Agda, Idris, F#, and this
thread: http://www.reddit.com/r/haskell/comments/2z5l9y/question_type_system_and_ndimensional_vectors/

Best regards,
Marcin Mrotek

From anuragohri92 at gmail.com  Wed Mar 18 01:51:26 2015
From: anuragohri92 at gmail.com (Anurag Ohri)
Date: Wed, 18 Mar 2015 07:21:26 +0530
Subject: [Haskell-beginners] Have I just broken Haskell?
In-Reply-To: <CACwMPm9NEjKXS61eoxmFuVbxm+ZYKeVL0PTms3z1f_oGh_OMqw@mail.gmail.com>
References: <5508A2D1.1090604@gmail.com>
 <CACwMPm9NEjKXS61eoxmFuVbxm+ZYKeVL0PTms3z1f_oGh_OMqw@mail.gmail.com>
Message-ID: <CAONtWhy=w-rHn6Ycw7o22LTby9uWdomMhHwZJZer5kBVigPfvw@mail.gmail.com>

I recently encountered the same Exception. It occurs when you open a file
but don't close it using "hClose".
On Mar 18, 2015 3:46 AM, "Bob Ippolito" <bob at redivi.com> wrote:

> On Tue, Mar 17, 2015 at 2:55 PM, Mark Carter <alt.mcarter at gmail.com>
> wrote:
>
>> Persuant to my previous question, I have made some progress, but there is
>> something odd going on.
>>
>> Suppose we want to implement the following algorithm, which is a little
>> simpler than the original problem:
>>
>> The user types the name of a file as input. Haskell then puts the text
>> "it works" to that file. I tried this:
>>
>> import System.IO
>> let openFileWrite name = openFile name WriteMode
>> h :: IO Handle
>> h <- fmap openFileWrite getLine -- you need to type in a file name
>>
>> This looks wrong, because h presumably needs to be a fixed Handle. Being
>> an IO Handle seems to imply it is mutable. Haskell doesn't complain, though.
>>
>
> This is wrong because the type of `h` should be Handle, not IO Handle.
> `fmap` is not the correct function to use here, you use `fmap` to lift a
> pure function into some Functor. What you need to do is sequence these two
> IO actions, like this:
>
> name <- getLine
> h <- openFileWrite name
>
> Which is equivalent to this:
>
> h <- getLine >>= openFileWrite
>
> or this (`=<<` is just a flipped `>>=` operator):
>
> h <- openFileWrite =<< getLine
>
> -bob
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From chaddai.fouche at gmail.com  Wed Mar 18 06:20:39 2015
From: chaddai.fouche at gmail.com (=?UTF-8?B?Q2hhZGRhw68gRm91Y2jDqQ==?=)
Date: Wed, 18 Mar 2015 07:20:39 +0100
Subject: [Haskell-beginners] Have I just broken Haskell?
In-Reply-To: <5508A2D1.1090604@gmail.com>
References: <5508A2D1.1090604@gmail.com>
Message-ID: <CANfjZRZ4nRFf0WaQNbv9c99LRfYnpCy35HzQ8ANDxdcpePKN6Q@mail.gmail.com>

On Tue, Mar 17, 2015 at 10:55 PM, Mark Carter <alt.mcarter at gmail.com> wrote:

> Persuant to my previous question, I have made some progress, but there is
> something odd going on.
>
> Suppose we want to implement the following algorithm, which is a little
> simpler than the original problem:
>
> The user types the name of a file as input. Haskell then puts the text "it
> works" to that file. I tried this:
>
> import System.IO
> let openFileWrite name = openFile name WriteMode
> h :: IO Handle
> h <- fmap openFileWrite getLine -- you need to type in a file name
>
> This looks wrong, because h presumably needs to be a fixed Handle. Being
> an IO Handle seems to imply it is mutable. Haskell doesn't complain, though.
>

No it doesn't, nothing is "mutable" in Haskell, you can have IORef or STRef
whose content can be mutated in the IO or ST monad but the IORef or STRef
themselves won't be mutable. IO Handle is not "a mutable Handle", it's an
IO action that returns an Handle... And that's exactly the type of h in
your code !

That's because this code doesn't open anything, it read a line and thanks
to fmap, it applies openFileWrite to this line, giving you an "IO Handle"
in h, an action that would give you an Handle if it was executed... but it
is not executed, it is just stocked in h.
The type of "fmap openFileWrite getLine" is IO (IO Handle), not as you
probably wanted IO Handle.

As Bob said, if you want an Handle, you shouldn't use fmap but rather
(>>=). Or you could do :

act :: IO handle <- fmap openFileWrite getLine
h :: Handle <- act

but that's really not the usual way to do it (on the other hand it's
interesting because it shows you that "IO a" are just values, you can
manipulate them just like normal values.

Basically your code was "correct", it just didn't give you an Handle (that
would have been clear as soon as you tried to use h as an Handle) !
-- 
Jeda?
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From martin.drautzburg at web.de  Wed Mar 18 21:49:50 2015
From: martin.drautzburg at web.de (martin)
Date: Wed, 18 Mar 2015 22:49:50 +0100
Subject: [Haskell-beginners] Finding the lowest element greater or equal than
Message-ID: <5509F2FE.4030708@web.de>

Hello all,

I want to find the lowest element in a collection of items which is greatet or equal to a given element. There can be
more than one such element in which case I wouldn't care which one I get. I still want to be able to iterate over the
elements following the found element. In a way I want the equivalent of

select ...
from table
where table.x > n


How would I do this efficiently?

I tried using an ordered list, but I would still have to traverse half the list on average.

I suppose I can do this with some sort of tree, but i don't want to write all the code myself. But I couldn't find
anything off the shelf.

From k-bx at k-bx.com  Wed Mar 18 22:07:27 2015
From: k-bx at k-bx.com (Konstantine Rybnikov)
Date: Thu, 19 Mar 2015 00:07:27 +0200
Subject: [Haskell-beginners] Finding the lowest element greater or equal
	than
In-Reply-To: <5509F2FE.4030708@web.de>
References: <5509F2FE.4030708@web.de>
Message-ID: <CAAbahfSxPZAA2UXAWT00OPH2fsY6kPPb+3dUHhxy6Xjv31AzSw@mail.gmail.com>

Hi Martin,

What you want depends on more details. If you just have a list of items,
you can access its first item which is less than some "n" by doing:

headMay (filter (< n) xs)

Would this solution satisfy you? If not -- what condition would not be met?

You mentioned something regarding that with ordered list you'd have to
traverse half of the list on average. What did you mean by that? (I mean,
if you have ordered list, then either its first element is what you need,
or none of them are)

On Wed, Mar 18, 2015 at 11:49 PM, martin <martin.drautzburg at web.de> wrote:

> Hello all,
>
> I want to find the lowest element in a collection of items which is
> greatet or equal to a given element. There can be
> more than one such element in which case I wouldn't care which one I get.
> I still want to be able to iterate over the
> elements following the found element. In a way I want the equivalent of
>
> select ...
> from table
> where table.x > n
>
>
> How would I do this efficiently?
>
> I tried using an ordered list, but I would still have to traverse half the
> list on average.
>
> I suppose I can do this with some sort of tree, but i don't want to write
> all the code myself. But I couldn't find
> anything off the shelf.
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From bob at redivi.com  Wed Mar 18 22:42:09 2015
From: bob at redivi.com (Bob Ippolito)
Date: Wed, 18 Mar 2015 15:42:09 -0700
Subject: [Haskell-beginners] Finding the lowest element greater or equal
	than
In-Reply-To: <5509F2FE.4030708@web.de>
References: <5509F2FE.4030708@web.de>
Message-ID: <CACwMPm_J69Eyqws_a-_oXv7o2CT2Fw6n8aJqQqtkcQcjq24jWQ@mail.gmail.com>

On Wed, Mar 18, 2015 at 2:49 PM, martin <martin.drautzburg at web.de> wrote:

>
> I want to find the lowest element in a collection of items which is
> greatet or equal to a given element. There can be
> more than one such element in which case I wouldn't care which one I get.
> I still want to be able to iterate over the
> elements following the found element. In a way I want the equivalent of
>
> select ...
> from table
> where table.x > n
>
>
> How would I do this efficiently?
>
> I tried using an ordered list, but I would still have to traverse half the
> list on average.
>

You could do it in log time (using a binary search algorithm) with an
ordered Vector or Array. Lists are not an appropriate data structure for
random access.


> I suppose I can do this with some sort of tree, but i don't want to write
> all the code myself. But I couldn't find
> anything off the shelf.
>

Data.Map supports that operation in log time with splitLookup. You can use
the values of the map to count the occurrences of a given element. If all
of the elements are unique you can save a bit of space and use Data.Set
instead, which has a splitMember function.

-bob
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From vale.cofershabica at gmail.com  Fri Mar 20 19:52:46 2015
From: vale.cofershabica at gmail.com (Vale Cofer-Shabica)
Date: Fri, 20 Mar 2015 15:52:46 -0400
Subject: [Haskell-beginners] suggestions for re-implementing mapM
Message-ID: <CAAzfV4R1hevE6PfWD+f-YyuVpa7VJRT=jGH_CgYpsN6LUYQnDA@mail.gmail.com>

Hello all,

I've been reading through "Tackling the Awkward Squad" [1] and am
implementing the definitions "left as exercises" as I go. For section 2.2,
I define:

    putLine :: [Char] -> IO ()
    putLine :: mapM_ putChar

where

    mapM_ :: Monad m => (a -> m b) -> a -> m ()
    mapM_ f [] = return ()
    mapM_ f (x:xs) = (f x) >> (mapM_ f xs)

which works without difficulty. For the sake of learning, I decided to
implement mapM as well. My definition (below) works, but seems really
in-elegant. I checked the prelude source and found mapM defined in terms of
sequence, which has some do-notation I'm not so clear on. I'm trying to
avoid using do notation in my implementation because it still feels like
magic. I'll work on de-sugaring do notation again once I have a solid
handle on (>>=), (>>), and return. Any suggestions for cleaning this up
would be much appreciated!

    mapM :: Monad m => (a -> m b) -> [a] -> m [b]
    mapM f [] = return []
    mapM f (x:xs) = consMM (f x) (mapM f xs)

    consMM :: Monad m => m a -> m [a] -> m [a]
    consMM mx mxs = mx >>= ((flip consM)  mxs) where
        consM x mxs = mxs>>=(\xs -> return (x:xs))

Thank you,
vale

[1] Suggested by apfelmus in a recent email to the list:
http://research.microsoft.com/en-us/um/people/simonpj/papers/marktoberdorf/
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From toad3k at gmail.com  Fri Mar 20 20:07:56 2015
From: toad3k at gmail.com (David McBride)
Date: Fri, 20 Mar 2015 16:07:56 -0400
Subject: [Haskell-beginners] suggestions for re-implementing mapM
In-Reply-To: <CAAzfV4R1hevE6PfWD+f-YyuVpa7VJRT=jGH_CgYpsN6LUYQnDA@mail.gmail.com>
References: <CAAzfV4R1hevE6PfWD+f-YyuVpa7VJRT=jGH_CgYpsN6LUYQnDA@mail.gmail.com>
Message-ID: <CAN+Tr43Pv+P-ek-TERAbEBxBSaGptS1s_OoysY1Lvo-z8pVxTQ@mail.gmail.com>

Sometimes it is easier to write it with do notation and then rewrite it
back in normal terms:

mapM :: Monad m => (a -> m b) -> [a] -> m [b]
mapM _ [] = return []
mapM f (x:xs) = do
  x' <- f x
  xs' <- mapM f xs
  return $ x':xs'

You can rewrite the second part step by step as:

mapM f (x:xs) = f x >>= \x' -> mapM f xs >>= \xs' -> return (x' : xs')

Also the base package does not use do notation.  It defines it much more
elegantly:

mapM            :: Monad m => (a -> m b) -> [a] -> m [b]mapM f as
 =  sequence (map f as)




On Fri, Mar 20, 2015 at 3:52 PM, Vale Cofer-Shabica <
vale.cofershabica at gmail.com> wrote:

> Hello all,
>
> I've been reading through "Tackling the Awkward Squad" [1] and am
> implementing the definitions "left as exercises" as I go. For section 2.2,
> I define:
>
>     putLine :: [Char] -> IO ()
>     putLine :: mapM_ putChar
>
> where
>
>     mapM_ :: Monad m => (a -> m b) -> a -> m ()
>     mapM_ f [] = return ()
>     mapM_ f (x:xs) = (f x) >> (mapM_ f xs)
>
> which works without difficulty. For the sake of learning, I decided to
> implement mapM as well. My definition (below) works, but seems really
> in-elegant. I checked the prelude source and found mapM defined in terms of
> sequence, which has some do-notation I'm not so clear on. I'm trying to
> avoid using do notation in my implementation because it still feels like
> magic. I'll work on de-sugaring do notation again once I have a solid
> handle on (>>=), (>>), and return. Any suggestions for cleaning this up
> would be much appreciated!
>
>     mapM :: Monad m => (a -> m b) -> [a] -> m [b]
>     mapM f [] = return []
>     mapM f (x:xs) = consMM (f x) (mapM f xs)
>
>     consMM :: Monad m => m a -> m [a] -> m [a]
>     consMM mx mxs = mx >>= ((flip consM)  mxs) where
>         consM x mxs = mxs>>=(\xs -> return (x:xs))
>
> Thank you,
> vale
>
> [1] Suggested by apfelmus in a recent email to the list:
> http://research.microsoft.com/en-us/um/people/simonpj/papers/marktoberdorf/
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From stuart.hungerford at gmail.com  Fri Mar 20 21:39:37 2015
From: stuart.hungerford at gmail.com (Stuart Hungerford)
Date: Sat, 21 Mar 2015 08:39:37 +1100
Subject: [Haskell-beginners] Multi-parameter type classes and ambiguous type
	variables...
Message-ID: <CAG+kMrF3ZufgNwfkDADrQAFp-5qKmvuKbnZF0nADYwQ4wxh2oA@mail.gmail.com>

Hi,

As a learning exercise I'm modelling some algebraic structures as
Haskell typeclasses. Some of these are multi-parameter type classes.
Here's a very simplified version of the type class relationships:

class MM a where
  one :: a

class AM a where
  zero :: a

class (AM a, MM a) => SR a

class (AM a) => AG a where
  inv :: a -> a

class (SR a) => R a where
  neg :: a -> a

class (R r, AG g) => M r g where
  sca :: r -> g -> g

check :: (Eq g, M r g) => g -> Bool
check x = sca one x == x

The problem I have is that GHC is finding the "r" type variable in the
"check" function ambiguous. Given my still limited Haskell knowledge
I'm not surprised this is happening. What I would like to know is how
experienced Haskellers handle this situation in practice: is there an
idiomatic way of disambiguating "r" or is it a sign of poor type class
design?

Thanks,

Stu

From sumit.sahrawat.apm13 at iitbhu.ac.in  Sat Mar 21 07:37:52 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Sat, 21 Mar 2015 13:07:52 +0530
Subject: [Haskell-beginners] Multi-parameter type classes and ambiguous
 type variables...
In-Reply-To: <CAG+kMrF3ZufgNwfkDADrQAFp-5qKmvuKbnZF0nADYwQ4wxh2oA@mail.gmail.com>
References: <CAG+kMrF3ZufgNwfkDADrQAFp-5qKmvuKbnZF0nADYwQ4wxh2oA@mail.gmail.com>
Message-ID: <CAJbEW8P2DnFg-juCCV8Fw9qVGG6dbc_pm2rkn_ryHCG8KhjDUg@mail.gmail.com>

This might be better answered at the haskell-cafe. Sending to cafe.

On 21 March 2015 at 03:09, Stuart Hungerford <stuart.hungerford at gmail.com>
wrote:

> Hi,
>
> As a learning exercise I'm modelling some algebraic structures as
> Haskell typeclasses. Some of these are multi-parameter type classes.
> Here's a very simplified version of the type class relationships:
>
> class MM a where
>   one :: a
>
> class AM a where
>   zero :: a
>
> class (AM a, MM a) => SR a
>
> class (AM a) => AG a where
>   inv :: a -> a
>
> class (SR a) => R a where
>   neg :: a -> a
>
> class (R r, AG g) => M r g where
>   sca :: r -> g -> g
>
> check :: (Eq g, M r g) => g -> Bool
> check x = sca one x == x
>
> The problem I have is that GHC is finding the "r" type variable in the
> "check" function ambiguous. Given my still limited Haskell knowledge
> I'm not surprised this is happening. What I would like to know is how
> experienced Haskellers handle this situation in practice: is there an
> idiomatic way of disambiguating "r" or is it a sign of poor type class
> design?
>
> Thanks,
>
> Stu
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>



-- 
Regards

Sumit Sahrawat
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From stuart.hungerford at gmail.com  Sat Mar 21 20:15:52 2015
From: stuart.hungerford at gmail.com (Stuart Hungerford)
Date: Sun, 22 Mar 2015 07:15:52 +1100
Subject: [Haskell-beginners] [Haskell-cafe] Multi-parameter type classes
 and ambiguous type variables...
In-Reply-To: <CANQO64+=xDrtLcr8Rn+qD0HfO-egOGfxY97nA=to+xJwmW9Muw@mail.gmail.com>
References: <CAG+kMrF3ZufgNwfkDADrQAFp-5qKmvuKbnZF0nADYwQ4wxh2oA@mail.gmail.com>
 <CAJbEW8P2DnFg-juCCV8Fw9qVGG6dbc_pm2rkn_ryHCG8KhjDUg@mail.gmail.com>
 <CANQO64+=xDrtLcr8Rn+qD0HfO-egOGfxY97nA=to+xJwmW9Muw@mail.gmail.com>
Message-ID: <CAG+kMrHr6vTuvjcMvk+pZXdA8mHa-h0APaW8wMH8hgkgPMLQGg@mail.gmail.com>

On Sat, Mar 21, 2015 at 7:31 PM, Chris Wong <lambda.fairy at gmail.com> wrote:

> [...]
>>> check :: (Eq g, M r g) => g -> Bool
>>> check x = sca one x == x
>>>
>>> The problem I have is that GHC is finding the "r" type variable in the
>>> "check" function ambiguous. Given my still limited Haskell knowledge
>>> I'm not surprised this is happening. What I would like to know is how
>>> experienced Haskellers handle this situation in practice: is there an
>>> idiomatic way of disambiguating "r" or is it a sign of poor type class
>>> design?
>
> In the type signature:
>
>     check :: (Eq g, M r g) => g -> Bool
>
> you fix the type `g`, but not the type `r`. This causes an ambiguity
> in the program because if you had e.g.
>
>     instance M Float Vector where ...
>     instance M Int Vector where ...
>
> both in the same program, and you passed a Vector to `check`, GHC
> won't know which instance to choose.
>
> To solve this ambiguity, either fix `r` with an extra parameter:
>
>     check :: (Eq g, M r g) => r -> g -> Bool
>     check one' x = sca one' x == x
>
> Or declare that `r` is uniquely determined by `g` using a *functional
> dependency*:
>
>     class (R r, AG g) => M r g | g -> r where
>         sca :: r -> g -> g
>
> Or equivalently, using *associated types*:
>
>     class (AG g, R (Scalar g)) => M g where
>         type Scalar g :: *
>         sca :: Scalar g -> g -> g
>
>     check :: (Eq g, M g) => g -> Bool
>     check x = -- as before
>
> A Google search for these two terms should yield plenty of tutorials
> and examples.

Thanks for this excellent explanation.

> (By the way, I'd suggest using longer names like "Ring" and
> "AdditiveGroup" instead, as they're easier to read.)

Yes -- I stripped the example down, including the full names of the
type classes, for explaining the issue. I could well have left the
full names though.


Stu

From ram at rkrishnan.org  Sun Mar 22 13:16:14 2015
From: ram at rkrishnan.org (Ramakrishnan Muthukrishnan)
Date: Sun, 22 Mar 2015 18:46:14 +0530
Subject: [Haskell-beginners] Code review: Go challenge #1 in Haskell
Message-ID: <1427030174.3627139.243629570.07376C30@webmail.messagingengine.com>

Hello Haskellers:

A few weeks ago, there was this thing called "Go challenge" and a
problem was posted? on decoding a binary format and printing out the
drum sequences in the input binary file.

I made a Haskell solution and would love to get some code reviews.

 <https://github.com/vu3rdd/drum>

One of the input files given does not have the full bitstream and my
program cannot parse that yet. I had been thinking about using a Monad
transformer to make a `Get (Maybe) a' and output whatever it could parse
and leave the rest behind, instead of failing completely.

? http://golang-challenge.com/go-challenge1/

(fwiw, the challenge is over.)

Thanks in advance,
Ramakrishnan

From twashing at gmail.com  Sun Mar 22 16:33:18 2015
From: twashing at gmail.com (Timothy Washington)
Date: Sun, 22 Mar 2015 09:33:18 -0700
Subject: [Haskell-beginners] Dipping Toes Into Haskell
In-Reply-To: <CAADtM-aA04b=jTJwhr50k0mO=_YxYqXhEMJv0QG5e8-0Lk4ODA@mail.gmail.com>
References: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
 <CAD=7U2DFK_T9Z+iWs9S1QtQyZz+yRiNGYeboEcV0pj3GvmWZZQ@mail.gmail.com>
 <CAADtM-aA04b=jTJwhr50k0mO=_YxYqXhEMJv0QG5e8-0Lk4ODA@mail.gmail.com>
Message-ID: <CAADtM-ZiG-Z2r=uMm22LSjQw54p4j=MvP+psmRzP72zPvn_56Q@mail.gmail.com>

So I've finally had a chance to revisit my tictactoe game.


import Control.Lens

data Piece = X | O | E deriving Show
type Row = [Piece]
type Board = [Row]
*data Position = Int Int deriving Show*

-- put an X or O in a position
*move :: Board -> Piece -> Position -> Board*
move board piece position = board

main :: IO ()
main = putStrLn "Hello World"

-- using Lenses
-- https://github.com/ekmett/lens/wiki/Examples
-- http://lens.github.io/tutorial.html
-- 
http://blog.jakubarnold.cz/2014/07/14/lens-tutorial-introduction-part-1.html

--let r1 = (E,E,E)
--let r2 = (E,E,E)
--let r3 = (E,E,E)
--let board = (r1,r2,r3)


Now, if I want to make a move, I'll want to set a *piece* on a *position*,
on a *board*. Let's assume the board has the following shape.

?> let r1 = (E,E,E)
?> let r2 = (E,E,E)
?> let r3 = (E,E,E)
?> let board = (r1,r2,r3)
((E,E,E),(E,E,E),(E,E,E))


To return an updated board, I dug around and *i)* couldn't find a core
Haskell equivalent to Clojure's update-in
<http://clojuredocs.org/clojure.core/update-in>. *ii)* Zippers only deal
with trees of left / right nodes. *iii)* So that left me with Control.Lens
<https://github.com/ekmett/lens/wiki/Examples>. Now I can easily inspect
and update the board like so.

?> board^*._**2._1*
E
?> set (*_2._1*) 42 board
((E,E,E),(42,E,E),(E,E,E))


I can then parameterize _2 or _1 like so.

?> let a = _2
?> board ^. a
(E,E,E)


But I can't figure out how to include that parameter's type into a
*Position* type definition. I've tried these unsuccessfully.

*data Position = Int Int deriving Show  -- original type definition*
*data Position = Simple Lens (Int a) (Int a) deriving Show  -- failing try
1*

move :: Board -> Piece -> Position -> Board
move board piece position = set (position) piece board  -- want to use this
instead of "set (*_2._1*) 42 board"



Any ideas?


:)

Tim Washington
Interruptsoftware.com <http://interruptsoftware.com>


On Thu, Mar 12, 2015 at 5:18 PM, Timothy Washington <twashing at gmail.com>
wrote:

> Veeery nice. Thanks for your insights.
>
> The Typeclass stuff making a lot more sense now. And hence ghc-mod's
> (Interactive-Haskell's) behaviour makes more sense to me. It's working like
> a charm.
>
> I'll play around with nested list manipulation this evening.
>
>
> Cheers
>
> Tim Washington
> Interruptsoftware.com <http://interruptsoftware.com>
>
>
> On Thu, Mar 12, 2015 at 8:03 PM, Mike Meyer <mwm at mired.org> wrote:
>
>> On Thu, Mar 12, 2015 at 6:02 PM, Timothy Washington <twashing at gmail.com>
>> wrote:
>>
>>> To get started, I'm trying to implement a simple *tictactoe* game. And
>>> I would like to be able to represent a Piece on the board, as either the
>>> string "X" or "O". This is what I have so far.
>>>
>>> module Main where
>>>
>>> data Piece = X | O
>>> type Row = [Piece]
>>> type Board = [Row]
>>>
>>> -- put an X or O in a position
>>> move :: Board -> Piece -> Board
>>> move board piece = board
>>>
>>> -- check win vertically
>>> -- check win horizontally
>>> -- check win diagonally
>>>
>>> main :: IO ()
>>> main = putStrLn "Hello World"
>>>
>>>
>>>
>>> *A)* Now, I'd like to be able to *load code interactively*, preferably
>>> within emacs. However I don't have access to my types with *ghci* or *ghc-mod
>>> (Interactive-Haskell)*. In either case, this call fails with the below
>>> error.
>>>
>>>
>>> let p = Piece X
>>> <interactive>:20:9-13: Not in scope: data constructor `Piece'
>>>
>>>
>> Piece is the data TYPE. It has two constructors, X, and O. If you load
>> your module into ghci, you can do ":t X" to get the type of X, which is
>> Piece. You want to do "let p = X" here.
>>
>> Check your mode docs while editing the haskell file. There should be  a
>> command to load the current buffer into a ghci running in an interactive
>> emacs buffer.
>>
>>
>>> *B)* And how do I make a *custom datatype* that's one of two strings
>>> (enumeration of either "X" or "O"). Cabal builds and runs the abouve code,
>>> so I know it can compile. But I'm confused as to where X or O is defined,
>>> and how I would supply it as an input.
>>>
>>
>>
>> As noted, you can derive "Show" so that Piece types print as "X" and "O".
>> Similarly, you can derive "Read":
>>
>> data Piece = X | O deriving (Show, Read)
>>
>> so that you can then read "X" and get back an X when the context calls
>> for a Piece value:
>>
>> *Main> read "X" :: Piece
>>
>> X
>>
>> However, this isn't used a lot, as it tends to be slow and not very
>> flexible for more complex types. For instance, it will let you read in
>> lists of Pieces and lists of lists, but you'l lhave to use the list syntax,
>>  not something that looks like an actual board when printed.
>>
>>
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From lists at webconect.ch  Sun Mar 22 18:56:52 2015
From: lists at webconect.ch (Elias Diem)
Date: Sun, 22 Mar 2015 19:56:52 +0100
Subject: [Haskell-beginners] Support for Maildir
Message-ID: <20150322185652.GA5878@webconect.local>

Hi there

Am I right that there are no up-to-date packages/libraries 
to process email in Maildir format?

-- 
Greetings
Elias



From hjgtuyl at chello.nl  Sun Mar 22 22:25:32 2015
From: hjgtuyl at chello.nl (Henk-Jan van Tuyl)
Date: Sun, 22 Mar 2015 23:25:32 +0100
Subject: [Haskell-beginners] Code review: Go challenge #1 in Haskell
In-Reply-To: <1427030174.3627139.243629570.07376C30@webmail.messagingengine.com>
References: <1427030174.3627139.243629570.07376C30@webmail.messagingengine.com>
Message-ID: <op.xvw68zdepz0j5l@alquantor>

On Sun, 22 Mar 2015 14:16:14 +0100, Ramakrishnan Muthukrishnan  
<ram at rkrishnan.org> wrote:

> Hello Haskellers:
>
> A few weeks ago, there was this thing called "Go challenge" and a
> problem was posted? on decoding a binary format and printing out the
> drum sequences in the input binary file.
>
> I made a Haskell solution and would love to get some code reviews.
>
>  <https://github.com/vu3rdd/drum>


in line
   put _ = do BinaryPut.putWord8 0  -- we don't care about writing
the 'do' is not necessary, as there is only one action;
you could also write:
   put _ = undefined  -- we don't care about writing
or:
   put _ = error "put is not implemented"


The line
   (intercalate "\n" $ map show tracks')
can be simplified to
   (unlines $ map show tracks')


The line
   putStrLn (show (runGet get bs :: Splice))
can be simplified to
   print (runGet get bs :: Splice)
as print is the same as
   putStrLn . show


Instead of:
       else
       do
         track <- getTrack
         tracks' <- getTracks
         return (track:tracks')
you could write:
       else liftM2 (:) getTrack getTracks
or:
       else getTrack ^:^ getTracks
         where
           (^:^) = liftM2 (:)
(import Control.Monad first)


The function splitN is the same as chunksOf from the package split; I  
found this by entering the type of splitN as a search string in Hoogle.

You can use hlint (from Hackage) to get some hints for possible  
improvements of the code.

Regards,
Henk-Jan van Tuyl


-- 
Folding at home
What if you could share your unused computer power to help find a cure? In  
just 5 minutes you can join the world's biggest networked computer and get  
us closer sooner. Watch the video.
http://folding.stanford.edu/


http://Van.Tuyl.eu/
http://members.chello.nl/hjgtuyl/tourdemonad.html
Haskell programming
--

From ram at rkrishnan.org  Mon Mar 23 01:16:12 2015
From: ram at rkrishnan.org (Ramakrishnan Muthukrishnan)
Date: Mon, 23 Mar 2015 06:46:12 +0530
Subject: [Haskell-beginners] Code review: Go challenge #1 in Haskell
In-Reply-To: <op.xvw68zdepz0j5l@alquantor>
References: <1427030174.3627139.243629570.07376C30@webmail.messagingengine.com>
 <op.xvw68zdepz0j5l@alquantor>
Message-ID: <1427073372.3755259.243804762.7CE52D5A@webmail.messagingengine.com>

On Mon, Mar 23, 2015, at 03:55 AM, Henk-Jan van Tuyl wrote:
> On Sun, 22 Mar 2015 14:16:14 +0100, Ramakrishnan Muthukrishnan  
> <ram at rkrishnan.org> wrote:
> 
> > Hello Haskellers:
> >
> > A few weeks ago, there was this thing called "Go challenge" and a
> > problem was posted? on decoding a binary format and printing out the
> > drum sequences in the input binary file.
> >
> > I made a Haskell solution and would love to get some code reviews.
> >
> >  <https://github.com/vu3rdd/drum>
> 
> 
> in line
>    put _ = do BinaryPut.putWord8 0  -- we don't care about writing
> the 'do' is not necessary, as there is only one action;
> you could also write:
>    put _ = undefined  -- we don't care about writing
> or:
>    put _ = error "put is not implemented"
> 
> 
> The line
>    (intercalate "\n" $ map show tracks')
> can be simplified to
>    (unlines $ map show tracks')
> 
> 
> The line
>    putStrLn (show (runGet get bs :: Splice))
> can be simplified to
>    print (runGet get bs :: Splice)
> as print is the same as
>    putStrLn . show
> 
> 
> Instead of:
>        else
>        do
>          track <- getTrack
>          tracks' <- getTracks
>          return (track:tracks')
> you could write:
>        else liftM2 (:) getTrack getTracks
> or:
>        else getTrack ^:^ getTracks
>          where
>            (^:^) = liftM2 (:)
> (import Control.Monad first)

Hello Henk-Jan,

Thanks a lot for all the suggestions.

> You can use hlint (from Hackage) to get some hints for possible  
> improvements of the code.

Thanks. I ran HLint and it gave some more suggestions. Incorporated all
the suggestions, thanks a lot.

Ramakrishnan

From hutch-lists at recursive.ca  Mon Mar 23 11:57:58 2015
From: hutch-lists at recursive.ca (Bob Hutchison)
Date: Mon, 23 Mar 2015 07:57:58 -0400
Subject: [Haskell-beginners] Dipping Toes Into Haskell
In-Reply-To: <CAADtM-ZiG-Z2r=uMm22LSjQw54p4j=MvP+psmRzP72zPvn_56Q@mail.gmail.com>
References: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
 <CAD=7U2DFK_T9Z+iWs9S1QtQyZz+yRiNGYeboEcV0pj3GvmWZZQ@mail.gmail.com>
 <CAADtM-aA04b=jTJwhr50k0mO=_YxYqXhEMJv0QG5e8-0Lk4ODA@mail.gmail.com>
 <CAADtM-ZiG-Z2r=uMm22LSjQw54p4j=MvP+psmRzP72zPvn_56Q@mail.gmail.com>
Message-ID: <B4D9CDA3-C973-4355-873D-90379CAE995E@recursive.ca>

Hi Tim,

Staying from clojure? :-)

> On Mar 22, 2015, at 12:33 PM, Timothy Washington <twashing at gmail.com> wrote:
> 
> So I've finally had a chance to revisit my tictactoe game. 
> 
> import Control.Lens
> 
> data Piece = X | O | E deriving Show
> type Row = [Piece]
> type Board = [Row]
> data Position = Int Int deriving Show
> 
> -- put an X or O in a position
> move :: Board -> Piece -> Position -> Board
> move board piece position = board
> 
> main :: IO ()
> main = putStrLn "Hello World"
> 
> -- using Lenses
> -- https://github.com/ekmett/lens/wiki/Examples <https://github.com/ekmett/lens/wiki/Examples>
> -- http://lens.github.io/tutorial.html <http://lens.github.io/tutorial.html>
> -- http://blog.jakubarnold.cz/2014/07/14/lens-tutorial-introduction-part-1.html <http://blog.jakubarnold.cz/2014/07/14/lens-tutorial-introduction-part-1.html>
> 
> --let r1 = (E,E,E)
> --let r2 = (E,E,E)
> --let r3 = (E,E,E)
> --let board = (r1,r2,r3)
> 
> Now, if I want to make a move, I'll want to set a piece on a position, on a board. Let's assume the board has the following shape. 
> 
> ?> let r1 = (E,E,E)
> ?> let r2 = (E,E,E)
> ?> let r3 = (E,E,E)
> ?> let board = (r1,r2,r3)
> ((E,E,E),(E,E,E),(E,E,E))
> 
> To return an updated board, I dug around and i) couldn't find a core Haskell equivalent to Clojure's update-in <http://clojuredocs.org/clojure.core/update-in>. ii) Zippers only deal with trees of left / right nodes. iii) So that left me with Control.Lens <https://github.com/ekmett/lens/wiki/Examples>. Now I can easily inspect and update the board like so. 

Or you could use an two dimensional array of Positions instead (Data.Array).

> 
> ?> board^._2._1
> E
> ?> set (_2._1) 42 board
> ((E,E,E),(42,E,E),(E,E,E))
> 
> I can then parameterize _2 or _1 like so. 
> 
> ?> let a = _2
> ?> board ^. a
> (E,E,E)

You can probably find the type of _1, _2, _3 in the repl, and use that in your code below.

> 
> But I can't figure out how to include that parameter's type into a Position type definition. I've tried these unsuccessfully. 
> 
> data Position = Int Int deriving Show  -- original type definition
> data Position = Simple Lens (Int a) (Int a) deriving Show  -- failing try 1

These are very suspicious. Did you intend to define constructors ?Int? and ?Simple?? Maybe you meant something like:

data Position = Position Int Int deriving Show

In which case you?ll have a type called ?Position? and a constructor with the same name.

Cheers,
Bob

> 
> move :: Board -> Piece -> Position -> Board
> move board piece position = set (position) piece board  -- want to use this instead of "set (_2._1) 42 board"
> 
> 
> Any ideas? 
> 
> 
> :) 
> 
> Tim Washington 
> Interruptsoftware.com <http://interruptsoftware.com/> 
> 
> 
> On Thu, Mar 12, 2015 at 5:18 PM, Timothy Washington <twashing at gmail.com <mailto:twashing at gmail.com>> wrote:
> Veeery nice. Thanks for your insights. 
> 
> The Typeclass stuff making a lot more sense now. And hence ghc-mod's (Interactive-Haskell's) behaviour makes more sense to me. It's working like a charm. 
> 
> I'll play around with nested list manipulation this evening.
> 
> 
> Cheers 
> 
> Tim Washington 
> Interruptsoftware.com <http://interruptsoftware.com/> 
> 
> 
> On Thu, Mar 12, 2015 at 8:03 PM, Mike Meyer <mwm at mired.org <mailto:mwm at mired.org>> wrote:
> On Thu, Mar 12, 2015 at 6:02 PM, Timothy Washington <twashing at gmail.com <mailto:twashing at gmail.com>> wrote:
> To get started, I'm trying to implement a simple tictactoe game. And I would like to be able to represent a Piece on the board, as either the string "X" or "O". This is what I have so far. 
> 
> module Main where
> 
> data Piece = X | O
> type Row = [Piece]
> type Board = [Row]
> 
> -- put an X or O in a position
> move :: Board -> Piece -> Board
> move board piece = board
>   
> -- check win vertically
> -- check win horizontally
> -- check win diagonally
> 
> main :: IO ()
> main = putStrLn "Hello World"
> 
> 
> A) Now, I'd like to be able to load code interactively, preferably within emacs. However I don't have access to my types with ghci or ghc-mod (Interactive-Haskell). In either case, this call fails with the below error.
> 
> let p = Piece X
> <interactive>:20:9-13: Not in scope: data constructor `Piece'
> 
> Piece is the data TYPE. It has two constructors, X, and O. If you load your module into ghci, you can do ":t X" to get the type of X, which is Piece. You want to do "let p = X" here.
> 
> Check your mode docs while editing the haskell file. There should be  a command to load the current buffer into a ghci running in an interactive emacs buffer.
>  
> B) And how do I make a custom datatype that's one of two strings (enumeration of either "X" or "O"). Cabal builds and runs the abouve code, so I know it can compile. But I'm confused as to where X or O is defined, and how I would supply it as an input. 
> 
> 
> As noted, you can derive "Show" so that Piece types print as "X" and "O". Similarly, you can derive "Read":
> 
> data Piece = X | O deriving (Show, Read)
> 
> so that you can then read "X" and get back an X when the context calls for a Piece value:
> *Main> read "X" :: Piece
> 
> X
> 
> 
> However, this isn't used a lot, as it tends to be slow and not very flexible for more complex types. For instance, it will let you read in lists of Pieces and lists of lists, but you'l lhave to use the list syntax,  not something that looks like an actual board when printed.
> 
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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From vale.cofershabica at gmail.com  Mon Mar 23 19:03:15 2015
From: vale.cofershabica at gmail.com (Vale Cofer-Shabica)
Date: Mon, 23 Mar 2015 15:03:15 -0400
Subject: [Haskell-beginners] suggestions for re-implementing mapM
In-Reply-To: <CAN+Tr43Pv+P-ek-TERAbEBxBSaGptS1s_OoysY1Lvo-z8pVxTQ@mail.gmail.com>
References: <CAAzfV4R1hevE6PfWD+f-YyuVpa7VJRT=jGH_CgYpsN6LUYQnDA@mail.gmail.com>
 <CAN+Tr43Pv+P-ek-TERAbEBxBSaGptS1s_OoysY1Lvo-z8pVxTQ@mail.gmail.com>
Message-ID: <CAAzfV4QVtkne1d46L+=4hoZX_4buM=-smkVQcpe56jy3zEr2tg@mail.gmail.com>

Thanks, David

I appreciate the suggestions.

vale


On Fri, Mar 20, 2015 at 4:07 PM, David McBride <toad3k at gmail.com> wrote:

> Sometimes it is easier to write it with do notation and then rewrite it
> back in normal terms:
>
> mapM :: Monad m => (a -> m b) -> [a] -> m [b]
> mapM _ [] = return []
> mapM f (x:xs) = do
>   x' <- f x
>   xs' <- mapM f xs
>   return $ x':xs'
>
> You can rewrite the second part step by step as:
>
> mapM f (x:xs) = f x >>= \x' -> mapM f xs >>= \xs' -> return (x' : xs')
>
> Also the base package does not use do notation.  It defines it much more
> elegantly:
>
> mapM            :: Monad m => (a -> m b) -> [a] -> m [b]mapM f as       =  sequence (map f as)
>
>
>
>
> On Fri, Mar 20, 2015 at 3:52 PM, Vale Cofer-Shabica <
> vale.cofershabica at gmail.com> wrote:
>
>> Hello all,
>>
>> I've been reading through "Tackling the Awkward Squad" [1] and am
>> implementing the definitions "left as exercises" as I go. For section 2.2,
>> I define:
>>
>>     putLine :: [Char] -> IO ()
>>     putLine :: mapM_ putChar
>>
>> where
>>
>>     mapM_ :: Monad m => (a -> m b) -> a -> m ()
>>     mapM_ f [] = return ()
>>     mapM_ f (x:xs) = (f x) >> (mapM_ f xs)
>>
>> which works without difficulty. For the sake of learning, I decided to
>> implement mapM as well. My definition (below) works, but seems really
>> in-elegant. I checked the prelude source and found mapM defined in terms of
>> sequence, which has some do-notation I'm not so clear on. I'm trying to
>> avoid using do notation in my implementation because it still feels like
>> magic. I'll work on de-sugaring do notation again once I have a solid
>> handle on (>>=), (>>), and return. Any suggestions for cleaning this up
>> would be much appreciated!
>>
>>     mapM :: Monad m => (a -> m b) -> [a] -> m [b]
>>     mapM f [] = return []
>>     mapM f (x:xs) = consMM (f x) (mapM f xs)
>>
>>     consMM :: Monad m => m a -> m [a] -> m [a]
>>     consMM mx mxs = mx >>= ((flip consM)  mxs) where
>>         consM x mxs = mxs>>=(\xs -> return (x:xs))
>>
>> Thank you,
>> vale
>>
>> [1] Suggested by apfelmus in a recent email to the list:
>>
>> http://research.microsoft.com/en-us/um/people/simonpj/papers/marktoberdorf/
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From martin.drautzburg at web.de  Mon Mar 23 20:43:24 2015
From: martin.drautzburg at web.de (martin)
Date: Mon, 23 Mar 2015 21:43:24 +0100
Subject: [Haskell-beginners] Arrow vs. function
Message-ID: <55107AEC.6040705@web.de>

Hello all

I skimmed several Arrow tutorial and looked at the pretty diagrams and thought to myself "Hmm, these guys seem to take
input and produce output".

But that's what a function does. So what's the fundamental difference between an arrow and a fuction?

From johnw at newartisans.com  Mon Mar 23 20:57:29 2015
From: johnw at newartisans.com (John Wiegley)
Date: Mon, 23 Mar 2015 15:57:29 -0500
Subject: [Haskell-beginners] Arrow vs. function
In-Reply-To: <55107AEC.6040705@web.de> (martin's message of "Mon, 23 Mar 2015
 21:43:24 +0100")
References: <55107AEC.6040705@web.de>
Message-ID: <m2oanj4ekm.fsf@newartisans.com>

>>>>> martin  <martin.drautzburg at web.de> writes:

> But that's what a function does. So what's the fundamental difference
> between an arrow and a fuction?

Arrows abstract functions, allowing you to have constructions like Kleisli,
which are Arrows, but compose in the presence of effects using (<=<) rather
than (.).

John

From johnw at newartisans.com  Mon Mar 23 21:11:44 2015
From: johnw at newartisans.com (John Wiegley)
Date: Mon, 23 Mar 2015 16:11:44 -0500
Subject: [Haskell-beginners] Arrow vs. function
In-Reply-To: <m2oanj4ekm.fsf@newartisans.com> (John Wiegley's message of "Mon, 
 23 Mar 2015 15:57:29 -0500")
References: <55107AEC.6040705@web.de> <m2oanj4ekm.fsf@newartisans.com>
Message-ID: <m2fv8v4dwv.fsf@newartisans.com>

>>>>> John Wiegley <johnw at newartisans.com> writes:

> Arrows abstract functions, allowing you to have constructions like Kleisli,
> which are Arrows, but compose in the presence of effects using (<=<) rather
> than (.).

That is, you compose them using (<<<) as you would using any other Arrow, but
the Arrow instance is using a different form of composition under the hood.

John

From mike_k_houghton at yahoo.co.uk  Mon Mar 23 22:06:15 2015
From: mike_k_houghton at yahoo.co.uk (Mike Houghton)
Date: Mon, 23 Mar 2015 22:06:15 +0000
Subject: [Haskell-beginners] Mastermind
Message-ID: <4152A3D0-082D-4485-976F-163B504FAEAB@yahoo.co.uk>

Hi All,

I?m working through http://www.seas.upenn.edu/~cis194/hw/02-lists.pdf <http://www.seas.upenn.edu/~cis194/hw/02-lists.pdf> - the homework for the CIS 194 Haskell course. 
I ?stuck? on question 6! 
If anyone has done this I?d really appreciate a pointer to solving it.

The problem is, given these colors

colors = [Red, Green, Blue, Yellow, Orange, Purple]
we first need to be able to generate a list of all
the codes, ie all length n combinations of the 6 colors. In general, Mastermind games use codes of length 4, however in theory the code could be any length. We have not yet made any assumptions about the lengths of the codes, so why start now? Your function should take in a length and return all Codes of that length:

 allCodes :: Int -> [Code]
Hint: This exercise is a bit tricky. Try using a helper function that takes in all the codes of length n ? 1 and uses it to produce all codes of length n. You may find the concatMap function helpful. 

Now this 
[ [a,b,c,d] | a<-colors, b<-colors, c<-colors, d<-colors]

will work for codes of length 4 but clearly doesn?t provide the general solution.

I?m just not seeing it yet!!!!

Thanks
Mike
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From raabe at froglogic.com  Mon Mar 23 22:46:04 2015
From: raabe at froglogic.com (Frerich Raabe)
Date: Mon, 23 Mar 2015 23:46:04 +0100
Subject: [Haskell-beginners] Mastermind
In-Reply-To: <4152A3D0-082D-4485-976F-163B504FAEAB@yahoo.co.uk>
References: <4152A3D0-082D-4485-976F-163B504FAEAB@yahoo.co.uk>
Message-ID: <91bcc7caa1363aeb662e5cf31b440a22@roundcube.froglogic.com>

Hi Mike,

On 2015-03-23 23:06, Mike Houghton wrote:
> I?m working through http://www.seas.upenn.edu/~cis194/hw/02-lists.pdf [1] - 
> the homework for the CIS 194 Haskell course.
> I ?stuck? on question 6!
> If anyone has done this I?d really appreciate a pointer to solving it.

Just thinking out loud:

Consider that to get all lists of length 2, you could add (e.g. prepend) each 
of the six colors to each of the lists of length 1. And to get each of the 
lists of length 1 you prepend each of the six colors to each of the lists of 
length 0.

Does that help? :-)

-- 
Frerich Raabe - raabe at froglogic.com
www.froglogic.com - Multi-Platform GUI Testing

From ngocdaothanh at gmail.com  Mon Mar 23 22:57:46 2015
From: ngocdaothanh at gmail.com (Ngoc Dao)
Date: Tue, 24 Mar 2015 07:57:46 +0900
Subject: [Haskell-beginners] Mastermind
In-Reply-To: <91bcc7caa1363aeb662e5cf31b440a22@roundcube.froglogic.com>
References: <4152A3D0-082D-4485-976F-163B504FAEAB@yahoo.co.uk>
 <91bcc7caa1363aeb662e5cf31b440a22@roundcube.froglogic.com>
Message-ID: <CAHqXiKksUguR5Q2DRsdsK+u06shLAXGaWt_V2WhtELGf+w794Q@mail.gmail.com>

Mike,

You may have noticed Frerich was saying about recursion.
I will provide you some more topics/keywords so that you can
investigate further:

You should practise writing the recursion in 2 ways:
normal recursion and tail recursion (to avoid stackoverflow when the
recursion depth is large)

For the tail recursion, you use the accumulator pattern, which is very
common in functional programming.

On Tue, Mar 24, 2015 at 7:46 AM, Frerich Raabe <raabe at froglogic.com> wrote:
> Hi Mike,
>
> On 2015-03-23 23:06, Mike Houghton wrote:
>>
>> I?m working through http://www.seas.upenn.edu/~cis194/hw/02-lists.pdf [1]
>> - the homework for the CIS 194 Haskell course.
>> I ?stuck? on question 6!
>> If anyone has done this I?d really appreciate a pointer to solving it.
>
>
> Just thinking out loud:
>
> Consider that to get all lists of length 2, you could add (e.g. prepend)
> each of the six colors to each of the lists of length 1. And to get each of
> the lists of length 1 you prepend each of the six colors to each of the
> lists of length 0.
>
> Does that help? :-)
>
> --
> Frerich Raabe - raabe at froglogic.com
> www.froglogic.com - Multi-Platform GUI Testing
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

From mike_k_houghton at yahoo.co.uk  Mon Mar 23 23:13:46 2015
From: mike_k_houghton at yahoo.co.uk (Mike Houghton)
Date: Mon, 23 Mar 2015 23:13:46 +0000
Subject: [Haskell-beginners] Mastermind
In-Reply-To: <CAHqXiKksUguR5Q2DRsdsK+u06shLAXGaWt_V2WhtELGf+w794Q@mail.gmail.com>
References: <4152A3D0-082D-4485-976F-163B504FAEAB@yahoo.co.uk>
 <91bcc7caa1363aeb662e5cf31b440a22@roundcube.froglogic.com>
 <CAHqXiKksUguR5Q2DRsdsK+u06shLAXGaWt_V2WhtELGf+w794Q@mail.gmail.com>
Message-ID: <65AF3A76-44F2-46C9-82DD-D1E8B0FCEE4C@yahoo.co.uk>

Thanks guys - I think the light has just gone on!
;)

> On 23 Mar 2015, at 22:57, Ngoc Dao <ngocdaothanh at gmail.com> wrote:
> 
> Mike,
> 
> You may have noticed Frerich was saying about recursion.
> I will provide you some more topics/keywords so that you can
> investigate further:
> 
> You should practise writing the recursion in 2 ways:
> normal recursion and tail recursion (to avoid stackoverflow when the
> recursion depth is large)
> 
> For the tail recursion, you use the accumulator pattern, which is very
> common in functional programming.
> 
> On Tue, Mar 24, 2015 at 7:46 AM, Frerich Raabe <raabe at froglogic.com> wrote:
>> Hi Mike,
>> 
>> On 2015-03-23 23:06, Mike Houghton wrote:
>>> 
>>> I?m working through http://www.seas.upenn.edu/~cis194/hw/02-lists.pdf [1]
>>> - the homework for the CIS 194 Haskell course.
>>> I ?stuck? on question 6!
>>> If anyone has done this I?d really appreciate a pointer to solving it.
>> 
>> 
>> Just thinking out loud:
>> 
>> Consider that to get all lists of length 2, you could add (e.g. prepend)
>> each of the six colors to each of the lists of length 1. And to get each of
>> the lists of length 1 you prepend each of the six colors to each of the
>> lists of length 0.
>> 
>> Does that help? :-)
>> 
>> --
>> Frerich Raabe - raabe at froglogic.com
>> www.froglogic.com - Multi-Platform GUI Testing
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners


From michael at orlitzky.com  Mon Mar 23 23:53:25 2015
From: michael at orlitzky.com (Michael Orlitzky)
Date: Mon, 23 Mar 2015 19:53:25 -0400
Subject: [Haskell-beginners] Arrow vs. function
In-Reply-To: <55107AEC.6040705@web.de>
References: <55107AEC.6040705@web.de>
Message-ID: <5510A775.8080700@orlitzky.com>

On 03/23/2015 04:43 PM, martin wrote:
> Hello all
> 
> I skimmed several Arrow tutorial and looked at the pretty diagrams and thought to myself "Hmm, these guys seem to take
> input and produce output".
> 
> But that's what a function does. So what's the fundamental difference between an arrow and a fuction?

They're basically functions, and you won't steer yourself wrong thinking
about them like that.

By analogy, in Haskell, if you wanted to add two floating point numbers
you would need to define a floating point addition:

  floatAdd :: Float -> Float -> Float

Then if you wanted to add two integers, you'd need a second function:

  intAdd :: Int -> Int -> Int

The prelude already abstracts this away for you -- both Int and Float
are instances of the Num class, which gives you,

  (+) :: (Num a) => a -> a -> a

So at that point, you might ask yourself, what's the point of a Num?
Aren't they just (floating point, int, etc.) numbers? The answer is
"yes," and you don't want to have twenty different addition functions
for everything that is some kind of number, so it makes sense to group
all number-things into one typeclass.

Likewise, all function-things are grouped into the Arrow typeclass so
that you can compose them without worrying about the exact type.
Everything in the Arrow typeclass is some kind of function, and you can
compose them with the Arrow class methods.


From ngocdaothanh at gmail.com  Tue Mar 24 01:15:47 2015
From: ngocdaothanh at gmail.com (Ngoc Dao)
Date: Tue, 24 Mar 2015 10:15:47 +0900
Subject: [Haskell-beginners] Mastermind
In-Reply-To: <65AF3A76-44F2-46C9-82DD-D1E8B0FCEE4C@yahoo.co.uk>
References: <4152A3D0-082D-4485-976F-163B504FAEAB@yahoo.co.uk>
 <91bcc7caa1363aeb662e5cf31b440a22@roundcube.froglogic.com>
 <CAHqXiKksUguR5Q2DRsdsK+u06shLAXGaWt_V2WhtELGf+w794Q@mail.gmail.com>
 <65AF3A76-44F2-46C9-82DD-D1E8B0FCEE4C@yahoo.co.uk>
Message-ID: <CAHqXiK=oA721euHs_Hsz0rjUVJud5NdjosoE7+uNRqE0h7sXfA@mail.gmail.com>

Conceptually, you can think like this:
* Recursion is the only way to loop in functional programming.
* Things like map, filter, the sexy list comprehension etc. are just nice
helpers to help you loop without having to write the recursion yourself.
* When you can't see a straight forward way to use the helpers, you can
always fall back to write the recursion yourself.

It's a good exercise to spend about an hour to try to implement the map,
filter etc. yourself, using recursion, to see what their
implementations look like.


On Tuesday, March 24, 2015, Mike Houghton <mike_k_houghton at yahoo.co.uk>
wrote:

> Thanks guys - I think the light has just gone on!
> ;)
>
> > On 23 Mar 2015, at 22:57, Ngoc Dao <ngocdaothanh at gmail.com
> <javascript:;>> wrote:
> >
> > Mike,
> >
> > You may have noticed Frerich was saying about recursion.
> > I will provide you some more topics/keywords so that you can
> > investigate further:
> >
> > You should practise writing the recursion in 2 ways:
> > normal recursion and tail recursion (to avoid stackoverflow when the
> > recursion depth is large)
> >
> > For the tail recursion, you use the accumulator pattern, which is very
> > common in functional programming.
> >
> > On Tue, Mar 24, 2015 at 7:46 AM, Frerich Raabe <raabe at froglogic.com
> <javascript:;>> wrote:
> >> Hi Mike,
> >>
> >> On 2015-03-23 23:06, Mike Houghton wrote:
> >>>
> >>> I?m working through http://www.seas.upenn.edu/~cis194/hw/02-lists.pdf
> [1]
> >>> - the homework for the CIS 194 Haskell course.
> >>> I ?stuck? on question 6!
> >>> If anyone has done this I?d really appreciate a pointer to solving it.
> >>
> >>
> >> Just thinking out loud:
> >>
> >> Consider that to get all lists of length 2, you could add (e.g. prepend)
> >> each of the six colors to each of the lists of length 1. And to get
> each of
> >> the lists of length 1 you prepend each of the six colors to each of the
> >> lists of length 0.
> >>
> >> Does that help? :-)
> >>
> >> --
> >> Frerich Raabe - raabe at froglogic.com <javascript:;>
> >> www.froglogic.com - Multi-Platform GUI Testing
> >> _______________________________________________
> >> Beginners mailing list
> >> Beginners at haskell.org <javascript:;>
> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> > _______________________________________________
> > Beginners mailing list
> > Beginners at haskell.org <javascript:;>
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org <javascript:;>
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From twashing at gmail.com  Tue Mar 24 02:38:24 2015
From: twashing at gmail.com (Timothy Washington)
Date: Mon, 23 Mar 2015 19:38:24 -0700
Subject: [Haskell-beginners] Dipping Toes Into Haskell
In-Reply-To: <B4D9CDA3-C973-4355-873D-90379CAE995E@recursive.ca>
References: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
 <CAD=7U2DFK_T9Z+iWs9S1QtQyZz+yRiNGYeboEcV0pj3GvmWZZQ@mail.gmail.com>
 <CAADtM-aA04b=jTJwhr50k0mO=_YxYqXhEMJv0QG5e8-0Lk4ODA@mail.gmail.com>
 <CAADtM-ZiG-Z2r=uMm22LSjQw54p4j=MvP+psmRzP72zPvn_56Q@mail.gmail.com>
 <B4D9CDA3-C973-4355-873D-90379CAE995E@recursive.ca>
Message-ID: <CAADtM-a7rEN-5p50DY2+zovsVd-++M3qAAmqQt5caZ4W=qCLFA@mail.gmail.com>

Hey man...


On Mon, Mar 23, 2015 at 4:57 AM, Bob Hutchison <hutch-lists at recursive.ca>
wrote:

> Hi Tim,
>
> Straying from clojure? :-)
>
>
Lol, love it!! I'm a free spirit. Can't hold me down :)



> On Mar 22, 2015, at 12:33 PM, Timothy Washington <twashing at gmail.com>
> wrote:
>
> So I've finally had a chance to revisit my tictactoe game.
>
>
> import Control.Lens
>
> data Piece = X | O | E deriving Show
> type Row = [Piece]
> type Board = [Row]
> *data Position = Int Int deriving Show*
>
> -- put an X or O in a position
> *move :: Board -> Piece -> Position -> Board*
> move board piece position = board
>
> main :: IO ()
> main = putStrLn "Hello World"
>
> Now, if I want to make a move, I'll want to set a *piece* on a *position*,
> on a *board*. Let's assume the board has the following shape.
>
> ?> let r1 = (E,E,E)
> ?> let r2 = (E,E,E)
> ?> let r3 = (E,E,E)
> ?> let board = (r1,r2,r3)
> ((E,E,E),(E,E,E),(E,E,E))
>
>
> To return an updated board, I dug around and *i)* couldn't find a core
> Haskell equivalent to Clojure's update-in
> <http://clojuredocs.org/clojure.core/update-in>. *ii)* Zippers only deal
> with trees of left / right nodes. *iii)* So that left me with Control.Lens
> <https://github.com/ekmett/lens/wiki/Examples>. Now I can easily inspect
> and update the board like so.
>
>
> Or you could use an two dimensional array of Positions instead
> (Data.Array).
>

Hmm, probably. But the function's type signature below, seem to be the
first order of business, no?



> ?> board^*._**2._1*
> E
> ?> set (*_2._1*) 42 board
> ((E,E,E),(42,E,E),(E,E,E))
>
>
> I can then parameterize _2 or _1 like so.
>
> ?> let a = _2
> ?> board ^. a
> (E,E,E)
>
>
> You can probably find the type of _1, _2, _3 in the repl, and use that in
> your code below.
>

The types (:t ...) of both of these are:

   - a :: (Field2 s t a b, Functor f) => (a -> f b) -> s -> f t
   - _2 :: (Field2 s t a b, Functor f) => (a -> f b) -> s -> f t



> But I can't figure out how to include that parameter's type into a
> *Position* type definition. I've tried these unsuccessfully.
>
> *data Position = Int Int deriving Show  -- original type definition*
> *data Position = Simple Lens (Int a) (Int a) deriving Show  -- failing try
> 1*
>
>
> These are very suspicious. Did you intend to define constructors ?Int? and
> ?Simple?? Maybe you meant something like:
>
> data Position = Position Int Int deriving Show
>
> In which case you?ll have a type called ?Position? and a constructor with
> the same name.
>
> Cheers,
> Bob
>
>
So ultimately I want a function signature that lets me pass in a lens
position.

-- 1. these 2 don't compile together

data Position = Position Int Int deriving Show

move :: Board -> Piece -> Position -> Board
move board piece position = set (position) piece board



-- 2. and using the (:t ...) type definition abouve, none of these work

move :: Board -> Piece -> ((Field2 s t a b, Functor f) => (a -> f b) -> s
-> f t) -> Board
-- move :: Board -> Piece -> ((a -> f b) -> s -> f t) -> Board
-- move :: Board -> Piece -> (a -> f b) -> Board
-- move :: Board -> Piece -> (s -> f t) -> Board

move board piece position = set (position) piece board


-- 3. so the below code compiles, but doesn't do me much good... I need
Position to be a lens, such that I can use A) *set (position) piece board*
, instead of B) *set (_2._1) 42 board*

module Main where

import Control.Lens

data Piece = X | O | E deriving Show
type Row = [Piece]
type Board = [Row]
*data Position = Int Int deriving Show*

*move :: Board -> Piece -> Position -> Board*
move board piece position = board

main :: IO ()
main = putStrLn "Hello World"




Cheers mate :)

Tim Washington
Interruptsoftware.com <http://interruptsoftware.com/>
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From alt.mcarter at gmail.com  Tue Mar 24 09:48:09 2015
From: alt.mcarter at gmail.com (Mark Carter)
Date: Tue, 24 Mar 2015 09:48:09 +0000
Subject: [Haskell-beginners] Derived data
Message-ID: <551132D9.4030103@gmail.com>

I read inputs from files into various data types. Not all of the 
information is known at input time, so some of the data for a type needs 
to be derived. The question is: what is the best way of "guarding" 
between derived and underived data?

By that I mean, there is a function which will derive missing data, and 
other functions can only work on derived data. Working on underived data 
would be a mistake.

From mdorman at ironicdesign.com  Tue Mar 24 10:21:22 2015
From: mdorman at ironicdesign.com (Michael Alan Dorman)
Date: Tue, 24 Mar 2015 06:21:22 -0400
Subject: [Haskell-beginners] Derived data
In-Reply-To: <551132D9.4030103@gmail.com> (Mark Carter's message of "Tue, 24
 Mar 2015 09:48:09 +0000")
References: <551132D9.4030103@gmail.com>
Message-ID: <87k2y63dct.fsf@ironicdesign.com>

Mark Carter <alt.mcarter at gmail.com> writes:
> I read inputs from files into various data types. Not all of the
> information is known at input time, so some of the data for a type
> needs to be derived. The question is: what is the best way of
> "guarding" between derived and underived data?
>
> By that I mean, there is a function which will derive missing data,
> and other functions can only work on derived data. Working on
> underived data would be a mistake.

That seems like the straight-up definition of two data types:

    data Raw = Raw (Maybe Int) Text (Maybe Text)

    data Cooked = Cooked Text

    oven :: Raw -> Cooked

    -- Can't pass Raw data to consumer
    consumer :: Cooked -> Result

Mike.

From hutch-lists at recursive.ca  Tue Mar 24 13:13:03 2015
From: hutch-lists at recursive.ca (Bob Hutchison)
Date: Tue, 24 Mar 2015 09:13:03 -0400
Subject: [Haskell-beginners] Dipping Toes Into Haskell
In-Reply-To: <CAADtM-a7rEN-5p50DY2+zovsVd-++M3qAAmqQt5caZ4W=qCLFA@mail.gmail.com>
References: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
 <CAD=7U2DFK_T9Z+iWs9S1QtQyZz+yRiNGYeboEcV0pj3GvmWZZQ@mail.gmail.com>
 <CAADtM-aA04b=jTJwhr50k0mO=_YxYqXhEMJv0QG5e8-0Lk4ODA@mail.gmail.com>
 <CAADtM-ZiG-Z2r=uMm22LSjQw54p4j=MvP+psmRzP72zPvn_56Q@mail.gmail.com>
 <B4D9CDA3-C973-4355-873D-90379CAE995E@recursive.ca>
 <CAADtM-a7rEN-5p50DY2+zovsVd-++M3qAAmqQt5caZ4W=qCLFA@mail.gmail.com>
Message-ID: <CB469B54-30F4-4757-9C69-83D9D6160843@recursive.ca>


> On Mar 23, 2015, at 10:38 PM, Timothy Washington <twashing at gmail.com> wrote:
> 

[snip]

> So ultimately I want a function signature that lets me pass in a lens position. 
> 
> -- 1. these 2 don't compile together
> 
> data Position = Position Int Int deriving Show 
> 
> move :: Board -> Piece -> Position -> Board
> move board piece position = set (position) piece board
> 
> 
> -- 2. and using the (:t ...) type definition abouve, none of these work
> 
> move :: Board -> Piece -> ((Field2 s t a b, Functor f) => (a -> f b) -> s -> f t) -> Board
> -- move :: Board -> Piece -> ((a -> f b) -> s -> f t) -> Board
> -- move :: Board -> Piece -> (a -> f b) -> Board
> -- move :: Board -> Piece -> (s -> f t) -> Board
> 
> move board piece position = set (position) piece board
> 
> -- 3. so the below code compiles, but doesn't do me much good... I need Position to be a lens, such that I can use A) set (position) piece board , instead of B) set (_2._1) 42 board
> 
> module Main where
> 
> import Control.Lens
> 
> data Piece = X | O | E deriving Show
> type Row = [Piece]
> type Board = [Row]
> data Position = Int Int deriving Show
> 
> move :: Board -> Piece -> Position -> Board
> move board piece position = board

You seem set on lenses. So if you write that move function like this (in case anyone?s wondering, I happen to know this isn?t a homework question):

data Piece = X | O | E deriving Show

move :: a -> b1 -> ASetter a b a1 b1 -> b
move board position piece = board & position .~ piece

This?ll work. But how are you going to get the position? If you?re given an integer based coordinate, as you seem to want from your definitions of Position, then you?re going to have to do something ugly.

Why not just go with an array and be done with it? Something like this (with your original definition of Piece):

import Data.Array

data Piece' = X | O | E deriving Show
type Position' = (Int,Int)
type Board' = Array Position? Piece'

board' :: Board'
board' = array ((1,1),(3,3)) [((i,j), E) | i <- [1,2,3], j <- [1,2,3]]

move' :: Board' -> Piece' -> Position' -> Board'
move' board piece pos = board // [(pos, piece)]

If you want a slightly less ugly of looking at the board

import qualified Data.List.Split as S

pp board = mapM_ print $ S.chunksOf 3 $ elems board

will display the board something like:

[E,E,E]
[E,E,E]
[E,E,E]

I hope I didn?t say too much.

Cheers,
Bob

> 
> main :: IO ()
> main = putStrLn "Hello World"
> 
> 
> 
> Cheers mate :) 
> 
> Tim Washington 
> Interruptsoftware.com 
>  
> 
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners


From shishir.srivastava at gmail.com  Tue Mar 24 13:24:28 2015
From: shishir.srivastava at gmail.com (Shishir Srivastava)
Date: Tue, 24 Mar 2015 13:24:28 +0000
Subject: [Haskell-beginners] Haskell Compiler
Message-ID: <CALe5RTsuFG_fEzFY9tF3p-yy_5hi9VREJ75qCHCdOPVf8Lj2Bg@mail.gmail.com>

Hi,

What language is the Haskell compiler writter in ? Is it in Haskell itself
just like gcc is written in 'C' ?

Thanks,
Shishir Srivastava
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From sumit.sahrawat.apm13 at iitbhu.ac.in  Tue Mar 24 13:26:10 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Tue, 24 Mar 2015 18:56:10 +0530
Subject: [Haskell-beginners] Haskell Compiler
In-Reply-To: <CALe5RTsuFG_fEzFY9tF3p-yy_5hi9VREJ75qCHCdOPVf8Lj2Bg@mail.gmail.com>
References: <CALe5RTsuFG_fEzFY9tF3p-yy_5hi9VREJ75qCHCdOPVf8Lj2Bg@mail.gmail.com>
Message-ID: <CAJbEW8MhKkkUxZzi=XWWh5h4kvw8zZrQUG1o_J-5z0qe3pUXKA@mail.gmail.com>

https://github.com/ghc/ghc

It's written in haskell. And a lot of other stuff (including C).

On 24 March 2015 at 18:54, Shishir Srivastava <shishir.srivastava at gmail.com>
wrote:

> Hi,
>
> What language is the Haskell compiler writter in ? Is it in Haskell itself
> just like gcc is written in 'C' ?
>
> Thanks,
> Shishir Srivastava
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>


-- 
Regards

Sumit Sahrawat
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From abahadirdogan at gmail.com  Tue Mar 24 13:32:26 2015
From: abahadirdogan at gmail.com (=?UTF-8?B?QmFoYWTEsXIgRG/En2Fu?=)
Date: Tue, 24 Mar 2015 15:32:26 +0200
Subject: [Haskell-beginners] Haskell Compiler
In-Reply-To: <CALe5RTsuFG_fEzFY9tF3p-yy_5hi9VREJ75qCHCdOPVf8Lj2Bg@mail.gmail.com>
References: <CALe5RTsuFG_fEzFY9tF3p-yy_5hi9VREJ75qCHCdOPVf8Lj2Bg@mail.gmail.com>
Message-ID: <CAO9tUsRT0Vk3rjJf2gPV9e3rmV1Y7zhFB4GQb2GXQXVf+qMJHA@mail.gmail.com>

For a deeper understanding:
http://www.aosabook.org/en/ghc.html

On Tue, Mar 24, 2015 at 3:24 PM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Hi,
>
> What language is the Haskell compiler writter in ? Is it in Haskell itself
> just like gcc is written in 'C' ?
>
> Thanks,
> Shishir Srivastava
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From mwm at mired.org  Tue Mar 24 13:40:21 2015
From: mwm at mired.org (Mike Meyer)
Date: Tue, 24 Mar 2015 08:40:21 -0500
Subject: [Haskell-beginners] Haskell Compiler
In-Reply-To: <CALe5RTsuFG_fEzFY9tF3p-yy_5hi9VREJ75qCHCdOPVf8Lj2Bg@mail.gmail.com>
References: <CALe5RTsuFG_fEzFY9tF3p-yy_5hi9VREJ75qCHCdOPVf8Lj2Bg@mail.gmail.com>
Message-ID: <CAD=7U2B0t3FFJt7sKVjWyT-9e_iM4XTaEzcSumg-Jh_DBrkFPg@mail.gmail.com>

There's more than one Haskell compiler, just like there's more than one C
compiler. As others have noted, the most popular one (ghc) is written in
haskell. You can find a list of implementations in the wiki:
https://wiki.haskell.org/Implementations.

On Tue, Mar 24, 2015 at 8:24 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Hi,
>
> What language is the Haskell compiler writter in ? Is it in Haskell itself
> just like gcc is written in 'C' ?
>
> Thanks,
> Shishir Srivastava
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From twashing at gmail.com  Wed Mar 25 03:37:42 2015
From: twashing at gmail.com (Timothy Washington)
Date: Tue, 24 Mar 2015 20:37:42 -0700
Subject: [Haskell-beginners] Dipping Toes Into Haskell
In-Reply-To: <CB469B54-30F4-4757-9C69-83D9D6160843@recursive.ca>
References: <CAADtM-b4=WpP+pn0KbgRN=6Xv-3Y2Axh3OSjS81YvGVg+7EXKw@mail.gmail.com>
 <CAD=7U2DFK_T9Z+iWs9S1QtQyZz+yRiNGYeboEcV0pj3GvmWZZQ@mail.gmail.com>
 <CAADtM-aA04b=jTJwhr50k0mO=_YxYqXhEMJv0QG5e8-0Lk4ODA@mail.gmail.com>
 <CAADtM-ZiG-Z2r=uMm22LSjQw54p4j=MvP+psmRzP72zPvn_56Q@mail.gmail.com>
 <B4D9CDA3-C973-4355-873D-90379CAE995E@recursive.ca>
 <CAADtM-a7rEN-5p50DY2+zovsVd-++M3qAAmqQt5caZ4W=qCLFA@mail.gmail.com>
 <CB469B54-30F4-4757-9C69-83D9D6160843@recursive.ca>
Message-ID: <CAADtM-ZxTYa0jBF0cfaX-W+HtSRhpsaZHczgGMC+FEUj6C54MQ@mail.gmail.com>

Crickey. You're still the man, lol :)


On Tue, Mar 24, 2015 at 6:13 AM, Bob Hutchison <hutch-lists at recursive.ca>
wrote:

>
> > On Mar 23, 2015, at 10:38 PM, Timothy Washington <twashing at gmail.com>
> wrote:
> >
>
> [snip]
>
> ...
>
> You seem set on lenses. So if you write that move function like this (in
> case anyone?s wondering, I happen to know this isn?t a homework question):
>
> data Piece = X | O | E deriving Show
>
> move :: a -> b1 -> ASetter a b a1 b1 -> b
> move board position piece = board & position .~ piece
>
> This?ll work. But how are you going to get the position? If you?re given
> an integer based coordinate, as you seem to want from your definitions of
> Position, then you?re going to have to do something ugly.
>

This is a good point. I'm not particularly attached to lenses. That was
just the conclusion I reached, in order to reach into and manipulate my
original nested lists. I didn't see a way to do nested list updates in
Data.List, on hackage
<https://hackage.haskell.org/package/base-4.7.0.2/docs/Data-List.html>. But
now I see the difference between that and Data.Array
<https://hackage.haskell.org/package/array-0.5.0.0/docs/Data-Array.html>
(indexable,
etc).

I'll come back to Lenses later on, just because now I'm curious. I'm happy
for the Position to be "*data Position = SomeLensType SomeLensType deriving
Show*", where "*:t _2 => SomeLensType*". Or something else that
accommodates lens' data types.... but baby steps :)


Why not just go with an array and be done with it? Something like this
> (with your original definition of Piece):
>
> import Data.Array
>
> data Piece' = X | O | E deriving Show
> type Position' = (Int,Int)
> type Board' = Array Position? Piece'
>
> board' :: Board'
> board' = array ((1,1),(3,3)) [((i,j), E) | i <- [1,2,3], j <- [1,2,3]]
>
> move' :: Board' -> Piece' -> Position' -> Board'
> move' board piece pos = board // [(pos, piece)]
>

This works swimmingly. So excellent point.


If you want a slightly less ugly of looking at the board
>
> import qualified Data.List.Split as S
>
> pp board = mapM_ print $ S.chunksOf 3 $ elems board
>
> will display the board something like:
>
> [E,E,E]
> [E,E,E]
> [E,E,E]
>
> I hope I didn?t say too much.
>

Not at all. These are exactly the kinds of hints that I need. In fact, with
the *//* and *array* functions, this makes a lot of sense. And I used that
and the board beautifier chunk, to create this.

import Data.Array
import qualified Data.List.Split as S

data Piece' = X | O | E deriving Show
type Position' = (Int,Int)
type Board' = Array Position' Piece'

board' :: Board'
board' = array ((1,1),(3,3)) [((i,j), E) | i <- [1,2,3], j <- [1,2,3]]

ppb' :: Board' -> IO()
ppb' b = mapM_ print $ S.chunksOf 3 $ elems b

move' :: Board' -> Piece' -> Position' -> Board'
move' board piece pos = board // [(pos, piece)]


Now, I can pass in the board, piece, and position, to get my expected
result.


?> board'
array ((1,1),(3,3))
[((1,1),E),((1,2),E),((1,3),E),((2,1),E),((2,2),E),((2,3),E),((3,1),E),((3,2),E),((3,3),E)]

?> let b1 = move' board' *X* (1,3)
?> b1
array ((1,1),(3,3))
[((1,1),E),((1,2),E),((1,3),X),((2,1),E),((2,2),E),((2,3),E),((3,1),E),((3,2),E),((3,3),E)]

?> ppb' b1
[E,E,*X*]
[E,E,E]
[E,E,E]



Cheers,
> Bob



Cool !
Cheers :)

Tim Washington
Interruptsoftware.com <http://interruptsoftware.com/>
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From shishir.srivastava at gmail.com  Wed Mar 25 13:55:11 2015
From: shishir.srivastava at gmail.com (Shishir Srivastava)
Date: Wed, 25 Mar 2015 13:55:11 +0000
Subject: [Haskell-beginners] Empty list
Message-ID: <CALe5RTvbkq3rYPXL2AWabxroXy_OFssAhQ899bUu1KZSD1szDw@mail.gmail.com>

Hi,

Can someone please explain why this results in error -

[] `elem` [1,2,3]

Shouldn't the empty set by definition be the element of all sets including
a non-empty set ?

I am assuming 'Lists' are different from 'Sets' in Haskell, if yes, is
there a separate module for dealing/working with sets ?

Thanks,
Shishir Srivastava
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From timmelzer at gmail.com  Wed Mar 25 14:00:02 2015
From: timmelzer at gmail.com (Norbert Melzer)
Date: Wed, 25 Mar 2015 15:00:02 +0100
Subject: [Haskell-beginners] Empty list
In-Reply-To: <CALe5RTvbkq3rYPXL2AWabxroXy_OFssAhQ899bUu1KZSD1szDw@mail.gmail.com>
References: <CALe5RTvbkq3rYPXL2AWabxroXy_OFssAhQ899bUu1KZSD1szDw@mail.gmail.com>
Message-ID: <CA+bCVsu9Do3_Z6KzW6D4VBjYNQz1OG7nhfw8j_NF9Rj9ioNDDA@mail.gmail.com>

You are correct, a list is not a set. A list is a list of things, that can
be there multiple times. A set is a set of things, where nothing can be
twice. So take a look at Data.Set
Am 25.03.2015 14:55 schrieb "Shishir Srivastava" <
shishir.srivastava at gmail.com>:

> Hi,
>
> Can someone please explain why this results in error -
>
> [] `elem` [1,2,3]
>
> Shouldn't the empty set by definition be the element of all sets including
> a non-empty set ?
>
> I am assuming 'Lists' are different from 'Sets' in Haskell, if yes, is
> there a separate module for dealing/working with sets ?
>
> Thanks,
> Shishir Srivastava
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From shishir.srivastava at gmail.com  Wed Mar 25 14:02:34 2015
From: shishir.srivastava at gmail.com (Shishir Srivastava)
Date: Wed, 25 Mar 2015 14:02:34 +0000
Subject: [Haskell-beginners] Monoids and Groups
Message-ID: <CALe5RTu8pd8y1F=PukUec-r2uEfkczZRhKcQgOcPvfB9ugGquA@mail.gmail.com>

Hi,

Reading about Monoids it seems they derive a lot on the algebraic
structures of 'Groups' ?

Is it then correct to assume that Monoids can be used to represent 'Groups'
?

If not are there any standard haskell libraries which represent algebraic
structures such as 'Groups' , 'Fields' etc.

Thanks,
Shishir Srivastava
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From allbery.b at gmail.com  Wed Mar 25 14:03:29 2015
From: allbery.b at gmail.com (Brandon Allbery)
Date: Wed, 25 Mar 2015 10:03:29 -0400
Subject: [Haskell-beginners] Empty list
In-Reply-To: <CA+bCVsu9Do3_Z6KzW6D4VBjYNQz1OG7nhfw8j_NF9Rj9ioNDDA@mail.gmail.com>
References: <CALe5RTvbkq3rYPXL2AWabxroXy_OFssAhQ899bUu1KZSD1szDw@mail.gmail.com>
 <CA+bCVsu9Do3_Z6KzW6D4VBjYNQz1OG7nhfw8j_NF9Rj9ioNDDA@mail.gmail.com>
Message-ID: <CAKFCL4V4HeUSnEJhoeWo8BcqenFgcmr=pQifjkimzMa7=CCNCQ@mail.gmail.com>

On Wed, Mar 25, 2015 at 10:00 AM, Norbert Melzer <timmelzer at gmail.com>
wrote:

> You are correct, a list is not a set. A list is a list of things, that can
> be there multiple times. A set is a set of things, where nothing can be
> twice. So take a look at Data.Set
>

Note that this won't actually solve the original problem; Haskell is an
implementation of a strongly typed lambda calculus, not of number theory,
and Haskell collections cannot (easily) contain elements of different types
--- so the empty set is not an element of a set, and the empty list is not
an element of a list.

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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From allbery.b at gmail.com  Wed Mar 25 14:07:17 2015
From: allbery.b at gmail.com (Brandon Allbery)
Date: Wed, 25 Mar 2015 10:07:17 -0400
Subject: [Haskell-beginners] Monoids and Groups
In-Reply-To: <CALe5RTu8pd8y1F=PukUec-r2uEfkczZRhKcQgOcPvfB9ugGquA@mail.gmail.com>
References: <CALe5RTu8pd8y1F=PukUec-r2uEfkczZRhKcQgOcPvfB9ugGquA@mail.gmail.com>
Message-ID: <CAKFCL4WjR0hkd5VCvc57dYgOPfisbYGwgfwNpMdPep2mjXZE=A@mail.gmail.com>

On Wed, Mar 25, 2015 at 10:02 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Reading about Monoids it seems they derive a lot on the algebraic
> structures of 'Groups' ?
>

A monoid is a semigroup with an identity element, and as such inherits much
of its behavior from semigroups.

For historical reasons, a Haskell Monoid is not based on a notional Haskell
Semigroup. There are packages that add semigroups and other algebraic
structures, and even alternative Preludes that provide a reasonably
complete set of algebraic structures. Every so often you'll see
bikeshedding in the Haskell community over whether the default Prelude
should provide some or all of these. :)

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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From mwm at mired.org  Wed Mar 25 14:07:16 2015
From: mwm at mired.org (Mike Meyer)
Date: Wed, 25 Mar 2015 09:07:16 -0500
Subject: [Haskell-beginners] Empty list
In-Reply-To: <CALe5RTvbkq3rYPXL2AWabxroXy_OFssAhQ899bUu1KZSD1szDw@mail.gmail.com>
References: <CALe5RTvbkq3rYPXL2AWabxroXy_OFssAhQ899bUu1KZSD1szDw@mail.gmail.com>
Message-ID: <CAD=7U2A2Zx1vbDiKaHANz7857H5fPYzJuSk0drGOpL-LTZAJ_w@mail.gmail.com>

On Wed, Mar 25, 2015 at 8:55 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Hi,
>
> Can someone please explain why this results in error -
>
> [] `elem` [1,2,3]
>
> Shouldn't the empty set by definition be the element of all sets including
> a non-empty set ?
>

Norbert gave you the information about Lists and Data.Set, but you're wrong
about the properties of the empty set. The empty set is a SUBSET of all
sets. But it's only a MEMBER of supersets of {{}}.
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From joel.s.williamson at gmail.com  Wed Mar 25 14:10:05 2015
From: joel.s.williamson at gmail.com (Joel Williamson)
Date: Wed, 25 Mar 2015 14:10:05 +0000
Subject: [Haskell-beginners] Empty list
In-Reply-To: <CAKFCL4V4HeUSnEJhoeWo8BcqenFgcmr=pQifjkimzMa7=CCNCQ@mail.gmail.com>
References: <CALe5RTvbkq3rYPXL2AWabxroXy_OFssAhQ899bUu1KZSD1szDw@mail.gmail.com>
 <CA+bCVsu9Do3_Z6KzW6D4VBjYNQz1OG7nhfw8j_NF9Rj9ioNDDA@mail.gmail.com>
 <CAKFCL4V4HeUSnEJhoeWo8BcqenFgcmr=pQifjkimzMa7=CCNCQ@mail.gmail.com>
Message-ID: <CAJGxSeqLbLHY7nVb3++YWWrGVpP+HaA6zDRq=bCJKGguuty=7w@mail.gmail.com>

Shishir, you need to distinguish between membership (1 is an element of
[1,2,3], [] is not) and subsets (both [1] and [] are subsets of [1,2,3]).
elem checks the first property, intersection checks the second.

On Wed, 25 Mar 2015 10:03 Brandon Allbery <allbery.b at gmail.com> wrote:

> On Wed, Mar 25, 2015 at 10:00 AM, Norbert Melzer <timmelzer at gmail.com>
> wrote:
>
>> You are correct, a list is not a set. A list is a list of things, that
>> can be there multiple times. A set is a set of things, where nothing can be
>> twice. So take a look at Data.Set
>>
>
> Note that this won't actually solve the original problem; Haskell is an
> implementation of a strongly typed lambda calculus, not of number theory,
> and Haskell collections cannot (easily) contain elements of different types
> --- so the empty set is not an element of a set, and the empty list is not
> an element of a list.
>
> --
> brandon s allbery kf8nh                               sine nomine
> associates
> allbery.b at gmail.com
> ballbery at sinenomine.net
> unix, openafs, kerberos, infrastructure, xmonad
> http://sinenomine.net
>  _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From mwm at mired.org  Wed Mar 25 14:11:07 2015
From: mwm at mired.org (Mike Meyer)
Date: Wed, 25 Mar 2015 09:11:07 -0500
Subject: [Haskell-beginners] Empty list
In-Reply-To: <CAKFCL4V4HeUSnEJhoeWo8BcqenFgcmr=pQifjkimzMa7=CCNCQ@mail.gmail.com>
References: <CALe5RTvbkq3rYPXL2AWabxroXy_OFssAhQ899bUu1KZSD1szDw@mail.gmail.com>
 <CA+bCVsu9Do3_Z6KzW6D4VBjYNQz1OG7nhfw8j_NF9Rj9ioNDDA@mail.gmail.com>
 <CAKFCL4V4HeUSnEJhoeWo8BcqenFgcmr=pQifjkimzMa7=CCNCQ@mail.gmail.com>
Message-ID: <CAD=7U2Au1hbbzzDYqb6x92n2JendQd-dJ9e17dnzo7FEtrVmxA@mail.gmail.com>

On Wed, Mar 25, 2015 at 9:03 AM, Brandon Allbery <allbery.b at gmail.com>
wrote:

> On Wed, Mar 25, 2015 at 10:00 AM, Norbert Melzer <timmelzer at gmail.com>
> wrote:
>
>> You are correct, a list is not a set. A list is a list of things, that
>> can be there multiple times. A set is a set of things, where nothing can be
>> twice. So take a look at Data.Set
>>
>
> Note that this won't actually solve the original problem; Haskell is an
> implementation of a strongly typed lambda calculus, not of number theory,
> and Haskell collections cannot (easily) contain elements of different types
> --- so the empty set is not an element of a set, and the empty list is not
> an element of a list.
>

Did you forget a couple of "necessarily"'s?

Prelude Data.Set> [] `elem` [[]]
True
Prelude Data.Set> empty `member` (insert empty empty)
True
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From max.voit+mlhb at with-eyes.net  Wed Mar 25 14:12:49 2015
From: max.voit+mlhb at with-eyes.net (Max Voit)
Date: Wed, 25 Mar 2015 15:12:49 +0100
Subject: [Haskell-beginners] Empty list
In-Reply-To: <CALe5RTvbkq3rYPXL2AWabxroXy_OFssAhQ899bUu1KZSD1szDw@mail.gmail.com>
References: <CALe5RTvbkq3rYPXL2AWabxroXy_OFssAhQ899bUu1KZSD1szDw@mail.gmail.com>
Message-ID: <20150325151249.6f13bb54@veeloqu.lan>

Hi Shishir,

(in the following I'm ignoring the problem that lists have order and
elements may be in there multiple times) 

On Wed, 25 Mar 2015 13:55:11 +0000
Shishir Srivastava <shishir.srivastava at gmail.com> wrote:

> Shouldn't the empty set by definition be the element of all sets
> including a non-empty set ?

This is not even true in the mathematical sense. True are:
	a) all power sets include the empty set as an *element*
	b) the empty set is a *subset* of all sets

For case (a) you'd need a list of lists since a powerset is a set of
sets.
For (b) `elem` is not the correct function.

Max

From adam at adamflott.com  Wed Mar 25 15:37:52 2015
From: adam at adamflott.com (Adam Flott)
Date: Wed, 25 Mar 2015 11:37:52 -0400
Subject: [Haskell-beginners] parsec and a never terminating parser
Message-ID: <20150325113752.a6e3f7256ffb4fe9fecb6ddb@adamflott.com>

I'm having trouble understanding why my simple parser never terminates when
specific input is used.

For example, let's say the first column is a field which can be in one of 4
states, empty, omitted, other, and any arbitrary value. That is,

    data FieldState a = EmptyState | OmittedState | OtherState | FullState a
      deriving (Eq, Ord)

When attempting to use,

    $ echo "- " | ./parser
    "- \n"
    empty ('-')
    $ echo "^ " | ./parser
    "^ \n"
    omitted ('^')
    $ echo "~ " | ./parser
    "~ \n"
    other ('~')
    [ all of this is as expected ]
    $ echo "1 " | ./parser
    "1 \n"
    [ computer twiddles it's thumbs here until I manually terminate it ... ]
    ^C^C
    $

Does anyone know what's happening and now to alleviate it?


-- begin full code --

-- base
import Control.Applicative
import Data.Word

-- Hackage
import qualified Data.Text.Lazy as TL
import qualified Data.Text.Lazy.IO as TLIO
import Text.Parsec (parse)
import Text.Parsec.Text.Lazy (Parser)
import Text.Parser.Combinators
import Text.Parser.Char

data FieldState a = EmptyState | OmittedState | OtherState | FullState a
  deriving (Eq, Ord)

instance Functor FieldState where
    fmap f (FullState a)    = FullState (f a)
    fmap _ EmptyState       = EmptyState
    fmap _ OmittedState     = OmittedState
    fmap _ OtherState       = OtherState
    
instance Applicative FieldState where
    pure = FullState
    (FullState f) <*> (FullState x) = FullState (f x)
    _             <*> _             = EmptyState
    
instance Monad FieldState where
    (FullState x) >>= k = k x
    EmptyState    >>= _ = EmptyState
    OmittedState  >>= _ = OmittedState
    OtherState    >>= _ = OtherState

    (FullState _) >> k = k
    EmptyState    >> _ = EmptyState
    OmittedState  >> _ = OmittedState
    OtherState    >> _ = OtherState

    return  = FullState
    fail _  = EmptyState

instance Show (FieldState x) where
    show (EmptyState)   = "empty ('-')"
    show (OmittedState) = "omitted ('^')"
    show (OtherState)   = "other ('~')"
    show x' = show x'
    
data Counter = Counter Word64 deriving (Eq, Ord, Show)

parseNum :: (Num a) => Parser a
parseNum = do
    n <- rd <$> many digit
    return $ fromIntegral n
    where rd = read :: String -> Integer

parseCounter :: Parser Counter
parseCounter = Counter <$> parseNum

parseFieldStateOff :: Parser Char
parseFieldStateOff = char '-'

parseFieldStateOmitted :: Parser Char
parseFieldStateOmitted = char '^'

parseFieldStateOther :: Parser Char
parseFieldStateOther = char '~'

parseFieldState :: Parser a -> Parser (FieldState a)
parseFieldState p = (parseFieldStateOff >> return EmptyState)
                  <|> (parseFieldStateOmitted >> return OmittedState)
                  <|> (parseFieldStateOther >> return OtherState)
                  <|> (p >>= return . FullState)

main :: IO ()
main = do
    ls <- TLIO.getContents
    print ls
    mapM_ processLine (TL.lines ls)

processLine :: TL.Text -> IO ()
processLine line = case (parse (parseFieldState parseCounter) "" line) of
  Left err -> print err
  Right xs -> print xs
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From martin.drautzburg at web.de  Wed Mar 25 17:51:48 2015
From: martin.drautzburg at web.de (martin)
Date: Wed, 25 Mar 2015 18:51:48 +0100
Subject: [Haskell-beginners] Open-ended list comprehension
Message-ID: <5512F5B4.80203@web.de>

Hello all,

when I write my own recursions, I often end up with something like

a : b : c : (f x)

where f x returns a list. I assume this avoids the costly (++) function, right?

But when I use list comprehensions I always get a complete list and it is costly to append something to it. How can I
use the power of list comprehensions, but return an [a]->[a] instead of an [a]?

Is this a good idea at all, or will I pile up thunks?




From idivyanshu.ranjan at gmail.com  Wed Mar 25 18:00:54 2015
From: idivyanshu.ranjan at gmail.com (divyanshu ranjan)
Date: Wed, 25 Mar 2015 23:30:54 +0530
Subject: [Haskell-beginners] Open-ended list comprehension
In-Reply-To: <5512F5B4.80203@web.de>
References: <5512F5B4.80203@web.de>
Message-ID: <CAL9hw24PMGztqVAQxmUR7qfRDROHnfR80cYyhch1PZfjjbo-=A@mail.gmail.com>

Hi martin,

Check dlist (https://hackage.haskell.org/package/dlist-0.7.1.1).

On Wed, Mar 25, 2015 at 11:21 PM, martin <martin.drautzburg at web.de> wrote:

> Hello all,
>
> when I write my own recursions, I often end up with something like
>
> a : b : c : (f x)
>
> where f x returns a list. I assume this avoids the costly (++) function,
> right?
>
> But when I use list comprehensions I always get a complete list and it is
> costly to append something to it. How can I
> use the power of list comprehensions, but return an [a]->[a] instead of an
> [a]?
>
> Is this a good idea at all, or will I pile up thunks?
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From martin.drautzburg at web.de  Wed Mar 25 18:20:43 2015
From: martin.drautzburg at web.de (martin)
Date: Wed, 25 Mar 2015 19:20:43 +0100
Subject: [Haskell-beginners] Open-ended list comprehension
In-Reply-To: <CAL9hw24PMGztqVAQxmUR7qfRDROHnfR80cYyhch1PZfjjbo-=A@mail.gmail.com>
References: <5512F5B4.80203@web.de>
 <CAL9hw24PMGztqVAQxmUR7qfRDROHnfR80cYyhch1PZfjjbo-=A@mail.gmail.com>
Message-ID: <5512FC7B.8040407@web.de>

Am 03/25/2015 um 07:00 PM schrieb divyanshu ranjan:
> Hi martin,
> 
> Check dlist (https://hackage.haskell.org/package/dlist-0.7.1.1). 

I cannot see how I can create a dlist via a list comprehension, other than running fromList. But I assume this already
has o(n) complexity. It may pay after several appends, though.

What I am looking for is, if I have:

veryLongList = [x | x <- ...]

how can I use list a comprehension and end up with a dlist?




> 
> On Wed, Mar 25, 2015 at 11:21 PM, martin <martin.drautzburg at web.de <mailto:martin.drautzburg at web.de>> wrote:
> 
>     Hello all,
> 
>     when I write my own recursions, I often end up with something like
> 
>     a : b : c : (f x)
> 
>     where f x returns a list. I assume this avoids the costly (++) function, right?
> 
>     But when I use list comprehensions I always get a complete list and it is costly to append something to it. How can I
>     use the power of list comprehensions, but return an [a]->[a] instead of an [a]?
> 
>     Is this a good idea at all, or will I pile up thunks?
> 
> 
> 
>     _______________________________________________
>     Beginners mailing list
>     Beginners at haskell.org <mailto:Beginners at haskell.org>
>     http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> 
> 
> 
> 
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> 


From marcin.jan.mrotek at gmail.com  Wed Mar 25 18:42:06 2015
From: marcin.jan.mrotek at gmail.com (Marcin Mrotek)
Date: Wed, 25 Mar 2015 19:42:06 +0100
Subject: [Haskell-beginners] Open-ended list comprehension
In-Reply-To: <5512FC7B.8040407@web.de>
References: <5512F5B4.80203@web.de>
 <CAL9hw24PMGztqVAQxmUR7qfRDROHnfR80cYyhch1PZfjjbo-=A@mail.gmail.com>
 <5512FC7B.8040407@web.de>
Message-ID: <CAJcfPz=NBZG_XMop2Zxfc5Y+pesX+Su291iEb+hwM-VOPDxSSg@mail.gmail.com>

Hello,

DList is a monad, so you can use them like lists with the
MonadComprehensions extension. Alternatively, make use of the
MonadPlus functions, which give you the same power without the nice
syntax.

Best regards,
Marcin MRotek

From toad3k at gmail.com  Wed Mar 25 21:10:00 2015
From: toad3k at gmail.com (David McBride)
Date: Wed, 25 Mar 2015 17:10:00 -0400
Subject: [Haskell-beginners] parsec and a never terminating parser
In-Reply-To: <20150325113752.a6e3f7256ffb4fe9fecb6ddb@adamflott.com>
References: <20150325113752.a6e3f7256ffb4fe9fecb6ddb@adamflott.com>
Message-ID: <CAN+Tr43Gj_xaFbQ70n6u4QyjpvPkaNYmXs8vD5-DbFe6xJUJ_A@mail.gmail.com>

The problem is your show instance.

show x' = show x', means that when x' is a FullState, it shows it, which
causes an infinite loop.

You need to take out the default case and add something for FullState.

On Wed, Mar 25, 2015 at 11:37 AM, Adam Flott <adam at adamflott.com> wrote:

> I'm having trouble understanding why my simple parser never terminates when
> specific input is used.
>
> For example, let's say the first column is a field which can be in one of 4
> states, empty, omitted, other, and any arbitrary value. That is,
>
>     data FieldState a = EmptyState | OmittedState | OtherState | FullState
> a
>       deriving (Eq, Ord)
>
> When attempting to use,
>
>     $ echo "- " | ./parser
>     "- \n"
>     empty ('-')
>     $ echo "^ " | ./parser
>     "^ \n"
>     omitted ('^')
>     $ echo "~ " | ./parser
>     "~ \n"
>     other ('~')
>     [ all of this is as expected ]
>     $ echo "1 " | ./parser
>     "1 \n"
>     [ computer twiddles it's thumbs here until I manually terminate it ...
> ]
>     ^C^C
>     $
>
> Does anyone know what's happening and now to alleviate it?
>
>
> -- begin full code --
>
> -- base
> import Control.Applicative
> import Data.Word
>
> -- Hackage
> import qualified Data.Text.Lazy as TL
> import qualified Data.Text.Lazy.IO as TLIO
> import Text.Parsec (parse)
> import Text.Parsec.Text.Lazy (Parser)
> import Text.Parser.Combinators
> import Text.Parser.Char
>
> data FieldState a = EmptyState | OmittedState | OtherState | FullState a
>   deriving (Eq, Ord)
>
> instance Functor FieldState where
>     fmap f (FullState a)    = FullState (f a)
>     fmap _ EmptyState       = EmptyState
>     fmap _ OmittedState     = OmittedState
>     fmap _ OtherState       = OtherState
>
> instance Applicative FieldState where
>     pure = FullState
>     (FullState f) <*> (FullState x) = FullState (f x)
>     _             <*> _             = EmptyState
>
> instance Monad FieldState where
>     (FullState x) >>= k = k x
>     EmptyState    >>= _ = EmptyState
>     OmittedState  >>= _ = OmittedState
>     OtherState    >>= _ = OtherState
>
>     (FullState _) >> k = k
>     EmptyState    >> _ = EmptyState
>     OmittedState  >> _ = OmittedState
>     OtherState    >> _ = OtherState
>
>     return  = FullState
>     fail _  = EmptyState
>
> instance Show (FieldState x) where
>     show (EmptyState)   = "empty ('-')"
>     show (OmittedState) = "omitted ('^')"
>     show (OtherState)   = "other ('~')"
>     show x' = show x'
>
> data Counter = Counter Word64 deriving (Eq, Ord, Show)
>
> parseNum :: (Num a) => Parser a
> parseNum = do
>     n <- rd <$> many digit
>     return $ fromIntegral n
>     where rd = read :: String -> Integer
>
> parseCounter :: Parser Counter
> parseCounter = Counter <$> parseNum
>
> parseFieldStateOff :: Parser Char
> parseFieldStateOff = char '-'
>
> parseFieldStateOmitted :: Parser Char
> parseFieldStateOmitted = char '^'
>
> parseFieldStateOther :: Parser Char
> parseFieldStateOther = char '~'
>
> parseFieldState :: Parser a -> Parser (FieldState a)
> parseFieldState p = (parseFieldStateOff >> return EmptyState)
>                   <|> (parseFieldStateOmitted >> return OmittedState)
>                   <|> (parseFieldStateOther >> return OtherState)
>                   <|> (p >>= return . FullState)
>
> main :: IO ()
> main = do
>     ls <- TLIO.getContents
>     print ls
>     mapM_ processLine (TL.lines ls)
>
> processLine :: TL.Text -> IO ()
> processLine line = case (parse (parseFieldState parseCounter) "" line) of
>   Left err -> print err
>   Right xs -> print xs
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From adam at adamflott.com  Wed Mar 25 21:53:46 2015
From: adam at adamflott.com (Adam Flott)
Date: Wed, 25 Mar 2015 17:53:46 -0400
Subject: [Haskell-beginners] parsec and a never terminating parser
In-Reply-To: <CAN+Tr43Gj_xaFbQ70n6u4QyjpvPkaNYmXs8vD5-DbFe6xJUJ_A@mail.gmail.com>
References: <20150325113752.a6e3f7256ffb4fe9fecb6ddb@adamflott.com>
 <CAN+Tr43Gj_xaFbQ70n6u4QyjpvPkaNYmXs8vD5-DbFe6xJUJ_A@mail.gmail.com>
Message-ID: <20150325175346.3681c6cce129f507c8d1a769@adamflott.com>

On Wed, 25 Mar 2015 17:10:00 -0400
David McBride <toad3k at gmail.com> wrote:

> The problem is your show instance.
> 
> show x' = show x', means that when x' is a FullState, it shows it, which
> causes an infinite loop.
> 
> You need to take out the default case and add something for FullState.
> 

Doh! Nice catch. Thanks David!

For those keeping track, here's how to fix such a case:

    instance Show x => Show (FieldState x) where
        show (EmptyState)   = "empty ('-')"
        show (OmittedState) = "omitted ('^')"
        show (OtherState)   = "other ('~')"
        show (FullState x') = show x'


> On Wed, Mar 25, 2015 at 11:37 AM, Adam Flott <adam at adamflott.com> wrote:
> 
> > I'm having trouble understanding why my simple parser never terminates when
> > specific input is used.
> >
> > For example, let's say the first column is a field which can be in one of 4
> > states, empty, omitted, other, and any arbitrary value. That is,
> >
> >     data FieldState a = EmptyState | OmittedState | OtherState | FullState
> > a
> >       deriving (Eq, Ord)
> >
> > When attempting to use,
> >
> >     $ echo "- " | ./parser
> >     "- \n"
> >     empty ('-')
> >     $ echo "^ " | ./parser
> >     "^ \n"
> >     omitted ('^')
> >     $ echo "~ " | ./parser
> >     "~ \n"
> >     other ('~')
> >     [ all of this is as expected ]
> >     $ echo "1 " | ./parser
> >     "1 \n"
> >     [ computer twiddles it's thumbs here until I manually terminate it ...
> > ]
> >     ^C^C
> >     $
> >
> > Does anyone know what's happening and now to alleviate it?
> >
> >
> > -- begin full code --
> >
> > -- base
> > import Control.Applicative
> > import Data.Word
> >
> > -- Hackage
> > import qualified Data.Text.Lazy as TL
> > import qualified Data.Text.Lazy.IO as TLIO
> > import Text.Parsec (parse)
> > import Text.Parsec.Text.Lazy (Parser)
> > import Text.Parser.Combinators
> > import Text.Parser.Char
> >
> > data FieldState a = EmptyState | OmittedState | OtherState | FullState a
> >   deriving (Eq, Ord)
> >
> > instance Functor FieldState where
> >     fmap f (FullState a)    = FullState (f a)
> >     fmap _ EmptyState       = EmptyState
> >     fmap _ OmittedState     = OmittedState
> >     fmap _ OtherState       = OtherState
> >
> > instance Applicative FieldState where
> >     pure = FullState
> >     (FullState f) <*> (FullState x) = FullState (f x)
> >     _             <*> _             = EmptyState
> >
> > instance Monad FieldState where
> >     (FullState x) >>= k = k x
> >     EmptyState    >>= _ = EmptyState
> >     OmittedState  >>= _ = OmittedState
> >     OtherState    >>= _ = OtherState
> >
> >     (FullState _) >> k = k
> >     EmptyState    >> _ = EmptyState
> >     OmittedState  >> _ = OmittedState
> >     OtherState    >> _ = OtherState
> >
> >     return  = FullState
> >     fail _  = EmptyState
> >
> > instance Show (FieldState x) where
> >     show (EmptyState)   = "empty ('-')"
> >     show (OmittedState) = "omitted ('^')"
> >     show (OtherState)   = "other ('~')"
> >     show x' = show x'
> >
> > data Counter = Counter Word64 deriving (Eq, Ord, Show)
> >
> > parseNum :: (Num a) => Parser a
> > parseNum = do
> >     n <- rd <$> many digit
> >     return $ fromIntegral n
> >     where rd = read :: String -> Integer
> >
> > parseCounter :: Parser Counter
> > parseCounter = Counter <$> parseNum
> >
> > parseFieldStateOff :: Parser Char
> > parseFieldStateOff = char '-'
> >
> > parseFieldStateOmitted :: Parser Char
> > parseFieldStateOmitted = char '^'
> >
> > parseFieldStateOther :: Parser Char
> > parseFieldStateOther = char '~'
> >
> > parseFieldState :: Parser a -> Parser (FieldState a)
> > parseFieldState p = (parseFieldStateOff >> return EmptyState)
> >                   <|> (parseFieldStateOmitted >> return OmittedState)
> >                   <|> (parseFieldStateOther >> return OtherState)
> >                   <|> (p >>= return . FullState)
> >
> > main :: IO ()
> > main = do
> >     ls <- TLIO.getContents
> >     print ls
> >     mapM_ processLine (TL.lines ls)
> >
> > processLine :: TL.Text -> IO ()
> > processLine line = case (parse (parseFieldState parseCounter) "" line) of
> >   Left err -> print err
> >   Right xs -> print xs
> >
> > _______________________________________________
> > Beginners mailing list
> > Beginners at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >
> >


-- 

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From shishir.srivastava at gmail.com  Thu Mar 26 10:06:17 2015
From: shishir.srivastava at gmail.com (Shishir Srivastava)
Date: Thu, 26 Mar 2015 10:06:17 +0000
Subject: [Haskell-beginners] Maybe and Just
Message-ID: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>

Hi,

After reading and re-reading the haskell tutorials I don't happen to see a
very convincing or appealing reason for having these data types.

Can anyone please explain where Maybe and Just provide the sort of
functionality that cannot be achieved in other languages which don't have
these kind.

Thanks,
Shishir Srivastava
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From shishir.srivastava at gmail.com  Thu Mar 26 10:21:30 2015
From: shishir.srivastava at gmail.com (Shishir Srivastava)
Date: Thu, 26 Mar 2015 10:21:30 +0000
Subject: [Haskell-beginners] Type Parameters and Type Variables
Message-ID: <CALe5RTv-_nf=LWKKkamw3_Xe_LisNFW6UhaLEV41rRtJh1LjSA@mail.gmail.com>

What's the difference between the two.. ?

Shishir Srivastava
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From mdorman at ironicdesign.com  Thu Mar 26 10:26:15 2015
From: mdorman at ironicdesign.com (Michael Alan Dorman)
Date: Thu, 26 Mar 2015 06:26:15 -0400
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 (Shishir Srivastava's message of "Thu, 26 Mar 2015 10:06:17 +0000")
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
Message-ID: <87384soy0o.fsf@ironicdesign.com>

Shishir Srivastava <shishir.srivastava at gmail.com> writes:
> After reading and re-reading the haskell tutorials I don't happen to
> see a very convincing or appealing reason for having these data
> types.

To be clear: Maybe is the data *type*.  Just and Nothing are its data
*constructors*.

> Can anyone please explain where Maybe and Just provide the sort of
> functionality that cannot be achieved in other languages which don't
> have these kind.

The functionality can be achieved in other languages, certainly.  The
question is whether the clarity and safety is also achieved.

When I see (as a totally contrived example):

  fopen :: Maybe FileHandle

I know that that function may not be able to return a FileHandle value
all the time.  The compiler will, in fact, nag me if I do not write the
code that calls it in such a way that it acknowledges that possibility.

When I see:

  FILE * fopen ( const char * filename, const char * mode );

It is not immediately clear whether that can fail.  Sure, we can make
that inference, based on what we know about filesystems, etc., but the
compiler is never going to complain if I ignore the possibility.

In my experience, programmers in many languages end up resorting to
convention to try and work around these sorts of ambiguities.  Large
projects have strategies for naming functions that try to pass along
information out of band, or languages have a pervasive culture of "lint"
tools that try to use heuristics to make up for what the type system
doesn't make simple.

That said, I know that doing Maybe sorts of things in languages that
don't have, say, pattern matching, or the idea of a "failure monad",
gets to be a drag very quickly---manually unwrapping things is at best
awkward, having to re-wrap them just to unwrap them again in a sequence
of computations quickly leads one to believe "it's just not worth
it"---or you resort to exception handling, which has its own challenges
to do well.

Mike.

From maydwell at gmail.com  Thu Mar 26 10:27:53 2015
From: maydwell at gmail.com (Lyndon Maydwell)
Date: Thu, 26 Mar 2015 21:27:53 +1100
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
Message-ID: <CAM5QZtw0yvX4sQ9pAB2hJ-P-7guQH=vct2fmyEE7Q48QAdruKA@mail.gmail.com>

Hi Shishir,


Indeed, Maybe doesn't grant you additional operational functionality that
other languages lack. Most use-cases of Maybe can be matched in nearly all
mainstream languages through the use of NULL.

Maybe illustrates a desired lack of functionality in cases where it is
absent.

For example, in Java:

public static Integer myMethod(Integer a, Integer b) {
  // body}


What do we know about this method from the signature?

You would think it is a method that takes two integers and returns an
integer, but really it could take NULLs too, and could possibly return a
NULL.

Here is the equivalent Haskell signature:

myMethod :: Maybe Integer -> Maybe Integer -> IO (Maybe Integer)


We can see that Maybe allows us to replicate the possibility of NULL values
from other languages... But how is that special since other languages
already have this built-in.

Now look at the following Haskell signature:

myMethod :: Integer -> Integer -> Integer


Now what do we know about this?

We know that it takes two Integers and returns an Integer.

This is the power that comes from having Maybe as the canonical way to
represent the possibility of missing values.


On Thu, Mar 26, 2015 at 9:06 PM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Hi,
>
> After reading and re-reading the haskell tutorials I don't happen to see a
> very convincing or appealing reason for having these data types.
>
> Can anyone please explain where Maybe and Just provide the sort of
> functionality that cannot be achieved in other languages which don't have
> these kind.
>
> Thanks,
> Shishir Srivastava
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From mwm at mired.org  Thu Mar 26 10:28:40 2015
From: mwm at mired.org (Mike Meyer)
Date: Thu, 26 Mar 2015 05:28:40 -0500
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
Message-ID: <CAD=7U2AnRdhqa34ALMNPSbPmPTsZhojGkLVXq6dVg-wHYDb3OQ@mail.gmail.com>

On Thu, Mar 26, 2015 at 5:06 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Hi,
>
> After reading and re-reading the haskell tutorials I don't happen to see a
> very convincing or appealing reason for having these data types.
>
> Can anyone please explain where Maybe and Just provide the sort of
> functionality that cannot be achieved in other languages which don't have
> these kind.
>

When a function fails to produce the return value, it can either return a
distinguished value (a NULL pointer, or -1 to signal an error, or ...) or
raise an exception. The latter is sufficiently clumsy that it's not
uncommon to see languages with two versions of some functions: one that
raises an exception, and one that returns a distinguished value.

Haskell uses the Maybe type for the distinguished value, raising it to the
type level. If I call a function that expects a value that is NOT the
distinguished value (a valid pointer, or a valid array index, or ...) in
languages without something like a Maybe type, I find out about it when my
unit tests fail if I'm lucky, or I just  get the wrong answer if I'm not.
When I call a function that expects an X with a value of type Maybe X in
Haskell, it's a type error at compile time.
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From raabe at froglogic.com  Thu Mar 26 10:34:58 2015
From: raabe at froglogic.com (Frerich Raabe)
Date: Thu, 26 Mar 2015 11:34:58 +0100
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
Message-ID: <c852ac46357133be475d511b9518ddcb@roundcube.froglogic.com>

Hi,

On 2015-03-26 11:06, Shishir Srivastava wrote:
> After reading and re-reading the haskell tutorials I don't happen to see a 
> very convincing or appealing reason for having these data types.

First of all, only one of them is a type (or a 'type constructor'): "Maybe". 
"Just" on the other hand is a "data constructor", i.e. "Just 'x'" will yield 
a value of type "Maybe Char".

> Can anyone please explain where Maybe and Just provide the sort of 
> functionality that cannot be achieved in other languages which don't have 
> these kind.

They don't provide anything which other languages cannot achieve. 'Maybe' 
values merely indicate that you *maybe* get a value. This is useful for 
functions which might not be able to yield a meaningful result. Consider a 
'safeHead' function which works like 'head', returning the first element of a 
given list. What should it do for empty lists?

Something like 'Maybe' is useful because it

1. Makes potential errors clearly visible in the type system: If a function 
returns a 'Maybe Char', you know that it possible won't yield a Char (but 
rather 'Nothing'). Raising an exception is totally opaque to the caller.

2. Clearly separates the values which might represent failures from those 
which don't. Contrast this with e.g. Java, where every object of every type 
can be null. If every value is potentially 'null', you either have to check 
using if() everywhere or use assertions. With 'Maybe', the null'ness can be 
asserted by the compiler.

3. Relieves other types from having an explicit 'Invalid' value. Consider:

   data Color = Red | Green | Blue | Invalid

   -- Parses a color string; yields 'Invalid' if the string is malformed
   parseColor :: String -> Color
   parseColor = undefind

With a setup like this, *every* code dealing with 'Color' values will have to 
be able to handle 'Invalid' colors. It ripples through the system. OTOH, 
something like

   data Color = Red | Green | Blue

   -- Parses a color string; yields 'Nothing' if the string is malformed
   parseColor :: String -> Maybe Color
   parseColor = undefined

Clearly separates the valid colors, such that only the caller of 'parseColor' 
only has to check for 'Just x' values - and if it got one, it can pass the 
'x' value to other code.

For what it's worth, other languages have similiar concepts. For instance, 
C++ programmers can use 'boost::optional' (which, alas, didn't make it into 
C++14).

-- 
Frerich Raabe - raabe at froglogic.com
www.froglogic.com - Multi-Platform GUI Testing

From sumit.sahrawat.apm13 at iitbhu.ac.in  Thu Mar 26 10:48:52 2015
From: sumit.sahrawat.apm13 at iitbhu.ac.in (Sumit Sahrawat, Maths & Computing,
 IIT (BHU))
Date: Thu, 26 Mar 2015 16:18:52 +0530
Subject: [Haskell-beginners] Type Parameters and Type Variables
In-Reply-To: <CALe5RTv-_nf=LWKKkamw3_Xe_LisNFW6UhaLEV41rRtJh1LjSA@mail.gmail.com>
References: <CALe5RTv-_nf=LWKKkamw3_Xe_LisNFW6UhaLEV41rRtJh1LjSA@mail.gmail.com>
Message-ID: <CAJbEW8Mj-2pdDjs_cbvU9ntxnf8GaseGXkKDDBwYL94SFgZ+mg@mail.gmail.com>

When type variables are passed to a Type Constructor, they become type
parameters.
Just like when variables are passed to functions, they become parameters.
Operationally, both are equivalent.

On 26 March 2015 at 15:51, Shishir Srivastava <shishir.srivastava at gmail.com>
wrote:

> What's the difference between the two.. ?
>
> Shishir Srivastava
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>


-- 
Regards

Sumit Sahrawat
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From joel.neely at gmail.com  Thu Mar 26 11:54:29 2015
From: joel.neely at gmail.com (Joel Neely)
Date: Thu, 26 Mar 2015 06:54:29 -0500
Subject: [Haskell-beginners] Empty list
In-Reply-To: <CALe5RTvbkq3rYPXL2AWabxroXy_OFssAhQ899bUu1KZSD1szDw@mail.gmail.com>
References: <CALe5RTvbkq3rYPXL2AWabxroXy_OFssAhQ899bUu1KZSD1szDw@mail.gmail.com>
Message-ID: <CAEEzXAiEbC3UsdUQi7ojjA+xtVm5nftYu6pBQBoKoCYFWDcXkw@mail.gmail.com>

Given the distinction between lists and sets, you might also consider

Prelude Data.Set Data.List> [] `isInfixOf` [1,2,3]
True
Prelude Data.Set Data.List> (fromList []) `isSubsetOf` (fromList [1,2,3])
True

Of course `isInfixOf` is in general more restrictive than `isSubsetOf`, in
the sense that the infix relationship depends on ordering, which the subset
relationship does not.

Hope that helps,

-jn-



On Wed, Mar 25, 2015 at 8:55 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Hi,
>
> Can someone please explain why this results in error -
>
> [] `elem` [1,2,3]
>
> Shouldn't the empty set by definition be the element of all sets including
> a non-empty set ?
>
> I am assuming 'Lists' are different from 'Sets' in Haskell, if yes, is
> there a separate module for dealing/working with sets ?
>
> Thanks,
> Shishir Srivastava
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>


-- 
Beauty of style and harmony and grace and good rhythm depend on simplicity.
- Plato
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From joel.neely at gmail.com  Thu Mar 26 12:37:28 2015
From: joel.neely at gmail.com (Joel Neely)
Date: Thu, 26 Mar 2015 07:37:28 -0500
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
Message-ID: <CAEEzXAg_Y8Q2e2vTLYDkAx-xLPE-AYB07m72jT2p=NT17_C97g@mail.gmail.com>

Hi, Shishir,

As a recovering complexity addict (i.e. Java programmer ;-), I'd put it
this way.

There are many methods (on many types) that can either succeed and return a
meaningful value, or be unable to fulfill the intent of the caller's
request. Example include indexOf on String,  get on Map, and read on a
Reader. Every time such a partial function scenario occurs, the author must
decide what to do if the intent can't be fulfilled. That freedom of choice
has led to a proliferation of techniques.


   - indexOf returns -1, which is a valid int, but not a valid position
   within a string.
   - get returns null, which is the only value common to all reference
   types that means 'I have nothing'. Of course, that also prevents one from
   storing null in a Map and simply retrieving it. Worse, if that result is
   passed along elsewhere, one can eventually receive a NullPointerException
   occurring in a completely different part of the code.
   - And, *horrors!*, read responds in a totally different way, by forcing
   the caller to "jump" somewhere.


Maybe provides a general, reusable solution to all such cases.

   - It allows one to "stretch" any type to include an extra "out of band"
   value.
   - The type declaration clearly (uniformly) notifies anyone else that the
   function may have to return "lack of result".
   - The type system prevents a caller from simply assuming that a result
   is present, and then accidentally propagating failure.
   - Because Maybe is an instance of Functor, one can use map to safely
   apply a function (or do nothing gracefully) without littering the calling
   code with if-statements.
   - etc.


I don't think of Maybe as enabling a *computation* that is impossible
otherwise, but I do regard it as providing a *consistency and simplicity*
that is otherwise not present.

Hope this helps,
Joel


On Thu, Mar 26, 2015 at 5:06 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Hi,
>
> After reading and re-reading the haskell tutorials I don't happen to see a
> very convincing or appealing reason for having these data types.
>
> Can anyone please explain where Maybe and Just provide the sort of
> functionality that cannot be achieved in other languages which don't have
> these kind.
>
> Thanks,
> Shishir Srivastava
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>


-- 
Beauty of style and harmony and grace and good rhythm depend on simplicity.
- Plato
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From d12frosted at icloud.com  Thu Mar 26 13:17:37 2015
From: d12frosted at icloud.com (Boris)
Date: Thu, 26 Mar 2015 15:17:37 +0200
Subject: [Haskell-beginners] Monoids and Groups
In-Reply-To: <CALe5RTu8pd8y1F=PukUec-r2uEfkczZRhKcQgOcPvfB9ugGquA@mail.gmail.com>
References: <CALe5RTu8pd8y1F=PukUec-r2uEfkczZRhKcQgOcPvfB9ugGquA@mail.gmail.com>
Message-ID: <etPan.551406f1.6b8b4567.1fd@d12frosted.local>

Hi,

While they have similarities, they are not the same. Semigroup (S,*) is a set S with associative binary operation *. Monoid is a semigroup that has identity element i such that i * a = a and a * i = a. And group is a monoid in which every element of set has it?s own inverse: a * b = i and b * a = i where b is an inverse of a.

Let?s look at the definition of Monoid in haskell.

class Monoid a where
        mempty  :: a
        -- ^ Identity of 'mappend'
        mappend :: a -> a -> a
        -- ^ An associative operation
        mconcat :: [a] -> a

        -- ^ Fold a list using the monoid.
        -- For most types, the default definition for 'mconcat' will be
        -- used, but the function is included in the class definition so
        -- that an optimized version can be provided for specific types.

        mconcat = foldr mappend mempty
So we have a set of a, associative binary operation mappend and identity element mempty. The only difference between Monoid a in haskell and monoid in algebra is that Monoid in haskell has mconcat function in it?s definition. But you can ignore it.

I hope it helps.

Cheers, d12frosted


On March 25, 2015 at 16:03:14, Shishir Srivastava (shishir.srivastava at gmail.com) wrote:

Hi,?

Reading about Monoids it seems they derive a lot on the algebraic structures of 'Groups' ??

Is it then correct to assume that Monoids can be used to represent 'Groups' ?

If not are there any standard haskell libraries which represent algebraic structures such as 'Groups' , 'Fields' etc.

Thanks,
Shishir Srivastava

_______________________________________________  
Beginners mailing list  
Beginners at haskell.org  
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners  
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From mwm at mired.org  Thu Mar 26 13:36:30 2015
From: mwm at mired.org (Mike Meyer)
Date: Thu, 26 Mar 2015 08:36:30 -0500
Subject: [Haskell-beginners] Something like pydoc?
Message-ID: <CAD=7U2D_egH471tG_3igTCFryw8ndf2WxtJ-Zuddz2zXTduH6g@mail.gmail.com>

Python had the "pydoc" command, which would could be used to print a
module's docstings from the command line, and provided the "help" function
in the REPL. Clojure has "doc" in the REPL, at least if you told it where
the sources were.

I can't seem to find anything like that for Haskell - either at the command
line or in ghci. Does such exist? Is such possible?

Thanks,
<mike
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From toad3k at gmail.com  Thu Mar 26 13:44:19 2015
From: toad3k at gmail.com (David McBride)
Date: Thu, 26 Mar 2015 09:44:19 -0400
Subject: [Haskell-beginners] Something like pydoc?
In-Reply-To: <CAD=7U2D_egH471tG_3igTCFryw8ndf2WxtJ-Zuddz2zXTduH6g@mail.gmail.com>
References: <CAD=7U2D_egH471tG_3igTCFryw8ndf2WxtJ-Zuddz2zXTduH6g@mail.gmail.com>
Message-ID: <CAN+Tr40ArPA_qrapuc53GNHGfTmSnxkiB4K1rFipMK5gs+d+bw@mail.gmail.com>

There is a hoogle command: https://hackage.haskell.org/package/hoogle.

I don't use it, but I imagine you would have to set documention: True in
cabal.config, so you get documentation for any packages you have, and then
either use your browser, or put an alias in your .ghci to query it in your
repl, or just use the executable.

On Thu, Mar 26, 2015 at 9:36 AM, Mike Meyer <mwm at mired.org> wrote:

> Python had the "pydoc" command, which would could be used to print a
> module's docstings from the command line, and provided the "help" function
> in the REPL. Clojure has "doc" in the REPL, at least if you told it where
> the sources were.
>
> I can't seem to find anything like that for Haskell - either at the
> command line or in ghci. Does such exist? Is such possible?
>
> Thanks,
> <mike
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From mwm at mired.org  Thu Mar 26 14:27:13 2015
From: mwm at mired.org (Mike Meyer)
Date: Thu, 26 Mar 2015 09:27:13 -0500
Subject: [Haskell-beginners] Something like pydoc?
In-Reply-To: <CAN+Tr40ArPA_qrapuc53GNHGfTmSnxkiB4K1rFipMK5gs+d+bw@mail.gmail.com>
References: <CAD=7U2D_egH471tG_3igTCFryw8ndf2WxtJ-Zuddz2zXTduH6g@mail.gmail.com>
 <CAN+Tr40ArPA_qrapuc53GNHGfTmSnxkiB4K1rFipMK5gs+d+bw@mail.gmail.com>
Message-ID: <CAD=7U2D+qWRtd54cDKyfBgF0szdqSMO3r=GWPv1vdp3u4y-yfw@mail.gmail.com>

On Thu, Mar 26, 2015 at 8:44 AM, David McBride <toad3k at gmail.com> wrote:

> There is a hoogle command: https://hackage.haskell.org/package/hoogle.
>
> I don't use it, but I imagine you would have to set documention: True in
> cabal.config, so you get documentation for any packages you have, and then
> either use your browser, or put an alias in your .ghci to query it in your
> repl, or just use the executable.
>

The appropriate entries for .ghci are:

:def hoogle \x -> return $ ":!hoogle \"" ++ x ++ "\""
:def doc \x -> return $ ":!hoogle --info \"" ++ x ++ "\""

That's actually close enough. However, the python & clojure commands use
the current search path, not a database build from the online docs. I
suspect this means I want a cabal command.
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From shishir.srivastava at gmail.com  Thu Mar 26 15:26:40 2015
From: shishir.srivastava at gmail.com (Shishir Srivastava)
Date: Thu, 26 Mar 2015 15:26:40 +0000
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <87384soy0o.fsf@ironicdesign.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
Message-ID: <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>

ok..but what's with using the keyword 'Just' ? why cannot 'Maybe' be
defined like this

data Maybe a = a | Nothing

what's the point in having 'Just' keyword ?

Shishir


On Thu, Mar 26, 2015 at 10:26 AM, Michael Alan Dorman <
mdorman at ironicdesign.com> wrote:

> Shishir Srivastava <shishir.srivastava at gmail.com> writes:
> > After reading and re-reading the haskell tutorials I don't happen to
> > see a very convincing or appealing reason for having these data
> > types.
>
> To be clear: Maybe is the data *type*.  Just and Nothing are its data
> *constructors*.
>
> > Can anyone please explain where Maybe and Just provide the sort of
> > functionality that cannot be achieved in other languages which don't
> > have these kind.
>
> The functionality can be achieved in other languages, certainly.  The
> question is whether the clarity and safety is also achieved.
>
> When I see (as a totally contrived example):
>
>   fopen :: Maybe FileHandle
>
> I know that that function may not be able to return a FileHandle value
> all the time.  The compiler will, in fact, nag me if I do not write the
> code that calls it in such a way that it acknowledges that possibility.
>
> When I see:
>
>   FILE * fopen ( const char * filename, const char * mode );
>
> It is not immediately clear whether that can fail.  Sure, we can make
> that inference, based on what we know about filesystems, etc., but the
> compiler is never going to complain if I ignore the possibility.
>
> In my experience, programmers in many languages end up resorting to
> convention to try and work around these sorts of ambiguities.  Large
> projects have strategies for naming functions that try to pass along
> information out of band, or languages have a pervasive culture of "lint"
> tools that try to use heuristics to make up for what the type system
> doesn't make simple.
>
> That said, I know that doing Maybe sorts of things in languages that
> don't have, say, pattern matching, or the idea of a "failure monad",
> gets to be a drag very quickly---manually unwrapping things is at best
> awkward, having to re-wrap them just to unwrap them again in a sequence
> of computations quickly leads one to believe "it's just not worth
> it"---or you resort to exception handling, which has its own challenges
> to do well.
>
> Mike.
>
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From allbery.b at gmail.com  Thu Mar 26 15:46:45 2015
From: allbery.b at gmail.com (Brandon Allbery)
Date: Thu, 26 Mar 2015 11:46:45 -0400
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
 <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
Message-ID: <CAKFCL4U=EuZ+3ojXwGe+9jSBLsT+nHvGv=-phH+TLRr8x1oypA@mail.gmail.com>

On Thu, Mar 26, 2015 at 11:26 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> ok..but what's with using the keyword 'Just' ? why cannot 'Maybe' be
> defined like this
>
> data Maybe a = a | Nothing
>

Because type inference won't work if you don't have a distinction between
an a and a Maybe a. Also, you end up back where you started --- there's a
distinguished (non)value that appears to be a value of the type, which
leaves you no better off than with C's NULL or Perl's undef, losing the
type safety of "this can't fail" vs. "this may fail and you must deal with
the failure somehow".

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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From corentin.dupont at gmail.com  Thu Mar 26 15:48:31 2015
From: corentin.dupont at gmail.com (Corentin Dupont)
Date: Thu, 26 Mar 2015 16:48:31 +0100
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
 <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
Message-ID: <CAEyhvmrZcDRrhCvwVZ7T7i_NWr1Brt1TKE2YW7Mq2D2MGPaPBg@mail.gmail.com>

Hi Shishir,
I think that's a legitimate question.

By writing

data Maybe a = a | Nothing

you are saying that the type of the left hand side of the = is the same
that right hand side (you are defining a type basically).
Also you can only sum things of the same type.
So you are saying:
type "Maybe a" = type "a"
Which is wrong.
That's why "a" should be wrapped into something:
type of "Just a" is indeed "Maybe a".

"Just" is a type constructor:
Just :: a -> Maybe a
It allows you to build the Maybe.

Said that, "Just" is a vocabulary choice.
Personally I prefer the name choices of OCaml, Rust, Scala etc.: Option a =
None | Some a



On Thu, Mar 26, 2015 at 4:26 PM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> ok..but what's with using the keyword 'Just' ? why cannot 'Maybe' be
> defined like this
>
> data Maybe a = a | Nothing
>
> what's the point in having 'Just' keyword ?
>
> Shishir
>
>
> On Thu, Mar 26, 2015 at 10:26 AM, Michael Alan Dorman <
> mdorman at ironicdesign.com> wrote:
>
>> Shishir Srivastava <shishir.srivastava at gmail.com> writes:
>> > After reading and re-reading the haskell tutorials I don't happen to
>> > see a very convincing or appealing reason for having these data
>> > types.
>>
>> To be clear: Maybe is the data *type*.  Just and Nothing are its data
>> *constructors*.
>>
>> > Can anyone please explain where Maybe and Just provide the sort of
>> > functionality that cannot be achieved in other languages which don't
>> > have these kind.
>>
>> The functionality can be achieved in other languages, certainly.  The
>> question is whether the clarity and safety is also achieved.
>>
>> When I see (as a totally contrived example):
>>
>>   fopen :: Maybe FileHandle
>>
>> I know that that function may not be able to return a FileHandle value
>> all the time.  The compiler will, in fact, nag me if I do not write the
>> code that calls it in such a way that it acknowledges that possibility.
>>
>> When I see:
>>
>>   FILE * fopen ( const char * filename, const char * mode );
>>
>> It is not immediately clear whether that can fail.  Sure, we can make
>> that inference, based on what we know about filesystems, etc., but the
>> compiler is never going to complain if I ignore the possibility.
>>
>> In my experience, programmers in many languages end up resorting to
>> convention to try and work around these sorts of ambiguities.  Large
>> projects have strategies for naming functions that try to pass along
>> information out of band, or languages have a pervasive culture of "lint"
>> tools that try to use heuristics to make up for what the type system
>> doesn't make simple.
>>
>> That said, I know that doing Maybe sorts of things in languages that
>> don't have, say, pattern matching, or the idea of a "failure monad",
>> gets to be a drag very quickly---manually unwrapping things is at best
>> awkward, having to re-wrap them just to unwrap them again in a sequence
>> of computations quickly leads one to believe "it's just not worth
>> it"---or you resort to exception handling, which has its own challenges
>> to do well.
>>
>> Mike.
>>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From objitsu at gmail.com  Thu Mar 26 16:25:28 2015
From: objitsu at gmail.com (emacstheviking)
Date: Thu, 26 Mar 2015 16:25:28 +0000
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <87384soy0o.fsf@ironicdesign.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
Message-ID: <CAEiEuU+tx8zQ2zm=hhLGJaf+0rRbtRJJ4aNwVGVwbsZZ+SoOJg@mail.gmail.com>

For what its worth I wrote this a while back, a hopeless attempt to do
something "monad like" in PHP but reading it again..... well, it's no
wonder I like Haskell more than I like having to use PHP!

http://objitsu.blogspot.co.uk/2013/01/php-and-maybe-miserable-monads.html



On 26 March 2015 at 10:26, Michael Alan Dorman <mdorman at ironicdesign.com>
wrote:

> Shishir Srivastava <shishir.srivastava at gmail.com> writes:
> > After reading and re-reading the haskell tutorials I don't happen to
> > see a very convincing or appealing reason for having these data
> > types.
>
> To be clear: Maybe is the data *type*.  Just and Nothing are its data
> *constructors*.
>
> > Can anyone please explain where Maybe and Just provide the sort of
> > functionality that cannot be achieved in other languages which don't
> > have these kind.
>
> The functionality can be achieved in other languages, certainly.  The
> question is whether the clarity and safety is also achieved.
>
> When I see (as a totally contrived example):
>
>   fopen :: Maybe FileHandle
>
> I know that that function may not be able to return a FileHandle value
> all the time.  The compiler will, in fact, nag me if I do not write the
> code that calls it in such a way that it acknowledges that possibility.
>
> When I see:
>
>   FILE * fopen ( const char * filename, const char * mode );
>
> It is not immediately clear whether that can fail.  Sure, we can make
> that inference, based on what we know about filesystems, etc., but the
> compiler is never going to complain if I ignore the possibility.
>
> In my experience, programmers in many languages end up resorting to
> convention to try and work around these sorts of ambiguities.  Large
> projects have strategies for naming functions that try to pass along
> information out of band, or languages have a pervasive culture of "lint"
> tools that try to use heuristics to make up for what the type system
> doesn't make simple.
>
> That said, I know that doing Maybe sorts of things in languages that
> don't have, say, pattern matching, or the idea of a "failure monad",
> gets to be a drag very quickly---manually unwrapping things is at best
> awkward, having to re-wrap them just to unwrap them again in a sequence
> of computations quickly leads one to believe "it's just not worth
> it"---or you resort to exception handling, which has its own challenges
> to do well.
>
> Mike.
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From corentin.dupont at gmail.com  Thu Mar 26 17:31:34 2015
From: corentin.dupont at gmail.com (Corentin Dupont)
Date: Thu, 26 Mar 2015 18:31:34 +0100
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTs8T1H4OWhmCjbA0HnGJc_Tk0vCE83viCUU2bToGQSwsQ@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
 <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
 <CAEyhvmrZcDRrhCvwVZ7T7i_NWr1Brt1TKE2YW7Mq2D2MGPaPBg@mail.gmail.com>
 <CALe5RTs8T1H4OWhmCjbA0HnGJc_Tk0vCE83viCUU2bToGQSwsQ@mail.gmail.com>
Message-ID: <CAEyhvmowUSHSx9mV9HSngqTCy_WS_0-RusPHN7E9ge96z3kmtg@mail.gmail.com>

Not really, when you define the type "Maybe a":

data Maybe a = Just a | Nothing

Haskell is creating automatically two functions for you:

Just :: a -> Maybe a
Nothing :: Maybe a

In the first case, you can think of "Just" and "Nothing" as a sort of tag
identifying which element of the sum you have.
It the second case it's a function, with the same name.
A more informed person than me could say if they are indeed separated of if
they are the same thing in GHC...


On Thu, Mar 26, 2015 at 5:34 PM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> isn't that then cyclic dependency between 'Maybe' and 'Just' ...where the
> first one is defined in terms of second and vice-versa...?
>
> Shishir
>
>
> On Thu, Mar 26, 2015 at 3:48 PM, Corentin Dupont <
> corentin.dupont at gmail.com> wrote:
>
>> Hi Shishir,
>> I think that's a legitimate question.
>>
>> By writing
>>
>> data Maybe a = a | Nothing
>>
>> you are saying that the type of the left hand side of the = is the same
>> that right hand side (you are defining a type basically).
>> Also you can only sum things of the same type.
>> So you are saying:
>> type "Maybe a" = type "a"
>> Which is wrong.
>> That's why "a" should be wrapped into something:
>> type of "Just a" is indeed "Maybe a".
>>
>> "Just" is a type constructor:
>> Just :: a -> Maybe a
>> It allows you to build the Maybe.
>>
>> Said that, "Just" is a vocabulary choice.
>> Personally I prefer the name choices of OCaml, Rust, Scala etc.: Option
>> a = None | Some a
>>
>>
>>
>> On Thu, Mar 26, 2015 at 4:26 PM, Shishir Srivastava <
>> shishir.srivastava at gmail.com> wrote:
>>
>>> ok..but what's with using the keyword 'Just' ? why cannot 'Maybe' be
>>> defined like this
>>>
>>> data Maybe a = a | Nothing
>>>
>>> what's the point in having 'Just' keyword ?
>>>
>>> Shishir
>>>
>>>
>>> On Thu, Mar 26, 2015 at 10:26 AM, Michael Alan Dorman <
>>> mdorman at ironicdesign.com> wrote:
>>>
>>>> Shishir Srivastava <shishir.srivastava at gmail.com> writes:
>>>> > After reading and re-reading the haskell tutorials I don't happen to
>>>> > see a very convincing or appealing reason for having these data
>>>> > types.
>>>>
>>>> To be clear: Maybe is the data *type*.  Just and Nothing are its data
>>>> *constructors*.
>>>>
>>>> > Can anyone please explain where Maybe and Just provide the sort of
>>>> > functionality that cannot be achieved in other languages which don't
>>>> > have these kind.
>>>>
>>>> The functionality can be achieved in other languages, certainly.  The
>>>> question is whether the clarity and safety is also achieved.
>>>>
>>>> When I see (as a totally contrived example):
>>>>
>>>>   fopen :: Maybe FileHandle
>>>>
>>>> I know that that function may not be able to return a FileHandle value
>>>> all the time.  The compiler will, in fact, nag me if I do not write the
>>>> code that calls it in such a way that it acknowledges that possibility.
>>>>
>>>> When I see:
>>>>
>>>>   FILE * fopen ( const char * filename, const char * mode );
>>>>
>>>> It is not immediately clear whether that can fail.  Sure, we can make
>>>> that inference, based on what we know about filesystems, etc., but the
>>>> compiler is never going to complain if I ignore the possibility.
>>>>
>>>> In my experience, programmers in many languages end up resorting to
>>>> convention to try and work around these sorts of ambiguities.  Large
>>>> projects have strategies for naming functions that try to pass along
>>>> information out of band, or languages have a pervasive culture of "lint"
>>>> tools that try to use heuristics to make up for what the type system
>>>> doesn't make simple.
>>>>
>>>> That said, I know that doing Maybe sorts of things in languages that
>>>> don't have, say, pattern matching, or the idea of a "failure monad",
>>>> gets to be a drag very quickly---manually unwrapping things is at best
>>>> awkward, having to re-wrap them just to unwrap them again in a sequence
>>>> of computations quickly leads one to believe "it's just not worth
>>>> it"---or you resort to exception handling, which has its own challenges
>>>> to do well.
>>>>
>>>> Mike.
>>>>
>>>
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>
>
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From martin.drautzburg at web.de  Thu Mar 26 20:18:37 2015
From: martin.drautzburg at web.de (martin)
Date: Thu, 26 Mar 2015 21:18:37 +0100
Subject: [Haskell-beginners] ap  =  liftM2 id
Message-ID: <5514699D.6070209@web.de>

Hello all,

can someone explain

ap  =  liftM2 id

to me? liftM2 wants a binary funcation

liftM2  :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r

but id is unary. How does this work?

From andres at well-typed.com  Thu Mar 26 20:33:46 2015
From: andres at well-typed.com (=?UTF-8?Q?Andres_L=C3=B6h?=)
Date: Thu, 26 Mar 2015 21:33:46 +0100
Subject: [Haskell-beginners] ap = liftM2 id
In-Reply-To: <5514699D.6070209@web.de>
References: <5514699D.6070209@web.de>
Message-ID: <CALjd_v4eWhDxem+WVqBZBeUyRm1nqgMJu_H8yK9sQq5LTQLOBw@mail.gmail.com>

There's no such thing as a fixed arity of a function in Haskell. The
type of id is

> id :: a -> a

If you instantiate "a" with "b -> c", then it becomes

> id :: (b -> c) -> b -> c

and it suddenly takes two arguments. In fact, it then behaves like
function application. Try

> id not True

or

> not `id` True

and you'll see that it works as if you had used function application
or $ instead. So ap lifts function application, and if it helps, you
can think of it as instead being defined as

> ap = liftM2 ($)

Cheers,
  Andres


On Thu, Mar 26, 2015 at 9:18 PM, martin <martin.drautzburg at web.de> wrote:
> Hello all,
>
> can someone explain
>
> ap  =  liftM2 id
>
> to me? liftM2 wants a binary funcation
>
> liftM2  :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
>
> but id is unary. How does this work?
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners



-- 
Andres L?h, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com

Registered in England & Wales, OC335890
250 Ice Wharf, 17 New Wharf Road, London N1 9RF, England

From martin.drautzburg at web.de  Thu Mar 26 21:04:47 2015
From: martin.drautzburg at web.de (martin)
Date: Thu, 26 Mar 2015 22:04:47 +0100
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
Message-ID: <5514746F.6060702@web.de>

Am 03/26/2015 um 11:06 AM schrieb Shishir Srivastava:
> Hi, 
> 
> After reading and re-reading the haskell tutorials I don't happen to see a very convincing or appealing reason for
> having these data types. 
> 
> Can anyone please explain where Maybe and Just provide the sort of functionality that cannot be achieved in other
> languages which don't have these kind.

My intuition goes like that:

"Maybe" is like "nullable" (which doesn't really exist, but you sure can imagine what it means). In many programming
languages a variable of any type can contain "null", i.e. every type is nullable. This means that any function accepting
one of these types as a parameter must be prepared to see a null. You end up with many checks like

	if (x==null) {return null;}

Not so in haskell. An Int is really an Int and Ints cannot be null. Functions accepting Ints don't have to check for null.

But you can beef up any datatype to be nullable. You can create a TYPE "nullable Int" which would be called "Maybe Int"
in Haskell. Just like "nullable" alone, Maybe itself is not a type, but an up-beefer of another type. There are no
values of type Maybe, but only of Maybe Int, Maybe String etc. In that respect maybe is like "List". There are no values
of type "List", but only of List of Int, List of String. List is an up-beefer too (written as [] in Haskell)

Obviously there needs to be a distinct VALUE for "null". In Haskell this value is called "Nothing".

For the non-null VALUES you could think about  writing them verbatim, like "42". But from just looking at 42, you
couldn't tell what type it is. It could be an Int or a Maybe Int. This problem is somewhat akin to writing 42.0 when you
want to make sure it is a float (in imperative languages), or 42L to indicate it is a long.

So in the same vein, you distinguish an Int (42) from a Maybe Int (Just 42). Just 42 is a value of type Maybe Int, which
just (sic!) happens not to be null, but it is still not a plain Int, but a Maybe Int. Like 42.0 is a float which happens
not to have any digits after the decimal point but also is not a plain Int but a float.



From martin.drautzburg at web.de  Thu Mar 26 21:48:37 2015
From: martin.drautzburg at web.de (martin)
Date: Thu, 26 Mar 2015 22:48:37 +0100
Subject: [Haskell-beginners] ap = liftM2 id
In-Reply-To: <CALjd_v4eWhDxem+WVqBZBeUyRm1nqgMJu_H8yK9sQq5LTQLOBw@mail.gmail.com>
References: <5514699D.6070209@web.de>
 <CALjd_v4eWhDxem+WVqBZBeUyRm1nqgMJu_H8yK9sQq5LTQLOBw@mail.gmail.com>
Message-ID: <55147EB5.2010002@web.de>

Am 03/26/2015 um 09:33 PM schrieb Andres L?h:

I'll have to think about this. But other than that I am laughing my ass off after reading your paper about "Konjunktiv
III". I can hardly type.


> There's no such thing as a fixed arity of a function in Haskell. The
> type of id is
> 
>> id :: a -> a
> 
> If you instantiate "a" with "b -> c", then it becomes
> 
>> id :: (b -> c) -> b -> c
> 
> and it suddenly takes two arguments. In fact, it then behaves like
> function application. Try
> 
>> id not True
> 
> or
> 
>> not `id` True
> 
> and you'll see that it works as if you had used function application
> or $ instead. So ap lifts function application, and if it helps, you
> can think of it as instead being defined as
> 
>> ap = liftM2 ($)
> 
> Cheers,
>   Andres
> 
> 
> On Thu, Mar 26, 2015 at 9:18 PM, martin <martin.drautzburg at web.de> wrote:
>> Hello all,
>>
>> can someone explain
>>
>> ap  =  liftM2 id
>>
>> to me? liftM2 wants a binary funcation
>>
>> liftM2  :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
>>
>> but id is unary. How does this work?
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> 
> 
> 


From andres.loeh at gmail.com  Thu Mar 26 22:19:20 2015
From: andres.loeh at gmail.com (=?UTF-8?Q?Andres_L=C3=B6h?=)
Date: Thu, 26 Mar 2015 23:19:20 +0100
Subject: [Haskell-beginners] ap = liftM2 id
In-Reply-To: <55147EB5.2010002@web.de>
References: <5514699D.6070209@web.de>
 <CALjd_v4eWhDxem+WVqBZBeUyRm1nqgMJu_H8yK9sQq5LTQLOBw@mail.gmail.com>
 <55147EB5.2010002@web.de>
Message-ID: <CALjd_v4oHtkhFtoFwUqZC5_RfaKK--+M97Xse8cr14kQNo-mVA@mail.gmail.com>

> I'll have to think about this. But other than that I am laughing my ass off after reading your paper about "Konjunktiv
> III". I can hardly type.

Oh, that was written a long, long time ago. Glad you like it :)

Cheers,
  Andres

From greg at grahamtx.net  Thu Mar 26 23:05:56 2015
From: greg at grahamtx.net (Greg Graham)
Date: Thu, 26 Mar 2015 18:05:56 -0500
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <5514746F.6060702@web.de>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <5514746F.6060702@web.de>
Message-ID: <CAJdD3rtRCeyk=XUe+GE9_NQJzXc=Cik9ObmF5UuXXgNsDocZ=Q@mail.gmail.com>

On Thu, Mar 26, 2015 at 4:04 PM, martin <martin.drautzburg at web.de> wrote:
> Am 03/26/2015 um 11:06 AM schrieb Shishir Srivastava:
>> Hi,
>>
>> After reading and re-reading the haskell tutorials I don't happen to see a very convincing or appealing reason for
>> having these data types.
>>
>> Can anyone please explain where Maybe and Just provide the sort of functionality that cannot be achieved in other
>> languages which don't have these kind.

I'm reading the little book, _Maybe Haskell_ [1], and I'm finding it
very helpful on this topic. The whole Maybe paradigm is one of the
reasons I decided to learn Haskell.

I had previously started learning Go, and although I was initially
attracted by its simplicity, I came to dislike how it used return
values to indicate errors. Go typically returns a pair of values from
operations that can fail. One value is the normal return value, and
the other is an error code. At first look, I thought it was similar to
Haskell's Maybe. However, after doing a little Go programming, I
realized there's a big difference. The error codes in Go can be
ignored, sometimes accidentally, but you can't ignore Haskell's Maybe.
Sure, it's possible to respond poorly, but you do have to respond, all
because (Just 42) is not the same thing as (42).

1: https://robots.thoughtbot.com/maybe-haskell-our-newest-book

From rein.henrichs at gmail.com  Fri Mar 27 00:34:46 2015
From: rein.henrichs at gmail.com (Rein Henrichs)
Date: Thu, 26 Mar 2015 17:34:46 -0700
Subject: [Haskell-beginners] ap = liftM2 id
In-Reply-To: <CALjd_v4oHtkhFtoFwUqZC5_RfaKK--+M97Xse8cr14kQNo-mVA@mail.gmail.com>
References: <5514699D.6070209@web.de>
 <CALjd_v4eWhDxem+WVqBZBeUyRm1nqgMJu_H8yK9sQq5LTQLOBw@mail.gmail.com>
 <55147EB5.2010002@web.de>
 <CALjd_v4oHtkhFtoFwUqZC5_RfaKK--+M97Xse8cr14kQNo-mVA@mail.gmail.com>
Message-ID: <CAJp6G8xCJdgofSuvx7rzTG_w8x+54n034=EEg+a-B1ZoUOdMNw@mail.gmail.com>

Following up on Andres's explanation, we have

ap f x = liftM2 id mf ma                          -- definition of ap
       = do { f <- mf; a <- ma; return (id f x) } -- definition of liftM2
       = do { f <- mf; a <- ma; return (f x) }    -- definition of id, f x
y = (f x) y

We can see that id applied to f gives f which is then applied to a.
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From shishir.srivastava at gmail.com  Fri Mar 27 09:44:22 2015
From: shishir.srivastava at gmail.com (Shishir Srivastava)
Date: Fri, 27 Mar 2015 09:44:22 +0000
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CAEyhvmowUSHSx9mV9HSngqTCy_WS_0-RusPHN7E9ge96z3kmtg@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
 <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
 <CAEyhvmrZcDRrhCvwVZ7T7i_NWr1Brt1TKE2YW7Mq2D2MGPaPBg@mail.gmail.com>
 <CALe5RTs8T1H4OWhmCjbA0HnGJc_Tk0vCE83viCUU2bToGQSwsQ@mail.gmail.com>
 <CAEyhvmowUSHSx9mV9HSngqTCy_WS_0-RusPHN7E9ge96z3kmtg@mail.gmail.com>
Message-ID: <CALe5RTuW0igyeK5WbTUGg81dcHua6drpSqbCL7JEFOk1rhS8NA@mail.gmail.com>

Sorry, but I still have some grudges so to say with the way 'Maybe' is
defined.

By definition '*Maybe*' takes a type parameter which I will denote here as '
*tp*' for the sake of clarity like below -

data Maybe tp = Just tp | Nothing

Therefore '*tp*' is not representing a value perse such as '3' or 'XYZ' etc
but a data type such as '*Int*', '*Char*' etc.

But when we have to create a data type of '*Maybe*' we write '*Just 3*' and
not '*Just Int*' or '*Just Char*' which is how it's defined in the
definition (i.e. '*Just tp*' ) .

It is this discrepancy in definition and the actual value creation which is
slightly troubling me.

Thanks,
Shishir

Shishir Srivastava
 +44 (0) 750 127 5019


On Thu, Mar 26, 2015 at 5:31 PM, Corentin Dupont <corentin.dupont at gmail.com>
wrote:

> Not really, when you define the type "Maybe a":
>
> data Maybe a = Just a | Nothing
>
> Haskell is creating automatically two functions for you:
>
> Just :: a -> Maybe a
> Nothing :: Maybe a
>
> In the first case, you can think of "Just" and "Nothing" as a sort of tag
> identifying which element of the sum you have.
> It the second case it's a function, with the same name.
> A more informed person than me could say if they are indeed separated of
> if they are the same thing in GHC...
>
>
> On Thu, Mar 26, 2015 at 5:34 PM, Shishir Srivastava <
> shishir.srivastava at gmail.com> wrote:
>
>> isn't that then cyclic dependency between 'Maybe' and 'Just' ...where the
>> first one is defined in terms of second and vice-versa...?
>>
>> Shishir
>>
>>
>> On Thu, Mar 26, 2015 at 3:48 PM, Corentin Dupont <
>> corentin.dupont at gmail.com> wrote:
>>
>>> Hi Shishir,
>>> I think that's a legitimate question.
>>>
>>> By writing
>>>
>>> data Maybe a = a | Nothing
>>>
>>> you are saying that the type of the left hand side of the = is the same
>>> that right hand side (you are defining a type basically).
>>> Also you can only sum things of the same type.
>>> So you are saying:
>>> type "Maybe a" = type "a"
>>> Which is wrong.
>>> That's why "a" should be wrapped into something:
>>> type of "Just a" is indeed "Maybe a".
>>>
>>> "Just" is a type constructor:
>>> Just :: a -> Maybe a
>>> It allows you to build the Maybe.
>>>
>>> Said that, "Just" is a vocabulary choice.
>>> Personally I prefer the name choices of OCaml, Rust, Scala etc.: Option
>>> a = None | Some a
>>>
>>>
>>>
>>> On Thu, Mar 26, 2015 at 4:26 PM, Shishir Srivastava <
>>> shishir.srivastava at gmail.com> wrote:
>>>
>>>> ok..but what's with using the keyword 'Just' ? why cannot 'Maybe' be
>>>> defined like this
>>>>
>>>> data Maybe a = a | Nothing
>>>>
>>>> what's the point in having 'Just' keyword ?
>>>>
>>>> Shishir
>>>>
>>>>
>>>> On Thu, Mar 26, 2015 at 10:26 AM, Michael Alan Dorman <
>>>> mdorman at ironicdesign.com> wrote:
>>>>
>>>>> Shishir Srivastava <shishir.srivastava at gmail.com> writes:
>>>>> > After reading and re-reading the haskell tutorials I don't happen to
>>>>> > see a very convincing or appealing reason for having these data
>>>>> > types.
>>>>>
>>>>> To be clear: Maybe is the data *type*.  Just and Nothing are its data
>>>>> *constructors*.
>>>>>
>>>>> > Can anyone please explain where Maybe and Just provide the sort of
>>>>> > functionality that cannot be achieved in other languages which don't
>>>>> > have these kind.
>>>>>
>>>>> The functionality can be achieved in other languages, certainly.  The
>>>>> question is whether the clarity and safety is also achieved.
>>>>>
>>>>> When I see (as a totally contrived example):
>>>>>
>>>>>   fopen :: Maybe FileHandle
>>>>>
>>>>> I know that that function may not be able to return a FileHandle value
>>>>> all the time.  The compiler will, in fact, nag me if I do not write the
>>>>> code that calls it in such a way that it acknowledges that possibility.
>>>>>
>>>>> When I see:
>>>>>
>>>>>   FILE * fopen ( const char * filename, const char * mode );
>>>>>
>>>>> It is not immediately clear whether that can fail.  Sure, we can make
>>>>> that inference, based on what we know about filesystems, etc., but the
>>>>> compiler is never going to complain if I ignore the possibility.
>>>>>
>>>>> In my experience, programmers in many languages end up resorting to
>>>>> convention to try and work around these sorts of ambiguities.  Large
>>>>> projects have strategies for naming functions that try to pass along
>>>>> information out of band, or languages have a pervasive culture of
>>>>> "lint"
>>>>> tools that try to use heuristics to make up for what the type system
>>>>> doesn't make simple.
>>>>>
>>>>> That said, I know that doing Maybe sorts of things in languages that
>>>>> don't have, say, pattern matching, or the idea of a "failure monad",
>>>>> gets to be a drag very quickly---manually unwrapping things is at best
>>>>> awkward, having to re-wrap them just to unwrap them again in a sequence
>>>>> of computations quickly leads one to believe "it's just not worth
>>>>> it"---or you resort to exception handling, which has its own challenges
>>>>> to do well.
>>>>>
>>>>> Mike.
>>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
>>>> Beginners at haskell.org
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>>
>>
>
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From corentin.dupont at gmail.com  Fri Mar 27 09:53:23 2015
From: corentin.dupont at gmail.com (Corentin Dupont)
Date: Fri, 27 Mar 2015 10:53:23 +0100
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTuW0igyeK5WbTUGg81dcHua6drpSqbCL7JEFOk1rhS8NA@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
 <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
 <CAEyhvmrZcDRrhCvwVZ7T7i_NWr1Brt1TKE2YW7Mq2D2MGPaPBg@mail.gmail.com>
 <CALe5RTs8T1H4OWhmCjbA0HnGJc_Tk0vCE83viCUU2bToGQSwsQ@mail.gmail.com>
 <CAEyhvmowUSHSx9mV9HSngqTCy_WS_0-RusPHN7E9ge96z3kmtg@mail.gmail.com>
 <CALe5RTuW0igyeK5WbTUGg81dcHua6drpSqbCL7JEFOk1rhS8NA@mail.gmail.com>
Message-ID: <CAEyhvmrtC9S46cvSKynSH3-05YL6sFOCMrCy1Dyox4SPVJt9Og@mail.gmail.com>

You're right, "Just" is used at two different levels, it's just that it
have the same name is both cases.
One is at type level, the other at value level.
Since this is two separated levels, there is no risk of confusion and
having the same name is fine.

To be clear, you can write:

data Maybe tp = Just Int | Nothing

Then use it as:

my_value = Just 3

On Fri, Mar 27, 2015 at 10:44 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Sorry, but I still have some grudges so to say with the way 'Maybe' is
> defined.
>
> By definition '*Maybe*' takes a type parameter which I will denote here
> as '*tp*' for the sake of clarity like below -
>
> data Maybe tp = Just tp | Nothing
>
> Therefore '*tp*' is not representing a value perse such as '3' or 'XYZ'
> etc but a data type such as '*Int*', '*Char*' etc.
>
> But when we have to create a data type of '*Maybe*' we write '*Just 3*'
> and not '*Just Int*' or '*Just Char*' which is how it's defined in the
> definition (i.e. '*Just tp*' ) .
>
> It is this discrepancy in definition and the actual value creation which
> is slightly troubling me.
>
> Thanks,
> Shishir
>
> Shishir Srivastava
>  +44 (0) 750 127 5019
>
>
> On Thu, Mar 26, 2015 at 5:31 PM, Corentin Dupont <
> corentin.dupont at gmail.com> wrote:
>
>> Not really, when you define the type "Maybe a":
>>
>> data Maybe a = Just a | Nothing
>>
>> Haskell is creating automatically two functions for you:
>>
>> Just :: a -> Maybe a
>> Nothing :: Maybe a
>>
>> In the first case, you can think of "Just" and "Nothing" as a sort of tag
>> identifying which element of the sum you have.
>> It the second case it's a function, with the same name.
>> A more informed person than me could say if they are indeed separated of
>> if they are the same thing in GHC...
>>
>>
>> On Thu, Mar 26, 2015 at 5:34 PM, Shishir Srivastava <
>> shishir.srivastava at gmail.com> wrote:
>>
>>> isn't that then cyclic dependency between 'Maybe' and 'Just' ...where
>>> the first one is defined in terms of second and vice-versa...?
>>>
>>> Shishir
>>>
>>>
>>> On Thu, Mar 26, 2015 at 3:48 PM, Corentin Dupont <
>>> corentin.dupont at gmail.com> wrote:
>>>
>>>> Hi Shishir,
>>>> I think that's a legitimate question.
>>>>
>>>> By writing
>>>>
>>>> data Maybe a = a | Nothing
>>>>
>>>> you are saying that the type of the left hand side of the = is the same
>>>> that right hand side (you are defining a type basically).
>>>> Also you can only sum things of the same type.
>>>> So you are saying:
>>>> type "Maybe a" = type "a"
>>>> Which is wrong.
>>>> That's why "a" should be wrapped into something:
>>>> type of "Just a" is indeed "Maybe a".
>>>>
>>>> "Just" is a type constructor:
>>>> Just :: a -> Maybe a
>>>> It allows you to build the Maybe.
>>>>
>>>> Said that, "Just" is a vocabulary choice.
>>>> Personally I prefer the name choices of OCaml, Rust, Scala etc.:
>>>> Option a = None | Some a
>>>>
>>>>
>>>>
>>>> On Thu, Mar 26, 2015 at 4:26 PM, Shishir Srivastava <
>>>> shishir.srivastava at gmail.com> wrote:
>>>>
>>>>> ok..but what's with using the keyword 'Just' ? why cannot 'Maybe' be
>>>>> defined like this
>>>>>
>>>>> data Maybe a = a | Nothing
>>>>>
>>>>> what's the point in having 'Just' keyword ?
>>>>>
>>>>> Shishir
>>>>>
>>>>>
>>>>> On Thu, Mar 26, 2015 at 10:26 AM, Michael Alan Dorman <
>>>>> mdorman at ironicdesign.com> wrote:
>>>>>
>>>>>> Shishir Srivastava <shishir.srivastava at gmail.com> writes:
>>>>>> > After reading and re-reading the haskell tutorials I don't happen to
>>>>>> > see a very convincing or appealing reason for having these data
>>>>>> > types.
>>>>>>
>>>>>> To be clear: Maybe is the data *type*.  Just and Nothing are its data
>>>>>> *constructors*.
>>>>>>
>>>>>> > Can anyone please explain where Maybe and Just provide the sort of
>>>>>> > functionality that cannot be achieved in other languages which don't
>>>>>> > have these kind.
>>>>>>
>>>>>> The functionality can be achieved in other languages, certainly.  The
>>>>>> question is whether the clarity and safety is also achieved.
>>>>>>
>>>>>> When I see (as a totally contrived example):
>>>>>>
>>>>>>   fopen :: Maybe FileHandle
>>>>>>
>>>>>> I know that that function may not be able to return a FileHandle value
>>>>>> all the time.  The compiler will, in fact, nag me if I do not write
>>>>>> the
>>>>>> code that calls it in such a way that it acknowledges that
>>>>>> possibility.
>>>>>>
>>>>>> When I see:
>>>>>>
>>>>>>   FILE * fopen ( const char * filename, const char * mode );
>>>>>>
>>>>>> It is not immediately clear whether that can fail.  Sure, we can make
>>>>>> that inference, based on what we know about filesystems, etc., but the
>>>>>> compiler is never going to complain if I ignore the possibility.
>>>>>>
>>>>>> In my experience, programmers in many languages end up resorting to
>>>>>> convention to try and work around these sorts of ambiguities.  Large
>>>>>> projects have strategies for naming functions that try to pass along
>>>>>> information out of band, or languages have a pervasive culture of
>>>>>> "lint"
>>>>>> tools that try to use heuristics to make up for what the type system
>>>>>> doesn't make simple.
>>>>>>
>>>>>> That said, I know that doing Maybe sorts of things in languages that
>>>>>> don't have, say, pattern matching, or the idea of a "failure monad",
>>>>>> gets to be a drag very quickly---manually unwrapping things is at best
>>>>>> awkward, having to re-wrap them just to unwrap them again in a
>>>>>> sequence
>>>>>> of computations quickly leads one to believe "it's just not worth
>>>>>> it"---or you resort to exception handling, which has its own
>>>>>> challenges
>>>>>> to do well.
>>>>>>
>>>>>> Mike.
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>>
>>>>
>>>
>>
>
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From mwm at mired.org  Fri Mar 27 10:00:08 2015
From: mwm at mired.org (Mike Meyer)
Date: Fri, 27 Mar 2015 05:00:08 -0500
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTuW0igyeK5WbTUGg81dcHua6drpSqbCL7JEFOk1rhS8NA@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
 <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
 <CAEyhvmrZcDRrhCvwVZ7T7i_NWr1Brt1TKE2YW7Mq2D2MGPaPBg@mail.gmail.com>
 <CALe5RTs8T1H4OWhmCjbA0HnGJc_Tk0vCE83viCUU2bToGQSwsQ@mail.gmail.com>
 <CAEyhvmowUSHSx9mV9HSngqTCy_WS_0-RusPHN7E9ge96z3kmtg@mail.gmail.com>
 <CALe5RTuW0igyeK5WbTUGg81dcHua6drpSqbCL7JEFOk1rhS8NA@mail.gmail.com>
Message-ID: <CAD=7U2AJ0m_+-JTvcTLg_9zbtPU2d4cGFh_vb3T0qLxV5ZyReg@mail.gmail.com>

On Fri, Mar 27, 2015 at 4:44 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Sorry, but I still have some grudges so to say with the way 'Maybe' is
> defined.
>
> By definition '*Maybe*' takes a type parameter which I will denote here
> as '*tp*' for the sake of clarity like below -
>
> data Maybe tp = Just tp | Nothing
>
> Therefore '*tp*' is not representing a value perse such as '3' or 'XYZ'
> etc but a data type such as '*Int*', '*Char*' etc.
>
> But when we have to create a data type of '*Maybe*' we write '*Just 3*'
> and not '*Just Int*' or '*Just Char*' which is how it's defined in the
> definition (i.e. '*Just tp*' ) .
>
> It is this discrepancy in definition and the actual value creation which
> is slightly troubling me.
>

This was already explained, all be it in passing.

Maybe Int is a type. Just and and Nothing are functions. Nothing takes no
arguments. Just takes a single argument of the type given. There IS no type
that Just Int would represent. There are only the two types Int and Maybe
Int. 3 is of type Int. Just 3 is of type Maybe Int. Nothing is of type
Maybe Int as well.

Going back to the notion of "type plus error value", Int is the type, and
Maybe Int is the type with an error value, the error value being Nothing.

It might be educational to look at a second example. Try the Either types,
which are sometimes used in a manner similar to the Maybe types. But
instead of Nothing to indicate "an error occurred", you have Left tp to
indicate that an error occurred using the value of tp to provide
information about the error.
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From corentin.dupont at gmail.com  Fri Mar 27 10:05:42 2015
From: corentin.dupont at gmail.com (Corentin Dupont)
Date: Fri, 27 Mar 2015 11:05:42 +0100
Subject: [Haskell-beginners] Fwd:  Maybe and Just
In-Reply-To: <CAEyhvmrtC9S46cvSKynSH3-05YL6sFOCMrCy1Dyox4SPVJt9Og@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
 <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
 <CAEyhvmrZcDRrhCvwVZ7T7i_NWr1Brt1TKE2YW7Mq2D2MGPaPBg@mail.gmail.com>
 <CALe5RTs8T1H4OWhmCjbA0HnGJc_Tk0vCE83viCUU2bToGQSwsQ@mail.gmail.com>
 <CAEyhvmowUSHSx9mV9HSngqTCy_WS_0-RusPHN7E9ge96z3kmtg@mail.gmail.com>
 <CALe5RTuW0igyeK5WbTUGg81dcHua6drpSqbCL7JEFOk1rhS8NA@mail.gmail.com>
 <CAEyhvmrtC9S46cvSKynSH3-05YL6sFOCMrCy1Dyox4SPVJt9Og@mail.gmail.com>
Message-ID: <CAEyhvmospTjjP8dr6ZkeXLGSO2q4_Gr5OL7WjaLVA+QuQoJnZw@mail.gmail.com>

You're right, "Just" is used at two different levels, it's just that it
have the same name is both cases.
One is at type level, the other at value level.
Since this is two separated levels, there is no risk of confusion and
having the same name is fine.

To be clear, you can write:

data Maybe = Just Int | Nothing

Then use it as:

my_value = Just 3

It the first case, "Just" is a tag in the type definition, in the second
case it's a function constructing a value.
They just happen to have the same name (a choice of Haskell language).


On Fri, Mar 27, 2015 at 10:44 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Sorry, but I still have some grudges so to say with the way 'Maybe' is
> defined.
>
> By definition '*Maybe*' takes a type parameter which I will denote here
> as '*tp*' for the sake of clarity like below -
>
> data Maybe tp = Just tp | Nothing
>
> Therefore '*tp*' is not representing a value perse such as '3' or 'XYZ'
> etc but a data type such as '*Int*', '*Char*' etc.
>
> But when we have to create a data type of '*Maybe*' we write '*Just 3*'
> and not '*Just Int*' or '*Just Char*' which is how it's defined in the
> definition (i.e. '*Just tp*' ) .
>
> It is this discrepancy in definition and the actual value creation which
> is slightly troubling me.
>
> Thanks,
> Shishir
>
> Shishir Srivastava
>  +44 (0) 750 127 5019
>
>
> On Thu, Mar 26, 2015 at 5:31 PM, Corentin Dupont <
> corentin.dupont at gmail.com> wrote:
>
>> Not really, when you define the type "Maybe a":
>>
>> data Maybe a = Just a | Nothing
>>
>> Haskell is creating automatically two functions for you:
>>
>> Just :: a -> Maybe a
>> Nothing :: Maybe a
>>
>> In the first case, you can think of "Just" and "Nothing" as a sort of tag
>> identifying which element of the sum you have.
>> It the second case it's a function, with the same name.
>> A more informed person than me could say if they are indeed separated of
>> if they are the same thing in GHC...
>>
>>
>> On Thu, Mar 26, 2015 at 5:34 PM, Shishir Srivastava <
>> shishir.srivastava at gmail.com> wrote:
>>
>>> isn't that then cyclic dependency between 'Maybe' and 'Just' ...where
>>> the first one is defined in terms of second and vice-versa...?
>>>
>>> Shishir
>>>
>>>
>>> On Thu, Mar 26, 2015 at 3:48 PM, Corentin Dupont <
>>> corentin.dupont at gmail.com> wrote:
>>>
>>>> Hi Shishir,
>>>> I think that's a legitimate question.
>>>>
>>>> By writing
>>>>
>>>> data Maybe a = a | Nothing
>>>>
>>>> you are saying that the type of the left hand side of the = is the same
>>>> that right hand side (you are defining a type basically).
>>>> Also you can only sum things of the same type.
>>>> So you are saying:
>>>> type "Maybe a" = type "a"
>>>> Which is wrong.
>>>> That's why "a" should be wrapped into something:
>>>> type of "Just a" is indeed "Maybe a".
>>>>
>>>> "Just" is a type constructor:
>>>> Just :: a -> Maybe a
>>>> It allows you to build the Maybe.
>>>>
>>>> Said that, "Just" is a vocabulary choice.
>>>> Personally I prefer the name choices of OCaml, Rust, Scala etc.:
>>>> Option a = None | Some a
>>>>
>>>>
>>>>
>>>> On Thu, Mar 26, 2015 at 4:26 PM, Shishir Srivastava <
>>>> shishir.srivastava at gmail.com> wrote:
>>>>
>>>>> ok..but what's with using the keyword 'Just' ? why cannot 'Maybe' be
>>>>> defined like this
>>>>>
>>>>> data Maybe a = a | Nothing
>>>>>
>>>>> what's the point in having 'Just' keyword ?
>>>>>
>>>>> Shishir
>>>>>
>>>>>
>>>>> On Thu, Mar 26, 2015 at 10:26 AM, Michael Alan Dorman <
>>>>> mdorman at ironicdesign.com> wrote:
>>>>>
>>>>>> Shishir Srivastava <shishir.srivastava at gmail.com> writes:
>>>>>> > After reading and re-reading the haskell tutorials I don't happen to
>>>>>> > see a very convincing or appealing reason for having these data
>>>>>> > types.
>>>>>>
>>>>>> To be clear: Maybe is the data *type*.  Just and Nothing are its data
>>>>>> *constructors*.
>>>>>>
>>>>>> > Can anyone please explain where Maybe and Just provide the sort of
>>>>>> > functionality that cannot be achieved in other languages which don't
>>>>>> > have these kind.
>>>>>>
>>>>>> The functionality can be achieved in other languages, certainly.  The
>>>>>> question is whether the clarity and safety is also achieved.
>>>>>>
>>>>>> When I see (as a totally contrived example):
>>>>>>
>>>>>>   fopen :: Maybe FileHandle
>>>>>>
>>>>>> I know that that function may not be able to return a FileHandle value
>>>>>> all the time.  The compiler will, in fact, nag me if I do not write
>>>>>> the
>>>>>> code that calls it in such a way that it acknowledges that
>>>>>> possibility.
>>>>>>
>>>>>> When I see:
>>>>>>
>>>>>>   FILE * fopen ( const char * filename, const char * mode );
>>>>>>
>>>>>> It is not immediately clear whether that can fail.  Sure, we can make
>>>>>> that inference, based on what we know about filesystems, etc., but the
>>>>>> compiler is never going to complain if I ignore the possibility.
>>>>>>
>>>>>> In my experience, programmers in many languages end up resorting to
>>>>>> convention to try and work around these sorts of ambiguities.  Large
>>>>>> projects have strategies for naming functions that try to pass along
>>>>>> information out of band, or languages have a pervasive culture of
>>>>>> "lint"
>>>>>> tools that try to use heuristics to make up for what the type system
>>>>>> doesn't make simple.
>>>>>>
>>>>>> That said, I know that doing Maybe sorts of things in languages that
>>>>>> don't have, say, pattern matching, or the idea of a "failure monad",
>>>>>> gets to be a drag very quickly---manually unwrapping things is at best
>>>>>> awkward, having to re-wrap them just to unwrap them again in a
>>>>>> sequence
>>>>>> of computations quickly leads one to believe "it's just not worth
>>>>>> it"---or you resort to exception handling, which has its own
>>>>>> challenges
>>>>>> to do well.
>>>>>>
>>>>>> Mike.
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>>
>>>>
>>>
>>
>
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From akaberto at gmail.com  Fri Mar 27 10:07:16 2015
From: akaberto at gmail.com (akash g)
Date: Fri, 27 Mar 2015 15:37:16 +0530
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTuW0igyeK5WbTUGg81dcHua6drpSqbCL7JEFOk1rhS8NA@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
 <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
 <CAEyhvmrZcDRrhCvwVZ7T7i_NWr1Brt1TKE2YW7Mq2D2MGPaPBg@mail.gmail.com>
 <CALe5RTs8T1H4OWhmCjbA0HnGJc_Tk0vCE83viCUU2bToGQSwsQ@mail.gmail.com>
 <CAEyhvmowUSHSx9mV9HSngqTCy_WS_0-RusPHN7E9ge96z3kmtg@mail.gmail.com>
 <CALe5RTuW0igyeK5WbTUGg81dcHua6drpSqbCL7JEFOk1rhS8NA@mail.gmail.com>
Message-ID: <CALiga_c7C=v_6oCTCywFetgOQsMf06ACb6GFNxPF6cce5yA7oQ@mail.gmail.com>

Think of the Maybe type as

-- Valid syntax with GADTs, I think
data Maybe a where
   Nothing :: Maybe a
   Just       :: a -> Maybe a

You are confusing types with actual values :  (Just 3) is a value with
type, (Maybe Int).

Think of it this way: A value is an object with some type and you have
Maybe value constructors via Nothing and Just.





On Fri, Mar 27, 2015 at 3:14 PM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Sorry, but I still have some grudges so to say with the way 'Maybe' is
> defined.
>
> By definition '*Maybe*' takes a type parameter which I will denote here
> as '*tp*' for the sake of clarity like below -
>
> data Maybe tp = Just tp | Nothing
>
> Therefore '*tp*' is not representing a value perse such as '3' or 'XYZ'
> etc but a data type such as '*Int*', '*Char*' etc.
>
> But when we have to create a data type of '*Maybe*' we write '*Just 3*'
> and not '*Just Int*' or '*Just Char*' which is how it's defined in the
> definition (i.e. '*Just tp*' ) .
>
> It is this discrepancy in definition and the actual value creation which
> is slightly troubling me.
>
> Thanks,
> Shishir
>
> Shishir Srivastava
>  +44 (0) 750 127 5019
>
>
> On Thu, Mar 26, 2015 at 5:31 PM, Corentin Dupont <
> corentin.dupont at gmail.com> wrote:
>
>> Not really, when you define the type "Maybe a":
>>
>> data Maybe a = Just a | Nothing
>>
>> Haskell is creating automatically two functions for you:
>>
>> Just :: a -> Maybe a
>> Nothing :: Maybe a
>>
>> In the first case, you can think of "Just" and "Nothing" as a sort of tag
>> identifying which element of the sum you have.
>> It the second case it's a function, with the same name.
>> A more informed person than me could say if they are indeed separated of
>> if they are the same thing in GHC...
>>
>>
>> On Thu, Mar 26, 2015 at 5:34 PM, Shishir Srivastava <
>> shishir.srivastava at gmail.com> wrote:
>>
>>> isn't that then cyclic dependency between 'Maybe' and 'Just' ...where
>>> the first one is defined in terms of second and vice-versa...?
>>>
>>> Shishir
>>>
>>>
>>> On Thu, Mar 26, 2015 at 3:48 PM, Corentin Dupont <
>>> corentin.dupont at gmail.com> wrote:
>>>
>>>> Hi Shishir,
>>>> I think that's a legitimate question.
>>>>
>>>> By writing
>>>>
>>>> data Maybe a = a | Nothing
>>>>
>>>> you are saying that the type of the left hand side of the = is the same
>>>> that right hand side (you are defining a type basically).
>>>> Also you can only sum things of the same type.
>>>> So you are saying:
>>>> type "Maybe a" = type "a"
>>>> Which is wrong.
>>>> That's why "a" should be wrapped into something:
>>>> type of "Just a" is indeed "Maybe a".
>>>>
>>>> "Just" is a type constructor:
>>>> Just :: a -> Maybe a
>>>> It allows you to build the Maybe.
>>>>
>>>> Said that, "Just" is a vocabulary choice.
>>>> Personally I prefer the name choices of OCaml, Rust, Scala etc.:
>>>> Option a = None | Some a
>>>>
>>>>
>>>>
>>>> On Thu, Mar 26, 2015 at 4:26 PM, Shishir Srivastava <
>>>> shishir.srivastava at gmail.com> wrote:
>>>>
>>>>> ok..but what's with using the keyword 'Just' ? why cannot 'Maybe' be
>>>>> defined like this
>>>>>
>>>>> data Maybe a = a | Nothing
>>>>>
>>>>> what's the point in having 'Just' keyword ?
>>>>>
>>>>> Shishir
>>>>>
>>>>>
>>>>> On Thu, Mar 26, 2015 at 10:26 AM, Michael Alan Dorman <
>>>>> mdorman at ironicdesign.com> wrote:
>>>>>
>>>>>> Shishir Srivastava <shishir.srivastava at gmail.com> writes:
>>>>>> > After reading and re-reading the haskell tutorials I don't happen to
>>>>>> > see a very convincing or appealing reason for having these data
>>>>>> > types.
>>>>>>
>>>>>> To be clear: Maybe is the data *type*.  Just and Nothing are its data
>>>>>> *constructors*.
>>>>>>
>>>>>> > Can anyone please explain where Maybe and Just provide the sort of
>>>>>> > functionality that cannot be achieved in other languages which don't
>>>>>> > have these kind.
>>>>>>
>>>>>> The functionality can be achieved in other languages, certainly.  The
>>>>>> question is whether the clarity and safety is also achieved.
>>>>>>
>>>>>> When I see (as a totally contrived example):
>>>>>>
>>>>>>   fopen :: Maybe FileHandle
>>>>>>
>>>>>> I know that that function may not be able to return a FileHandle value
>>>>>> all the time.  The compiler will, in fact, nag me if I do not write
>>>>>> the
>>>>>> code that calls it in such a way that it acknowledges that
>>>>>> possibility.
>>>>>>
>>>>>> When I see:
>>>>>>
>>>>>>   FILE * fopen ( const char * filename, const char * mode );
>>>>>>
>>>>>> It is not immediately clear whether that can fail.  Sure, we can make
>>>>>> that inference, based on what we know about filesystems, etc., but the
>>>>>> compiler is never going to complain if I ignore the possibility.
>>>>>>
>>>>>> In my experience, programmers in many languages end up resorting to
>>>>>> convention to try and work around these sorts of ambiguities.  Large
>>>>>> projects have strategies for naming functions that try to pass along
>>>>>> information out of band, or languages have a pervasive culture of
>>>>>> "lint"
>>>>>> tools that try to use heuristics to make up for what the type system
>>>>>> doesn't make simple.
>>>>>>
>>>>>> That said, I know that doing Maybe sorts of things in languages that
>>>>>> don't have, say, pattern matching, or the idea of a "failure monad",
>>>>>> gets to be a drag very quickly---manually unwrapping things is at best
>>>>>> awkward, having to re-wrap them just to unwrap them again in a
>>>>>> sequence
>>>>>> of computations quickly leads one to believe "it's just not worth
>>>>>> it"---or you resort to exception handling, which has its own
>>>>>> challenges
>>>>>> to do well.
>>>>>>
>>>>>> Mike.
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>>
>>>>
>>>
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From shishir.srivastava at gmail.com  Fri Mar 27 10:14:39 2015
From: shishir.srivastava at gmail.com (Shishir Srivastava)
Date: Fri, 27 Mar 2015 10:14:39 +0000
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CAEyhvmospTjjP8dr6ZkeXLGSO2q4_Gr5OL7WjaLVA+QuQoJnZw@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
 <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
 <CAEyhvmrZcDRrhCvwVZ7T7i_NWr1Brt1TKE2YW7Mq2D2MGPaPBg@mail.gmail.com>
 <CALe5RTs8T1H4OWhmCjbA0HnGJc_Tk0vCE83viCUU2bToGQSwsQ@mail.gmail.com>
 <CAEyhvmowUSHSx9mV9HSngqTCy_WS_0-RusPHN7E9ge96z3kmtg@mail.gmail.com>
 <CALe5RTuW0igyeK5WbTUGg81dcHua6drpSqbCL7JEFOk1rhS8NA@mail.gmail.com>
 <CAEyhvmrtC9S46cvSKynSH3-05YL6sFOCMrCy1Dyox4SPVJt9Og@mail.gmail.com>
 <CAEyhvmospTjjP8dr6ZkeXLGSO2q4_Gr5OL7WjaLVA+QuQoJnZw@mail.gmail.com>
Message-ID: <CALe5RTu2UqzC39+TYO_oh5WSfHqkQiBKPztQstFA2YP3=hTvbg@mail.gmail.com>

Thanks Corentin. I think it makes more sense now that you've mentioned the
fact that '*Just*' in the type definition of '*Maybe*' is only a *tag*.

So would it be correct to re-define Maybe data type as follows -

data Maybe tp = Justin tp | Nothing

and then create the type value of *Maybe *as -

*Just 3*

*?*



Shishir Srivastava
 +44 (0) 750 127 5019


On Fri, Mar 27, 2015 at 10:05 AM, Corentin Dupont <corentin.dupont at gmail.com
> wrote:

>
> You're right, "Just" is used at two different levels, it's just that it
> have the same name is both cases.
> One is at type level, the other at value level.
> Since this is two separated levels, there is no risk of confusion and
> having the same name is fine.
>
> To be clear, you can write:
>
> data Maybe = Just Int | Nothing
>
> Then use it as:
>
> my_value = Just 3
>
> It the first case, "Just" is a tag in the type definition, in the second
> case it's a function constructing a value.
> They just happen to have the same name (a choice of Haskell language).
>
>
> On Fri, Mar 27, 2015 at 10:44 AM, Shishir Srivastava <
> shishir.srivastava at gmail.com> wrote:
>
>> Sorry, but I still have some grudges so to say with the way 'Maybe' is
>> defined.
>>
>> By definition '*Maybe*' takes a type parameter which I will denote here
>> as '*tp*' for the sake of clarity like below -
>>
>> data Maybe tp = Just tp | Nothing
>>
>> Therefore '*tp*' is not representing a value perse such as '3' or 'XYZ'
>> etc but a data type such as '*Int*', '*Char*' etc.
>>
>> But when we have to create a data type of '*Maybe*' we write '*Just 3*'
>> and not '*Just Int*' or '*Just Char*' which is how it's defined in the
>> definition (i.e. '*Just tp*' ) .
>>
>> It is this discrepancy in definition and the actual value creation which
>> is slightly troubling me.
>>
>> Thanks,
>> Shishir
>>
>> Shishir Srivastava
>>  +44 (0) 750 127 5019
>>
>>
>> On Thu, Mar 26, 2015 at 5:31 PM, Corentin Dupont <
>> corentin.dupont at gmail.com> wrote:
>>
>>> Not really, when you define the type "Maybe a":
>>>
>>> data Maybe a = Just a | Nothing
>>>
>>> Haskell is creating automatically two functions for you:
>>>
>>> Just :: a -> Maybe a
>>> Nothing :: Maybe a
>>>
>>> In the first case, you can think of "Just" and "Nothing" as a sort of
>>> tag identifying which element of the sum you have.
>>> It the second case it's a function, with the same name.
>>> A more informed person than me could say if they are indeed separated of
>>> if they are the same thing in GHC...
>>>
>>>
>>> On Thu, Mar 26, 2015 at 5:34 PM, Shishir Srivastava <
>>> shishir.srivastava at gmail.com> wrote:
>>>
>>>> isn't that then cyclic dependency between 'Maybe' and 'Just' ...where
>>>> the first one is defined in terms of second and vice-versa...?
>>>>
>>>> Shishir
>>>>
>>>>
>>>> On Thu, Mar 26, 2015 at 3:48 PM, Corentin Dupont <
>>>> corentin.dupont at gmail.com> wrote:
>>>>
>>>>> Hi Shishir,
>>>>> I think that's a legitimate question.
>>>>>
>>>>> By writing
>>>>>
>>>>> data Maybe a = a | Nothing
>>>>>
>>>>> you are saying that the type of the left hand side of the = is the
>>>>> same that right hand side (you are defining a type basically).
>>>>> Also you can only sum things of the same type.
>>>>> So you are saying:
>>>>> type "Maybe a" = type "a"
>>>>> Which is wrong.
>>>>> That's why "a" should be wrapped into something:
>>>>> type of "Just a" is indeed "Maybe a".
>>>>>
>>>>> "Just" is a type constructor:
>>>>> Just :: a -> Maybe a
>>>>> It allows you to build the Maybe.
>>>>>
>>>>> Said that, "Just" is a vocabulary choice.
>>>>> Personally I prefer the name choices of OCaml, Rust, Scala etc.:
>>>>> Option a = None | Some a
>>>>>
>>>>>
>>>>>
>>>>> On Thu, Mar 26, 2015 at 4:26 PM, Shishir Srivastava <
>>>>> shishir.srivastava at gmail.com> wrote:
>>>>>
>>>>>> ok..but what's with using the keyword 'Just' ? why cannot 'Maybe' be
>>>>>> defined like this
>>>>>>
>>>>>> data Maybe a = a | Nothing
>>>>>>
>>>>>> what's the point in having 'Just' keyword ?
>>>>>>
>>>>>> Shishir
>>>>>>
>>>>>>
>>>>>> On Thu, Mar 26, 2015 at 10:26 AM, Michael Alan Dorman <
>>>>>> mdorman at ironicdesign.com> wrote:
>>>>>>
>>>>>>> Shishir Srivastava <shishir.srivastava at gmail.com> writes:
>>>>>>> > After reading and re-reading the haskell tutorials I don't happen
>>>>>>> to
>>>>>>> > see a very convincing or appealing reason for having these data
>>>>>>> > types.
>>>>>>>
>>>>>>> To be clear: Maybe is the data *type*.  Just and Nothing are its data
>>>>>>> *constructors*.
>>>>>>>
>>>>>>> > Can anyone please explain where Maybe and Just provide the sort of
>>>>>>> > functionality that cannot be achieved in other languages which
>>>>>>> don't
>>>>>>> > have these kind.
>>>>>>>
>>>>>>> The functionality can be achieved in other languages, certainly.  The
>>>>>>> question is whether the clarity and safety is also achieved.
>>>>>>>
>>>>>>> When I see (as a totally contrived example):
>>>>>>>
>>>>>>>   fopen :: Maybe FileHandle
>>>>>>>
>>>>>>> I know that that function may not be able to return a FileHandle
>>>>>>> value
>>>>>>> all the time.  The compiler will, in fact, nag me if I do not write
>>>>>>> the
>>>>>>> code that calls it in such a way that it acknowledges that
>>>>>>> possibility.
>>>>>>>
>>>>>>> When I see:
>>>>>>>
>>>>>>>   FILE * fopen ( const char * filename, const char * mode );
>>>>>>>
>>>>>>> It is not immediately clear whether that can fail.  Sure, we can make
>>>>>>> that inference, based on what we know about filesystems, etc., but
>>>>>>> the
>>>>>>> compiler is never going to complain if I ignore the possibility.
>>>>>>>
>>>>>>> In my experience, programmers in many languages end up resorting to
>>>>>>> convention to try and work around these sorts of ambiguities.  Large
>>>>>>> projects have strategies for naming functions that try to pass along
>>>>>>> information out of band, or languages have a pervasive culture of
>>>>>>> "lint"
>>>>>>> tools that try to use heuristics to make up for what the type system
>>>>>>> doesn't make simple.
>>>>>>>
>>>>>>> That said, I know that doing Maybe sorts of things in languages that
>>>>>>> don't have, say, pattern matching, or the idea of a "failure monad",
>>>>>>> gets to be a drag very quickly---manually unwrapping things is at
>>>>>>> best
>>>>>>> awkward, having to re-wrap them just to unwrap them again in a
>>>>>>> sequence
>>>>>>> of computations quickly leads one to believe "it's just not worth
>>>>>>> it"---or you resort to exception handling, which has its own
>>>>>>> challenges
>>>>>>> to do well.
>>>>>>>
>>>>>>> Mike.
>>>>>>>
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> Beginners mailing list
>>>>>> Beginners at haskell.org
>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
>
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From timmelzer at gmail.com  Fri Mar 27 12:31:03 2015
From: timmelzer at gmail.com (Norbert Melzer)
Date: Fri, 27 Mar 2015 13:31:03 +0100
Subject: [Haskell-beginners] Writing a Parser with attoparsec that reads
	from a list of tokens
Message-ID: <CA+bCVsuC6F2Zm3Vb8Ac0S27tob1kNYdnmuodpc8PcA2xeKJE_g@mail.gmail.com>

Hi there!

I want to write some small toy language using attoparsec.

So I thought, first step tokenize. Let attoparsec consume the input stream
and produce a list of tokens.
Second step, parse tokens and produce the AST.

Using parsec this would be possible easily and is documented. But I want to
use attoparsec for this task, because I am interested in attoparsecs
capability to have the input in chunks.

TIA
Norbert
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From mrz.vtl at gmail.com  Fri Mar 27 13:38:50 2015
From: mrz.vtl at gmail.com (Maurizio Vitale)
Date: Fri, 27 Mar 2015 09:38:50 -0400
Subject: [Haskell-beginners] parser frontend
Message-ID: <CAAeLbQ+42zh3sRHgVc5e6XGif5ZP1vprX5m_NpSVU57ZGUQ_xQ@mail.gmail.com>

G'day,
  I'm trying to decide how to architect the frontend of a compiler I'm
using as an excuse for learning Haskell.

  Suppose we have a language (SystemVerilog) that has the notion of
multi-file compilation units.
This means that specific subset of files form separate scopes for certain
things.
For instance if we represent with lists of lists compilation units:
[[a,b,c], [d], [e,f]]
a macro definition in file b, would affect the rest of the file and file c,
but wouldn't have effect on files d,e or a.

Furthermore, if a toplevel construct is split between two files, say c and
d, the compiler should treat the original compilation unit specification as
if it was:
[[a,b,(c,d)][e,f]], where (c,d) means we're compiling the logical
concatenation of c and d.
If (c,d) is also incomplete, (c,d,e) should be tried and so on.

Now in well designed systems, all files can actually be compiled in
parallel and so I'd like to optimize for that case and recompile files
(either because we need side effects from files before them or because
they're incomplete. So I'm not to concerned if wasted work is done for the
degenerate cases.

Any suggestion on how to go about this?
Le's assume that parsing a file is done with a function:
p :: file -> Either Error (ast, [defs], [use]) where the [defs] are things
that might affects files down the line and [use] are things that, if
defined by some prior file, cause recompilation.

I'm interested in both a sequential and parallel solution and it would be
sweet if they where similar, with the proper abstractions.

Thanks a lot for any insight/ideas,

  Maurizio
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From hjgtuyl at chello.nl  Fri Mar 27 15:08:45 2015
From: hjgtuyl at chello.nl (Henk-Jan van Tuyl)
Date: Fri, 27 Mar 2015 16:08:45 +0100
Subject: [Haskell-beginners] Writing a Parser with attoparsec that reads
 from a list of tokens
In-Reply-To: <CA+bCVsuC6F2Zm3Vb8Ac0S27tob1kNYdnmuodpc8PcA2xeKJE_g@mail.gmail.com>
References: <CA+bCVsuC6F2Zm3Vb8Ac0S27tob1kNYdnmuodpc8PcA2xeKJE_g@mail.gmail.com>
Message-ID: <op.xv5wc0b4pz0j5l@alquantor>

On Fri, 27 Mar 2015 13:31:03 +0100, Norbert Melzer <timmelzer at gmail.com>  
wrote:

> Hi there!
>
> I want to write some small toy language using attoparsec.
>
> So I thought, first step tokenize. Let attoparsec consume the input  
> stream
> and produce a list of tokens.
> Second step, parse tokens and produce the AST.
>
> Using parsec this would be possible easily and is documented. But I want  
> to
> use attoparsec for this task, because I am interested in attoparsecs
> capability to have the input in chunks.

You can use the list of reverse dependencies for attoparsec
   http://packdeps.haskellers.com/reverse/attoparsec
to find usage examples.

Regards,
Henk-Jan van Tuyl


-- 
Folding at home
What if you could share your unused computer power to help find a cure? In  
just 5 minutes you can join the world's biggest networked computer and get  
us closer sooner. Watch the video.
http://folding.stanford.edu/


http://Van.Tuyl.eu/
http://members.chello.nl/hjgtuyl/tourdemonad.html
Haskell programming
--

From edwards.benj at gmail.com  Fri Mar 27 15:20:37 2015
From: edwards.benj at gmail.com (Benjamin Edwards)
Date: Fri, 27 Mar 2015 15:20:37 +0000
Subject: [Haskell-beginners] Writing a Parser with attoparsec that reads
 from a list of tokens
In-Reply-To: <op.xv5wc0b4pz0j5l@alquantor>
References: <CA+bCVsuC6F2Zm3Vb8Ac0S27tob1kNYdnmuodpc8PcA2xeKJE_g@mail.gmail.com>
 <op.xv5wc0b4pz0j5l@alquantor>
Message-ID: <CAN6k4ng7rNNUB5zk0M0pHEVt2hJV_m3AupYdW_OUyCCiAFp1Qw@mail.gmail.com>

The thing is you can write parsers for tokens, then write parsers that
consume those tokens by exclusively using the token parsers as the building
blocks rather than the raw steam, all using the same combinators and
mechanisms. This is much more flexible.

Ben

On Fri, 27 Mar 2015 at 15:08 Henk-Jan van Tuyl <hjgtuyl at chello.nl> wrote:

> On Fri, 27 Mar 2015 13:31:03 +0100, Norbert Melzer <timmelzer at gmail.com>
> wrote:
>
> > Hi there!
> >
> > I want to write some small toy language using attoparsec.
> >
> > So I thought, first step tokenize. Let attoparsec consume the input
> > stream
> > and produce a list of tokens.
> > Second step, parse tokens and produce the AST.
> >
> > Using parsec this would be possible easily and is documented. But I want
> > to
> > use attoparsec for this task, because I am interested in attoparsecs
> > capability to have the input in chunks.
>
> You can use the list of reverse dependencies for attoparsec
>    http://packdeps.haskellers.com/reverse/attoparsec
> to find usage examples.
>
> Regards,
> Henk-Jan van Tuyl
>
>
> --
> Folding at home
> What if you could share your unused computer power to help find a cure? In
> just 5 minutes you can join the world's biggest networked computer and get
> us closer sooner. Watch the video.
> http://folding.stanford.edu/
>
>
> http://Van.Tuyl.eu/
> http://members.chello.nl/hjgtuyl/tourdemonad.html
> Haskell programming
> --
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From joel.neely at gmail.com  Fri Mar 27 19:01:56 2015
From: joel.neely at gmail.com (Joel Neely)
Date: Fri, 27 Mar 2015 14:01:56 -0500
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CALe5RTu2UqzC39+TYO_oh5WSfHqkQiBKPztQstFA2YP3=hTvbg@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
 <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
 <CAEyhvmrZcDRrhCvwVZ7T7i_NWr1Brt1TKE2YW7Mq2D2MGPaPBg@mail.gmail.com>
 <CALe5RTs8T1H4OWhmCjbA0HnGJc_Tk0vCE83viCUU2bToGQSwsQ@mail.gmail.com>
 <CAEyhvmowUSHSx9mV9HSngqTCy_WS_0-RusPHN7E9ge96z3kmtg@mail.gmail.com>
 <CALe5RTuW0igyeK5WbTUGg81dcHua6drpSqbCL7JEFOk1rhS8NA@mail.gmail.com>
 <CAEyhvmrtC9S46cvSKynSH3-05YL6sFOCMrCy1Dyox4SPVJt9Og@mail.gmail.com>
 <CAEyhvmospTjjP8dr6ZkeXLGSO2q4_Gr5OL7WjaLVA+QuQoJnZw@mail.gmail.com>
 <CALe5RTu2UqzC39+TYO_oh5WSfHqkQiBKPztQstFA2YP3=hTvbg@mail.gmail.com>
Message-ID: <CAEEzXAiSY4_5q7NACWU2s_cfr-Nx95Wy8iT1f1Xc=khTkGL75Q@mail.gmail.com>

Shishir,

Would I be right to guess that you're coming from an OO background? If so,
I'd suggest thinking about the difference between declaring a constructor
and invoking it.

I suggest that you can read the Haskell declaration

data Maybe tp = Just tp | Nothing


as declaring ?a data type and specifying two constructors that can return
an instance (in the OO sense of the word) of that data type. But when the
expression

Just "FOO"


is invoking one of those constructors on a value of type String, so the
resulting value has type Maybe String.

Because Maybe is an instance of Show (in the Haskell sense of the word),
there's a show function that can be applied to that value to produce
something that looks like the line above. That's roughly equivalent to
having a toString method on a Java class that returns a string that looks
like a constructor call that could produce the actual object.

-jn-

On Fri, Mar 27, 2015 at 5:14 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Thanks Corentin. I think it makes more sense now that you've mentioned the
> fact that '*Just*' in the type definition of '*Maybe*' is only a *tag*.
>
> So would it be correct to re-define Maybe data type as follows -
>
> data Maybe tp = Justin tp | Nothing
>
> and then create the type value of *Maybe *as -
>
> *Just 3*
>
> *?*
>
>
>
> Shishir Srivastava
>  +44 (0) 750 127 5019
>
>
> On Fri, Mar 27, 2015 at 10:05 AM, Corentin Dupont <
> corentin.dupont at gmail.com> wrote:
>
>>
>> You're right, "Just" is used at two different levels, it's just that it
>> have the same name is both cases.
>> One is at type level, the other at value level.
>> Since this is two separated levels, there is no risk of confusion and
>> having the same name is fine.
>>
>> To be clear, you can write:
>>
>> data Maybe = Just Int | Nothing
>>
>> Then use it as:
>>
>> my_value = Just 3
>>
>> It the first case, "Just" is a tag in the type definition, in the second
>> case it's a function constructing a value.
>> They just happen to have the same name (a choice of Haskell language).
>>
>>
>> On Fri, Mar 27, 2015 at 10:44 AM, Shishir Srivastava <
>> shishir.srivastava at gmail.com> wrote:
>>
>>> Sorry, but I still have some grudges so to say with the way 'Maybe' is
>>> defined.
>>>
>>> By definition '*Maybe*' takes a type parameter which I will denote here
>>> as '*tp*' for the sake of clarity like below -
>>>
>>> data Maybe tp = Just tp | Nothing
>>>
>>> Therefore '*tp*' is not representing a value perse such as '3' or 'XYZ'
>>> etc but a data type such as '*Int*', '*Char*' etc.
>>>
>>> But when we have to create a data type of '*Maybe*' we write '*Just 3*'
>>> and not '*Just Int*' or '*Just Char*' which is how it's defined in the
>>> definition (i.e. '*Just tp*' ) .
>>>
>>> It is this discrepancy in definition and the actual value creation which
>>> is slightly troubling me.
>>>
>>> Thanks,
>>> Shishir
>>>
>>> Shishir Srivastava
>>>  +44 (0) 750 127 5019
>>>
>>>
>>> On Thu, Mar 26, 2015 at 5:31 PM, Corentin Dupont <
>>> corentin.dupont at gmail.com> wrote:
>>>
>>>> Not really, when you define the type "Maybe a":
>>>>
>>>> data Maybe a = Just a | Nothing
>>>>
>>>> Haskell is creating automatically two functions for you:
>>>>
>>>> Just :: a -> Maybe a
>>>> Nothing :: Maybe a
>>>>
>>>> In the first case, you can think of "Just" and "Nothing" as a sort of
>>>> tag identifying which element of the sum you have.
>>>> It the second case it's a function, with the same name.
>>>> A more informed person than me could say if they are indeed separated
>>>> of if they are the same thing in GHC...
>>>>
>>>>
>>>> On Thu, Mar 26, 2015 at 5:34 PM, Shishir Srivastava <
>>>> shishir.srivastava at gmail.com> wrote:
>>>>
>>>>> isn't that then cyclic dependency between 'Maybe' and 'Just' ...where
>>>>> the first one is defined in terms of second and vice-versa...?
>>>>>
>>>>> Shishir
>>>>>
>>>>>
>>>>> On Thu, Mar 26, 2015 at 3:48 PM, Corentin Dupont <
>>>>> corentin.dupont at gmail.com> wrote:
>>>>>
>>>>>> Hi Shishir,
>>>>>> I think that's a legitimate question.
>>>>>>
>>>>>> By writing
>>>>>>
>>>>>> data Maybe a = a | Nothing
>>>>>>
>>>>>> you are saying that the type of the left hand side of the = is the
>>>>>> same that right hand side (you are defining a type basically).
>>>>>> Also you can only sum things of the same type.
>>>>>> So you are saying:
>>>>>> type "Maybe a" = type "a"
>>>>>> Which is wrong.
>>>>>> That's why "a" should be wrapped into something:
>>>>>> type of "Just a" is indeed "Maybe a".
>>>>>>
>>>>>> "Just" is a type constructor:
>>>>>> Just :: a -> Maybe a
>>>>>> It allows you to build the Maybe.
>>>>>>
>>>>>> Said that, "Just" is a vocabulary choice.
>>>>>> Personally I prefer the name choices of OCaml, Rust, Scala etc.:
>>>>>> Option a = None | Some a
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Thu, Mar 26, 2015 at 4:26 PM, Shishir Srivastava <
>>>>>> shishir.srivastava at gmail.com> wrote:
>>>>>>
>>>>>>> ok..but what's with using the keyword 'Just' ? why cannot 'Maybe' be
>>>>>>> defined like this
>>>>>>>
>>>>>>> data Maybe a = a | Nothing
>>>>>>>
>>>>>>> what's the point in having 'Just' keyword ?
>>>>>>>
>>>>>>> Shishir
>>>>>>>
>>>>>>>
>>>>>>> On Thu, Mar 26, 2015 at 10:26 AM, Michael Alan Dorman <
>>>>>>> mdorman at ironicdesign.com> wrote:
>>>>>>>
>>>>>>>> Shishir Srivastava <shishir.srivastava at gmail.com> writes:
>>>>>>>> > After reading and re-reading the haskell tutorials I don't happen
>>>>>>>> to
>>>>>>>> > see a very convincing or appealing reason for having these data
>>>>>>>> > types.
>>>>>>>>
>>>>>>>> To be clear: Maybe is the data *type*.  Just and Nothing are its
>>>>>>>> data
>>>>>>>> *constructors*.
>>>>>>>>
>>>>>>>> > Can anyone please explain where Maybe and Just provide the sort of
>>>>>>>> > functionality that cannot be achieved in other languages which
>>>>>>>> don't
>>>>>>>> > have these kind.
>>>>>>>>
>>>>>>>> The functionality can be achieved in other languages, certainly.
>>>>>>>> The
>>>>>>>> question is whether the clarity and safety is also achieved.
>>>>>>>>
>>>>>>>> When I see (as a totally contrived example):
>>>>>>>>
>>>>>>>>   fopen :: Maybe FileHandle
>>>>>>>>
>>>>>>>> I know that that function may not be able to return a FileHandle
>>>>>>>> value
>>>>>>>> all the time.  The compiler will, in fact, nag me if I do not write
>>>>>>>> the
>>>>>>>> code that calls it in such a way that it acknowledges that
>>>>>>>> possibility.
>>>>>>>>
>>>>>>>> When I see:
>>>>>>>>
>>>>>>>>   FILE * fopen ( const char * filename, const char * mode );
>>>>>>>>
>>>>>>>> It is not immediately clear whether that can fail.  Sure, we can
>>>>>>>> make
>>>>>>>> that inference, based on what we know about filesystems, etc., but
>>>>>>>> the
>>>>>>>> compiler is never going to complain if I ignore the possibility.
>>>>>>>>
>>>>>>>> In my experience, programmers in many languages end up resorting to
>>>>>>>> convention to try and work around these sorts of ambiguities.  Large
>>>>>>>> projects have strategies for naming functions that try to pass along
>>>>>>>> information out of band, or languages have a pervasive culture of
>>>>>>>> "lint"
>>>>>>>> tools that try to use heuristics to make up for what the type system
>>>>>>>> doesn't make simple.
>>>>>>>>
>>>>>>>> That said, I know that doing Maybe sorts of things in languages that
>>>>>>>> don't have, say, pattern matching, or the idea of a "failure monad",
>>>>>>>> gets to be a drag very quickly---manually unwrapping things is at
>>>>>>>> best
>>>>>>>> awkward, having to re-wrap them just to unwrap them again in a
>>>>>>>> sequence
>>>>>>>> of computations quickly leads one to believe "it's just not worth
>>>>>>>> it"---or you resort to exception handling, which has its own
>>>>>>>> challenges
>>>>>>>> to do well.
>>>>>>>>
>>>>>>>> Mike.
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> Beginners mailing list
>>>>>>> Beginners at haskell.org
>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>


-- 
Beauty of style and harmony and grace and good rhythm depend on simplicity.
- Plato
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From cppljevans at suddenlink.net  Fri Mar 27 19:57:00 2015
From: cppljevans at suddenlink.net (Larry Evans)
Date: Fri, 27 Mar 2015 14:57:00 -0500
Subject: [Haskell-beginners] Maybe and Just
In-Reply-To: <CAEEzXAiSY4_5q7NACWU2s_cfr-Nx95Wy8iT1f1Xc=khTkGL75Q@mail.gmail.com>
References: <CALe5RTsU5_QDnh7RPyi+4VYMDgs0aM=fMvNxLKfZPb-A3ccUHA@mail.gmail.com>
 <87384soy0o.fsf@ironicdesign.com>
 <CALe5RTtSst=tuOG4Ef5PRatD-A8PaT2Xf+bhdA71ku_bb1iLfA@mail.gmail.com>
 <CAEyhvmrZcDRrhCvwVZ7T7i_NWr1Brt1TKE2YW7Mq2D2MGPaPBg@mail.gmail.com>
 <CALe5RTs8T1H4OWhmCjbA0HnGJc_Tk0vCE83viCUU2bToGQSwsQ@mail.gmail.com>
 <CAEyhvmowUSHSx9mV9HSngqTCy_WS_0-RusPHN7E9ge96z3kmtg@mail.gmail.com>
 <CALe5RTuW0igyeK5WbTUGg81dcHua6drpSqbCL7JEFOk1rhS8NA@mail.gmail.com>
 <CAEyhvmrtC9S46cvSKynSH3-05YL6sFOCMrCy1Dyox4SPVJt9Og@mail.gmail.com>
 <CAEyhvmospTjjP8dr6ZkeXLGSO2q4_Gr5OL7WjaLVA+QuQoJnZw@mail.gmail.com>
 <CALe5RTu2UqzC39+TYO_oh5WSfHqkQiBKPztQstFA2YP3=hTvbg@mail.gmail.com>
 <CAEEzXAiSY4_5q7NACWU2s_cfr-Nx95Wy8iT1f1Xc=khTkGL75Q@mail.gmail.com>
Message-ID: <5515B60C.3040005@suddenlink.net>



On 03/27/2015 02:01 PM, Joel Neely wrote:
> Shishir,
> 
> Would I be right to guess that you're coming from an OO background? If
> so, I'd suggest thinking about the difference between declaring a
> constructor and invoking it.
> 
> I suggest that you can read the Haskell declaration
> 
>     data Maybe tp = Just tp | Nothing
> 
> 
> as declaring ?a data type and specifying two constructors that can
> return an instance (in the OO sense of the word) of that data type. But
> when the expression
> 
>     Just "FOO"
> 
> 
> is invoking one of those constructors on a value of type String, so the
> resulting value has type Maybe String.
> 
[snip]
Or, Maybe can be thought of as an abstract class with concrete
subclasses, Just and Nothing.

Or, Maybe can be thought of like a tagged union.  In c++ syntax:

  enum MaybeTag{ JustTag, NothingTag};
  template<typename ValType>
  struct JustType
  {
    ValType my_val;
    JustType(ValType a_val)
    : my_val(a_val)
    {}
  };
  struct NothingType
  {};
  template<typename ValType>
  struct Maybe
  {
  private:
      MaybeTag
    my_tag;
      union possible_values
      { JustType<ValType> just_val;
        NothingType nothing_val;
        possible_values(ValType a_val)
          : just_val(a_val)
          {}
        possible_values()
          : nothing_val()
          {}
      }
    my_val;
    Maybe(ValType a_val)
      : my_tag(JustTag)
      , my_val(a_val)
      {
      }
    Maybe()
      : my_tag(NothingTag)
      , my_val()
      {}
  public:
      MaybeTag
    tag()const
      { return my_tag;
      }
      static Maybe<ValType>
    Just(ValType a_val)
      { return Maybe<ValType>(a_val);
      }
      static Maybe<ValType>
    Nothing()
      { return Maybe<ValType>();
      }
  };

#include <iostream>

int main()
{
  auto mj=Maybe<int>::Just(5);
  std::cout<<"mj.tag="<<mj.tag()<<"\n";
  auto mn=Maybe<int>::Nothing();
  std::cout<<"mn.tag="<<mn.tag()<<"\n";
  return 0;
}

The following is the compile and run of above code:

g++ -std=c++11 tagged_union.cpp -o tagged_union.exe
./tagged_union.exe
mj.tag=0
mn.tag=1



From animeshsaxena at icloud.com  Sat Mar 28 04:10:02 2015
From: animeshsaxena at icloud.com (Animesh Saxena)
Date: Sat, 28 Mar 2015 12:10:02 +0800
Subject: [Haskell-beginners] Automatic Differentiation
Message-ID: <F62AF753-E976-4EC3-962B-FFAF3BC06098@icloud.com>

I was implementing automatic differentiation in haskell and was able to code the calculation part, but I wanted to extend it to show the symbols instead of just the final value.
Let me explain and copy/paste the code?

3  data ADif a = ADif a a deriving (Eq)

10 instance Floating x => Floating (ADif x) where
11           pi = ADif pi 0
12           exp    (ADif x x') = ADif (exp    x) (x' * exp x)
13           log    (ADif x x') = ADif (log    x) (x' / x)
14           sqrt   (ADif x x') = ADif (sqrt   x) (x' / (2 * sqrt x))
15           sin    (ADif x x') = ADif (sin    x) (x' * cos x)
16           cos    (ADif x x') = ADif (cos    x) (x' * (- sin x))
?.And so on all the functions


27 instance Num x => Num (ADif x) where
28         ADif x x' + ADif y y' = ADif (x+y) (x'+y')
29         ADif x x' * ADif y y' = ADif (x*y) (y'*x + x'*y)
30         fromInteger x = fromInteger x

let myfunction x = exp (log (sin x))
*Main> myfunction (ADif 2 1)
-0.4161468365471424

I verified that this is the correct solution by hand! (& well mathematica too!)

Anyway now I was hoping to print the actual symbols, so I was googling around for extending ?Show? typeclass for floating and Num, kinda similar pattern. Is this the right approach? Or I need to rethink the problem?
Basically my aim is to do something like mathematica where if I specify D[f[x],x] then I get the answer in symbols. 

-Animesh

From chaddai.fouche at gmail.com  Sat Mar 28 07:19:06 2015
From: chaddai.fouche at gmail.com (=?UTF-8?B?Q2hhZGRhw68gRm91Y2jDqQ==?=)
Date: Sat, 28 Mar 2015 08:19:06 +0100
Subject: [Haskell-beginners] Automatic Differentiation
In-Reply-To: <F62AF753-E976-4EC3-962B-FFAF3BC06098@icloud.com>
References: <F62AF753-E976-4EC3-962B-FFAF3BC06098@icloud.com>
Message-ID: <CANfjZRZ9a1SwfnOEeEuPSWeDsOx5+vWzhs9RaMox_Px4sRni1Q@mail.gmail.com>

On Sat, Mar 28, 2015 at 5:10 AM, Animesh Saxena <animeshsaxena at icloud.com>
wrote:

> I was implementing automatic differentiation in haskell and was able to
> code the calculation part, but I wanted to extend it to show the symbols
> instead of just the final value.
>
> Anyway now I was hoping to print the actual symbols, so I was googling
> around for extending ?Show? typeclass for floating and Num, kinda similar
> pattern. Is this the right approach? Or I need to rethink the problem?
> Basically my aim is to do something like mathematica where if I specify
> D[f[x],x] then I get the answer in symbols.
>
>
The idea would be to write a new type that transport an human readable
representation as well as the actual value, then make it a Num, Floating
and so on instance so that you could just use ADif with this type to get a
representation of your action. You can look at simple-reflect
<https://hackage.haskell.org/package/simple-reflect> for a simple
implementation of this idea you can directly use with your ADif. If instead
of a String, you transport an operation tree, you may even simplify your
result and get back almost a symbolic differentiation from your automatic
differentiation !

-- 
Jeda?
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From k-bx at k-bx.com  Sat Mar 28 11:13:55 2015
From: k-bx at k-bx.com (Konstantine Rybnikov)
Date: Sat, 28 Mar 2015 13:13:55 +0200
Subject: [Haskell-beginners] parser frontend
In-Reply-To: <CAAeLbQ+42zh3sRHgVc5e6XGif5ZP1vprX5m_NpSVU57ZGUQ_xQ@mail.gmail.com>
References: <CAAeLbQ+42zh3sRHgVc5e6XGif5ZP1vprX5m_NpSVU57ZGUQ_xQ@mail.gmail.com>
Message-ID: <CAAbahfRPL5dg-V50+TNn5OM-OoLR8iTmwu7_5fzt4uPVSy6Log@mail.gmail.com>

Maurizio,

It's not completely clear how your compiler knows which files depend on
which. Does it read files one by one and "updates" dependency list? What
about situation when toplevel construct is split in two files, how does
this single-file API handle it?

On Fri, Mar 27, 2015 at 3:38 PM, Maurizio Vitale <mrz.vtl at gmail.com> wrote:

> G'day,
>   I'm trying to decide how to architect the frontend of a compiler I'm
> using as an excuse for learning Haskell.
>
>   Suppose we have a language (SystemVerilog) that has the notion of
> multi-file compilation units.
> This means that specific subset of files form separate scopes for certain
> things.
> For instance if we represent with lists of lists compilation units:
> [[a,b,c], [d], [e,f]]
> a macro definition in file b, would affect the rest of the file and file
> c, but wouldn't have effect on files d,e or a.
>
> Furthermore, if a toplevel construct is split between two files, say c and
> d, the compiler should treat the original compilation unit specification as
> if it was:
> [[a,b,(c,d)][e,f]], where (c,d) means we're compiling the logical
> concatenation of c and d.
> If (c,d) is also incomplete, (c,d,e) should be tried and so on.
>
> Now in well designed systems, all files can actually be compiled in
> parallel and so I'd like to optimize for that case and recompile files
> (either because we need side effects from files before them or because
> they're incomplete. So I'm not to concerned if wasted work is done for the
> degenerate cases.
>
> Any suggestion on how to go about this?
> Le's assume that parsing a file is done with a function:
> p :: file -> Either Error (ast, [defs], [use]) where the [defs] are things
> that might affects files down the line and [use] are things that, if
> defined by some prior file, cause recompilation.
>
> I'm interested in both a sequential and parallel solution and it would be
> sweet if they where similar, with the proper abstractions.
>
> Thanks a lot for any insight/ideas,
>
>   Maurizio
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From mrz.vtl at gmail.com  Sat Mar 28 14:56:04 2015
From: mrz.vtl at gmail.com (Maurizio Vitale)
Date: Sat, 28 Mar 2015 10:56:04 -0400
Subject: [Haskell-beginners] parser frontend
In-Reply-To: <CAAbahfRPL5dg-V50+TNn5OM-OoLR8iTmwu7_5fzt4uPVSy6Log@mail.gmail.com>
References: <CAAeLbQ+42zh3sRHgVc5e6XGif5ZP1vprX5m_NpSVU57ZGUQ_xQ@mail.gmail.com>
 <CAAbahfRPL5dg-V50+TNn5OM-OoLR8iTmwu7_5fzt4uPVSy6Log@mail.gmail.com>
Message-ID: <CAAeLbQLfAsUk4HWuxf-MyWc2C0PPgEcbGHQxyUKa-KE=P-OHyg@mail.gmail.com>

I did some reading and realized that the question is  a bit too wide. I've
got now Simon Marlow's book on parallel/concurrent haskell and going
through it.

Files depend on each other because one defines something that the other
uses. There's more in SystemVerilog, but assume #define and #ifdef pairs.
So the dependency is only implicit: you know it only after you've parsed
both files.
Nevertheless, a well organized system will #include files with definitions
(and do the same for other SystemVerilog things that cause dependencies) so
I'm ok with optimistically assume no dependency and compute a list of
"problems" that cause recompilation.
So if:
file A:
...
#define foo
...
file B:
...
#ifdef foo
...
#endif
...
and the command line has the order fileA, fileB we compile both files in
parallel, but the result of compiling A will be (ast, [foo], []) and the
result of compiling B will be (ast/Error, [], [foo]) since foo is in the
use list of B and in the def list of A we know we have to recompile B (and
compiling B doesn't pass a def list to a possible C until we're satisfied
with it). So when there're dependencies, I'm ok with recompiling multiple
times.

As for the second question, splitting a toplevel across files is even more
perverse and should be extremely rare (I think is there only for historical
reasons: the first Verilog simulators assumed the entire design available
and basically did a 'cat f1..fN' before compiling. Again, I'm happy with
redo work in this case. If compilation units are [ [a,b,c], [d,e] ] and you
have a parse error in c after having consumed the entire file and d has
also a parse error, we restart with a new specification [ [a,b,(c,d)] [e]]
where (c,d) means concatenation of the content of file c and d. Parse for a
and b are still valid. Parse for e might be valid. But even if we
invalidate and redo everything in exchange for simple code, I'm ok with it.

My question can be abstracted a bit more in terms of list processing.
We have a list of lists [[a]]. We need to process each leaf element by
mapping a function p a e which process an element of type 'a' in an
environment 'e' which comes form the evaluation of previous elements. Up to
here, would be normal monadic code to hide 'e'. In addition since in our
domain, we assume 'e' `intersect` 'u' is often empty (where 'u' are the
uses that we discover while processing an element) we'd like to start the
processing of all elements in parallel and redo the ones that turned out to
be invalidated by previous ones. I'd like to find a solution to this that
is in good Haskell style.

We can forget the toplevel split across files for now, that's can be dealt
in a outer layer that restarts everything throwing away all the work.


On Sat, Mar 28, 2015 at 7:13 AM, Konstantine Rybnikov <k-bx at k-bx.com> wrote:

> Maurizio,
>
> It's not completely clear how your compiler knows which files depend on
> which. Does it read files one by one and "updates" dependency list? What
> about situation when toplevel construct is split in two files, how does
> this single-file API handle it?
>
> On Fri, Mar 27, 2015 at 3:38 PM, Maurizio Vitale <mrz.vtl at gmail.com>
> wrote:
>
>> G'day,
>>   I'm trying to decide how to architect the frontend of a compiler I'm
>> using as an excuse for learning Haskell.
>>
>>   Suppose we have a language (SystemVerilog) that has the notion of
>> multi-file compilation units.
>> This means that specific subset of files form separate scopes for certain
>> things.
>> For instance if we represent with lists of lists compilation units:
>> [[a,b,c], [d], [e,f]]
>> a macro definition in file b, would affect the rest of the file and file
>> c, but wouldn't have effect on files d,e or a.
>>
>> Furthermore, if a toplevel construct is split between two files, say c
>> and d, the compiler should treat the original compilation unit
>> specification as if it was:
>> [[a,b,(c,d)][e,f]], where (c,d) means we're compiling the logical
>> concatenation of c and d.
>> If (c,d) is also incomplete, (c,d,e) should be tried and so on.
>>
>> Now in well designed systems, all files can actually be compiled in
>> parallel and so I'd like to optimize for that case and recompile files
>> (either because we need side effects from files before them or because
>> they're incomplete. So I'm not to concerned if wasted work is done for the
>> degenerate cases.
>>
>> Any suggestion on how to go about this?
>> Le's assume that parsing a file is done with a function:
>> p :: file -> Either Error (ast, [defs], [use]) where the [defs] are
>> things that might affects files down the line and [use] are things that, if
>> defined by some prior file, cause recompilation.
>>
>> I'm interested in both a sequential and parallel solution and it would be
>> sweet if they where similar, with the proper abstractions.
>>
>> Thanks a lot for any insight/ideas,
>>
>>   Maurizio
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From mrz.vtl at gmail.com  Sat Mar 28 19:40:18 2015
From: mrz.vtl at gmail.com (Maurizio Vitale)
Date: Sat, 28 Mar 2015 15:40:18 -0400
Subject: [Haskell-beginners] parser frontend
In-Reply-To: <CAAeLbQ+42zh3sRHgVc5e6XGif5ZP1vprX5m_NpSVU57ZGUQ_xQ@mail.gmail.com>
References: <CAAeLbQ+42zh3sRHgVc5e6XGif5ZP1vprX5m_NpSVU57ZGUQ_xQ@mail.gmail.com>
Message-ID: <CAAeLbQKgjfrRcO06WcqHEZKqgeCD4Ny+JD92aPpBRkVTS2PZFA@mail.gmail.com>

I think this goes a very long way towards what I need:
https://github.com/ekmett/speculation

If anybody has any suggestion on other libraries that can be of use when
trying to parallelize a parser, I'd be very interested in hearing it.

Thanks,

  Maurizio

On Fri, Mar 27, 2015 at 9:38 AM, Maurizio Vitale <mrz.vtl at gmail.com> wrote:

> G'day,
>   I'm trying to decide how to architect the frontend of a compiler I'm
> using as an excuse for learning Haskell.
>
>   Suppose we have a language (SystemVerilog) that has the notion of
> multi-file compilation units.
> This means that specific subset of files form separate scopes for certain
> things.
> For instance if we represent with lists of lists compilation units:
> [[a,b,c], [d], [e,f]]
> a macro definition in file b, would affect the rest of the file and file
> c, but wouldn't have effect on files d,e or a.
>
> Furthermore, if a toplevel construct is split between two files, say c and
> d, the compiler should treat the original compilation unit specification as
> if it was:
> [[a,b,(c,d)][e,f]], where (c,d) means we're compiling the logical
> concatenation of c and d.
> If (c,d) is also incomplete, (c,d,e) should be tried and so on.
>
> Now in well designed systems, all files can actually be compiled in
> parallel and so I'd like to optimize for that case and recompile files
> (either because we need side effects from files before them or because
> they're incomplete. So I'm not to concerned if wasted work is done for the
> degenerate cases.
>
> Any suggestion on how to go about this?
> Le's assume that parsing a file is done with a function:
> p :: file -> Either Error (ast, [defs], [use]) where the [defs] are things
> that might affects files down the line and [use] are things that, if
> defined by some prior file, cause recompilation.
>
> I'm interested in both a sequential and parallel solution and it would be
> sweet if they where similar, with the proper abstractions.
>
> Thanks a lot for any insight/ideas,
>
>   Maurizio
>
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From jeffbrown.the at gmail.com  Sun Mar 29 01:34:00 2015
From: jeffbrown.the at gmail.com (Jeffrey Brown)
Date: Sat, 28 Mar 2015 18:34:00 -0700
Subject: [Haskell-beginners] failure $ cabal install wx
Message-ID: <CAEc4Ma2LDut2M2E7YvYaiWy5fnXqHR6tqjLq-MAtSES5r0y_pA@mail.gmail.com>

Hi!

I first sent this to Haskell-Cafe, then thought this would be a better
destination.

I switched OSs, from Ubuntu to OpenSuse 13.2, to fix some other problems. I
since installed The Haskell Platform, and would like to install wxHaskell.

The first time I tried that under OpenSuse, cabal install wx provided more
output, installing a lot of things. Since then it has only given this:

    jeff at linux-ee07:~> cabal install wx
    Resolving dependencies...
    [1 of 1] Compiling Main             (
/tmp/wxc-0.91.0.0-16672/wxc-0.91.0.0/Setup.hs,
/tmp/wxc-0.91.0.0-16672/wxc-0.91.0.0/dist/setup/Main.o )
    Linking /tmp/wxc-0.91.0.0-16672/wxc-0.91.0.0/dist/setup/setup ...
    Configuring wxc-0.91.0.0...

                        Warning: No config found to match:
/usr/bin/wx-config --version=2.9 --version-full
                                         in /usr/lib64/wx/config
                        If you require this configuration, please install
the desired
                        library build.  If this is part of an automated
configuration
                        test and no other errors occur, you may safely
ignore it.
                        You may use wx-config --list to see all configs
available in
                        the default prefix.

    readProcess failed: readProcess: wx-config "--version=2.9"
"--version-full" (exit 1): failed
    Configuring wxc to build against wxWidgets 3.0.2.0

    Building wxc
    /usr/bin/gcc -Wl,--hash-size=31 -Wl,--reduce-memory-overheads
-Wl,--disable-new-dtags -Isrc/include -I/usr/include/wx-3.0
-I/usr/lib64/wx/include/gtk3-unicode-3.0 -D__WXGTK__ -DWXUSINGDLL
-DwxDEBUG_LEVEL=0 -D_FILE_OFFSET_BITS=64 -DwxcREFUSE_MEDIACTRL -fPIC -c
src/cpp/apppath.cpp -o dist/build/src/cpp/apppath.o
    gcc: error trying to exec 'cc1plus': execvp: No such file or directory
    Failed to install wxc-0.91.0.0
    cabal: Error: some packages failed to install:
    wx-0.91.0.0 depends on wxc-0.91.0.0 which failed to install.
    wxc-0.91.0.0 failed during the building phase. The exception was:
    ExitFailure 1
    wxcore-0.91.0.0 depends on wxc-0.91.0.0 which failed to install.
    jeff at linux-ee07:~>

The file /usr/bin/wx-config exists. It may have gotten there today when I
installed some wx packages via zypper (OpenSuse's equivalent to apt-get). I
don't remember which those packages were, but in the wx-configg file appear
these lines:

    this_version="3.0"
    [ -z "$output_option_release"       ] || echo "3.0"
    [ -z "$output_option_version"       ] || echo "3.0.2"
    [ -z "$output_option_version_full"  ] || echo "3.0.2.0"

so I believe the version of wx I have is 3.0.2.0.

I'm using a Lenovo Thinkpad Edge 535 or 545.

Thanks,
Jeff
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From allbery.b at gmail.com  Sun Mar 29 01:38:27 2015
From: allbery.b at gmail.com (Brandon Allbery)
Date: Sat, 28 Mar 2015 21:38:27 -0400
Subject: [Haskell-beginners] failure $ cabal install wx
In-Reply-To: <CAEc4Ma2LDut2M2E7YvYaiWy5fnXqHR6tqjLq-MAtSES5r0y_pA@mail.gmail.com>
References: <CAEc4Ma2LDut2M2E7YvYaiWy5fnXqHR6tqjLq-MAtSES5r0y_pA@mail.gmail.com>
Message-ID: <CAKFCL4WjXj0Q581ZQ1OH_VFFzeLZxb59MET+SZ8Yb5DrHJGQxg@mail.gmail.com>

On Sat, Mar 28, 2015 at 9:34 PM, Jeffrey Brown <jeffbrown.the at gmail.com>
wrote:

>     gcc: error trying to exec 'cc1plus': execvp: No such file or directory
>

That part at least is because installing gcc on SuSE doesn't install the
C++ compiler components. You'll need to find the package containing the C++
components.

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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From jeffbrown.the at gmail.com  Sun Mar 29 17:34:10 2015
From: jeffbrown.the at gmail.com (Jeffrey Brown)
Date: Sun, 29 Mar 2015 10:34:10 -0700
Subject: [Haskell-beginners] failure $ cabal install wx
In-Reply-To: <CAKFCL4WjXj0Q581ZQ1OH_VFFzeLZxb59MET+SZ8Yb5DrHJGQxg@mail.gmail.com>
References: <CAEc4Ma2LDut2M2E7YvYaiWy5fnXqHR6tqjLq-MAtSES5r0y_pA@mail.gmail.com>
 <CAKFCL4WjXj0Q581ZQ1OH_VFFzeLZxb59MET+SZ8Yb5DrHJGQxg@mail.gmail.com>
Message-ID: <CAEc4Ma1SdY=cqxUEveTTOy0DuNjcaqdWQHv3nHE3bkv0cT6_EQ@mail.gmail.com>

Thank you, Brandon.

I tried installing a ton of wx libraries via zypper, and it didn't work.
Then I gave up and reverted to Kubuntu.

On Sat, Mar 28, 2015 at 6:38 PM, Brandon Allbery <allbery.b at gmail.com>
wrote:

> On Sat, Mar 28, 2015 at 9:34 PM, Jeffrey Brown <jeffbrown.the at gmail.com>
> wrote:
>
>>     gcc: error trying to exec 'cc1plus': execvp: No such file or directory
>>
>
> That part at least is because installing gcc on SuSE doesn't install the
> C++ compiler components. You'll need to find the package containing the C++
> components.
>
> --
> brandon s allbery kf8nh                               sine nomine
> associates
> allbery.b at gmail.com
> ballbery at sinenomine.net
> unix, openafs, kerberos, infrastructure, xmonad
> http://sinenomine.net
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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From dot_wangyushi at yeah.net  Mon Mar 30 06:22:20 2015
From: dot_wangyushi at yeah.net (m00nlight)
Date: Mon, 30 Mar 2015 14:22:20 +0800 (CST)
Subject: [Haskell-beginners] Arithmetic expression parsing problem
Message-ID: <12e4319a.292.14c69585d2d.Coremail.dot_wangyushi@yeah.net>

Dear Haskellers,

I am new to programming in haskell, recently I came up with an task to write an simple arithemtic evaluator in
haskell. I try to write it using Text.Parsec, it can handle binary operation and some unary operation, but will
give information of parsing error in some case. The following is my code



import Text.Parsec
import Text.Parsec.Expr
import Text.Parsec.Combinator
import Data.Functor

data Exp = Num Int
         | Add Exp Exp
         | Sub Exp Exp
         | Mul Exp Exp
         | Div Exp Exp
         | Pos Exp
         | Neg Exp

expr = buildExpressionParser table factor


table = [[op "*" (Mul) AssocRight, op "/" (Div) AssocRight]
        ,[op "+" (Add) AssocLeft, op "-" (Sub) AssocLeft]
        ,[prefix "-" (Neg), prefix "+" (Pos)]]
        where op s f assoc = Infix (f <$ string s) assoc
              prefix s f = Prefix (f <$ string s)


factor = between (char '(') (char ')') expr <|> (Num . read <$> many1 digit)

eval :: (Num a, Integral a) => Exp -> a
eval e = case e of
    Num x   -> fromIntegral x
    Pos a   -> eval a
    Neg a   -> negate $ eval a
    Add a b -> eval a   +   eval b
    Sub a b -> eval a   -   eval b
    Mul a b -> eval a   *   eval b
    Div a b -> eval a `div` eval b

solution :: (Num a, Integral a) => String -> a
solution = either (const (error "Did not parse")) eval . parse expr ""

The following is some test,



*Main> solution "-4/(2+3)"
0
*Main> solution "-4/(2-3)"
4
*Main> solution "-4/-2"
*** Exception: Did not parse
*Main> solution "16/-4"
*** Exception: Did not parse
*Main> solution "-16/4"
-4
*Main>

Can anyone teach me how to solve this?  Thanks in advanced.




--m00nlight


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From fa-ml at ariis.it  Mon Mar 30 07:05:14 2015
From: fa-ml at ariis.it (Francesco Ariis)
Date: Mon, 30 Mar 2015 09:05:14 +0200
Subject: [Haskell-beginners] Arithmetic expression parsing problem
In-Reply-To: <12e4319a.292.14c69585d2d.Coremail.dot_wangyushi@yeah.net>
References: <12e4319a.292.14c69585d2d.Coremail.dot_wangyushi@yeah.net>
Message-ID: <20150330070514.GA9832@x60s.casa>

On Mon, Mar 30, 2015 at 02:22:20PM +0800, m00nlight wrote:
> I am new to programming in haskell, recently I came up with an task to
> write an simple arithemtic evaluator in haskell. I try to write it using
> Text.Parsec, it can handle binary operation and some unary operation,
> but will give information of parsing error in some case. The following
> is my code
> 
> [..]
> Can anyone teach me how to solve this?  Thanks in advanced.

Hello,

first things first, I slightly modified your `solution` function, so it
can fail with meaningful error messages

    solution :: (Num a, Integral a) => String -> a
    solution = either (error . show) eval . parse expr ""

Then I added to `<?>` operators at the end of your parsing expression
(again, for easier diagnostic).

    factor = between (char '(') (char ')') expr
             <|> (Num . read <$> many1 digit) <?> "factor"

    expr = buildExpressionParser table factor <?> "expr"

Now the error message is:

    ?> solution "-4/-2"
    *** Exception: (line 1, column 4):
    unexpected "-"
    expecting factor

So Parsec was expecting a "factor" element but found itself with '-'.
The first alternative of factor is not what we want (an expression
surrounded by parentheses).
The second part should be it (a 'number'). Now it is easy to recognise
the problem: `digit` only accepts digits and not '-' as a prefix.


When parsing stuff, <?>, small functions and early testing (with
hspec[1]) saved me much time and pain debugging.


[1] http://hspec.github.io/
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From dot_wangyushi at yeah.net  Mon Mar 30 08:10:18 2015
From: dot_wangyushi at yeah.net (m00nlight)
Date: Mon, 30 Mar 2015 16:10:18 +0800 (CST)
Subject: [Haskell-beginners] Arithmetic expression parsing problem
In-Reply-To: <20150330070514.GA9832@x60s.casa>
References: <12e4319a.292.14c69585d2d.Coremail.dot_wangyushi@yeah.net>
 <20150330070514.GA9832@x60s.casa>
Message-ID: <48ca1d90.323.14c69bb33cf.Coremail.dot_wangyushi@yeah.net>

Hi Francesco,

Thanks for your explaination, so how can I modify the parser to handle this case?
Can you give me some more details of how to modify it?

Thanks in advanced.







--m00nlight



?2015?03?30 15?05?, "Francesco Ariis"<fa-ml at ariis.it>??:

On Mon, Mar 30, 2015 at 02:22:20PM +0800, m00nlight wrote:
> I am new to programming in haskell, recently I came up with an task to
> write an simple arithemtic evaluator in haskell. I try to write it using
> Text.Parsec, it can handle binary operation and some unary operation,
> but will give information of parsing error in some case. The following
> is my code
>
> [..]
> Can anyone teach me how to solve this?  Thanks in advanced.

Hello,

first things first, I slightly modified your `solution` function, so it
can fail with meaningful error messages

   solution :: (Num a, Integral a) => String -> a
   solution = either (error . show) eval . parse expr ""

Then I added to `<?>` operators at the end of your parsing expression
(again, for easier diagnostic).

   factor = between (char '(') (char ')') expr
            <|> (Num . read <$> many1 digit) <?> "factor"

   expr = buildExpressionParser table factor <?> "expr"

Now the error message is:

   ?> solution "-4/-2"
   *** Exception: (line 1, column 4):
   unexpected "-"
   expecting factor

So Parsec was expecting a "factor" element but found itself with '-'.
The first alternative of factor is not what we want (an expression
surrounded by parentheses).
The second part should be it (a 'number'). Now it is easy to recognise
the problem: `digit` only accepts digits and not '-' as a prefix.


When parsing stuff, <?>, small functions and early testing (with
hspec[1]) saved me much time and pain debugging.


[1] http://hspec.github.io/
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From martin.drautzburg at web.de  Mon Mar 30 11:22:44 2015
From: martin.drautzburg at web.de (martin)
Date: Mon, 30 Mar 2015 13:22:44 +0200
Subject: [Haskell-beginners] Why does QuickCheck insist on this class
	constraint?
Message-ID: <55193204.90305@web.de>

Hello all,

Ghc complainend about

    Could not deduce (Arbitrary a1) arising from a use of ?arbitrary?
    Could not deduce (Eq a1) arising from a use of ?arbitrary?

in the following code ..

data CList a = CList [Change a]
             deriving Show

instance (Arbitrary c, Eq c) => Arbitrary (CList c)
        where
            arbitrary = do
                ts <- orderedList      :: Gen[Time]
                vs <- listOf arbitrary :: Arbitrary c => Gen [c]
                return $ CList $ zipWith (\t v -> Chg t v) (nub ts) vs


instance (Arbitrary a, Eq a) => Arbitrary (Temporal a)
        where
            arbitrary = do
                d  <- arbitrary
                (CList cs) <- arbitrary :: (Arbitrary a, Eq a) => Gen (CList a) -- <===
                return (Temporal d cs)

..if I leave out the class constraints in the line maked with "<===". Why is that so, i.e. why isn't the constraint four
lines up (in the instance declaration) sufficient?

From fa-ml at ariis.it  Mon Mar 30 11:38:04 2015
From: fa-ml at ariis.it (Francesco Ariis)
Date: Mon, 30 Mar 2015 13:38:04 +0200
Subject: [Haskell-beginners] Arithmetic expression parsing problem
In-Reply-To: <48ca1d90.323.14c69bb33cf.Coremail.dot_wangyushi@yeah.net>
References: <12e4319a.292.14c69585d2d.Coremail.dot_wangyushi@yeah.net>
 <20150330070514.GA9832@x60s.casa>
 <48ca1d90.323.14c69bb33cf.Coremail.dot_wangyushi@yeah.net>
Message-ID: <20150330113804.GA20503@x60s.casa>

On Mon, Mar 30, 2015 at 04:10:18PM +0800, m00nlight wrote:
> Hi Francesco,
> 
> Thanks for your explaination, so how can I modify the parser to handle this case?
> Can you give me some more details of how to modify it?
> 
> Thanks in advanced.

Can't now, but take a look at Parsec.Token.reservedOp! You might want to
read what it does and use it instead of `string`

From jeffbrown.the at gmail.com  Tue Mar 31 02:14:35 2015
From: jeffbrown.the at gmail.com (Jeffrey Brown)
Date: Mon, 30 Mar 2015 19:14:35 -0700
Subject: [Haskell-beginners] Mutable collection of gui objects (wxHaskell)
Message-ID: <CAEc4Ma2WbxWzUnYD4xooOD=vG_iTGcrDp-0upvOAxcgkNoq_MQ@mail.gmail.com>

Hi,

On Github, Jeremy O'Donoghue provides wx example code. From his collection
I am trying to modify Layout.hs [1] to permit a variable number, rather
than a fixed number, of text entry boxes. I shrank the program, mostly by
removing features, until it was this:

  main = start $ do
    f      <- frame  [text := "Layout test"]
    p      <- panel  f []
    xinput <- textEntry p [text := "100"]
    yinput <- textEntry p [text := "100"]
    myVar <- varCreate [xinput,yinput]
    set f [ layout := container p $ margin 10 $
      column 5 [boxed "coordinates" (grid 5 5
          [[hfill $ widget xinput], [hfill $ widget yinput]] -- replacing
        ) ] ]
    return ()

I want to replace the line marked "replacing". Rather than hard-coding the
number of text entry boxes (2), I want it to deal with a mutable collection
of them, in myVar.

I tried this:
  [ fmap (\e -> hfill $ widget e) $ varGet myVar ]
and got this error:
  Layout.hs:23:11:
      Couldn't match type `IO' with `[]'
      Expected type: [Layout]
        Actual type: IO Layout
      In the expression: fmap (\ e -> hfill $ widget e) $ varGet myVar
      In the third argument of `grid', namely
        `[fmap (\ e -> hfill $ widget e) $ varGet myVar]'
      In the second argument of `boxed', namely
        `(grid 5 5 [fmap (\ e -> hfill $ widget e) $ varGet myVar])'

So then I tried something I thought would be equivalent:
  [[ (hfill $ widget e) | e <- (varGet myVar)]]
and got a different error:
  Layout.hs:23:39:
      Couldn't match expected type `[w0]'
                  with actual type `IO [TextCtrl ()]'
      In the return type of a call of `varGet'
      In the expression: (varGet myVar)
      In a stmt of a list comprehension: e <- (varGet myVar)

I kind of understand the problem is that when I varGet myVar, I end up with
type IO Layout, rather than type Layout, but I don't know what to do about
it.

Thanks,
Jeff


[1] https://github.com/jodonoghue/wxHaskell/blob/master/samples/wx/Layout.hs
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From tkoster at gmail.com  Tue Mar 31 03:26:53 2015
From: tkoster at gmail.com (Thomas Koster)
Date: Tue, 31 Mar 2015 14:26:53 +1100
Subject: [Haskell-beginners] Is there an aeson-like XML serialization
	library?
Message-ID: <CAG1wH7DD-XBuhCnNnq=vc5yKGHMEc0QNyCmpp1N7QJ8Y5b4sJg@mail.gmail.com>

Hi list,

I want to write a simple XML web service in Haskell that services a .NET
WCF client, but am struggling with choosing the right XML library.

"aeson" [1] has been a pleasure to use, so I am looking for an XML
serialization library similar to aeson, where I can write
ToElement/FromElement instances using simple applicative combinators,
working with a high-level document model (i.e. not a parse tree), just like
I have done in the past with ToJSON/FromJSON.

Does such a library exist?

I need the library to understand XML namespaces and the mandatory
predefined entities, but I do not need any other extensions like XML
Schemas, XPath or XSLT. Preferably, "xmlns" attributes should be handled
specially and namespace prefixes resolved by the parser. "xmlns" attributes
should be placed automatically by the renderer (I don't care what prefixes
it chooses). I do not want to have to keep track of seen namespace prefixes
while I am traversing the document, or to manually place "xmlns" attributes
on elements for rendering. The "xml" [2] library is giving me grief at the
moment with how unspecial its treatment of namespaces is.

HXT appears to be beyond my skill level at the moment. There appear to be
too many new things I would have to learn all at once before I could use it
for this simple task. Cf. aeson, which I was able to use for practical
applications the day I learned what an applicative functor was.

[1] https://hackage.haskell.org/package/aeson
[2] https://hackage.haskell.org/package/xml

Thanks,
--
Thomas Koster
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From martin.drautzburg at web.de  Tue Mar 31 05:22:32 2015
From: martin.drautzburg at web.de (martin)
Date: Tue, 31 Mar 2015 07:22:32 +0200
Subject: [Haskell-beginners] Arrow vs. function
In-Reply-To: <m2oanj4ekm.fsf@newartisans.com>
References: <55107AEC.6040705@web.de> <m2oanj4ekm.fsf@newartisans.com>
Message-ID: <551A2F18.2040603@web.de>

Am 03/23/2015 um 09:57 PM schrieb John Wiegley:
>>>>>> martin  <martin.drautzburg at web.de> writes:
> 
>> But that's what a function does. So what's the fundamental difference
>> between an arrow and a fuction?
> 
> Arrows abstract functions, allowing you to have constructions like Kleisli,
> which are Arrows, but compose in the presence of effects using (<=<) rather
> than (.).

I recall reading something like "under composition an arrow returns a new version of itself". It this correct? This
would indeed be something a function does not do.


From martin.drautzburg at web.de  Tue Mar 31 05:33:56 2015
From: martin.drautzburg at web.de (martin)
Date: Tue, 31 Mar 2015 07:33:56 +0200
Subject: [Haskell-beginners] Are Arrows good for simulation purposes
Message-ID: <551A31C4.3030002@web.de>

Hello all,

little do I know about arrows, but the "stream processor" metaphor suggests, that they can be used to simulate a network
of things, services or data. Is this correct?

I came across the following thought: When I simulate a billard game with a DES, I would compute collisions of balls with
each other and with the banks, creatign a set of events, of which only the earliest will be considered to compute the
next state. This is pure DES and does not seem to be a good candidate for arrows. In this case I'd be more interested in
composing collision detectors than in stream processing.

OTOH, when I move parcels around and process then in various stages, then a "stream processor" would make perfect sense
to me. In that case, would I abandon the DES paradigm entirely (including the notion of an event queue) and just model
the network and let it run?


From michael at snoyman.com  Tue Mar 31 05:39:28 2015
From: michael at snoyman.com (Michael Snoyman)
Date: Tue, 31 Mar 2015 05:39:28 +0000
Subject: [Haskell-beginners] Is there an aeson-like XML serialization
	library?
In-Reply-To: <CAG1wH7DD-XBuhCnNnq=vc5yKGHMEc0QNyCmpp1N7QJ8Y5b4sJg@mail.gmail.com>
References: <CAG1wH7DD-XBuhCnNnq=vc5yKGHMEc0QNyCmpp1N7QJ8Y5b4sJg@mail.gmail.com>
Message-ID: <CAKA2JgJzdchrUUboFBpY=+0VYYN-7BWs3PN1zc1c8rBkr_wfOw@mail.gmail.com>

It doesn't have the typeclass stuff built in, but xml-conduit has proper
support for XML namespaces.

On Tue, Mar 31, 2015, 6:27 AM Thomas Koster <tkoster at gmail.com> wrote:

> Hi list,
>
> I want to write a simple XML web service in Haskell that services a .NET
> WCF client, but am struggling with choosing the right XML library.
>
> "aeson" [1] has been a pleasure to use, so I am looking for an XML
> serialization library similar to aeson, where I can write
> ToElement/FromElement instances using simple applicative combinators,
> working with a high-level document model (i.e. not a parse tree), just like
> I have done in the past with ToJSON/FromJSON.
>
> Does such a library exist?
>
> I need the library to understand XML namespaces and the mandatory
> predefined entities, but I do not need any other extensions like XML
> Schemas, XPath or XSLT. Preferably, "xmlns" attributes should be handled
> specially and namespace prefixes resolved by the parser. "xmlns" attributes
> should be placed automatically by the renderer (I don't care what prefixes
> it chooses). I do not want to have to keep track of seen namespace prefixes
> while I am traversing the document, or to manually place "xmlns" attributes
> on elements for rendering. The "xml" [2] library is giving me grief at the
> moment with how unspecial its treatment of namespaces is.
>
> HXT appears to be beyond my skill level at the moment. There appear to be
> too many new things I would have to learn all at once before I could use it
> for this simple task. Cf. aeson, which I was able to use for practical
> applications the day I learned what an applicative functor was.
>
> [1] https://hackage.haskell.org/package/aeson
> [2] https://hackage.haskell.org/package/xml
>
> Thanks,
> --
> Thomas Koster
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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From hjgtuyl at chello.nl  Tue Mar 31 07:48:54 2015
From: hjgtuyl at chello.nl (Henk-Jan van Tuyl)
Date: Tue, 31 Mar 2015 09:48:54 +0200
Subject: [Haskell-beginners] Mutable collection of gui objects
	(wxHaskell)
In-Reply-To: <CAEc4Ma2WbxWzUnYD4xooOD=vG_iTGcrDp-0upvOAxcgkNoq_MQ@mail.gmail.com>
References: <CAEc4Ma2WbxWzUnYD4xooOD=vG_iTGcrDp-0upvOAxcgkNoq_MQ@mail.gmail.com>
Message-ID: <op.xwcqnxsppz0j5l@alquantor>

On Tue, 31 Mar 2015 04:14:35 +0200, Jeffrey Brown  
<jeffbrown.the at gmail.com> wrote:

:
> I am trying to modify Layout.hs [1] to permit a variable number, rather
> than a fixed number, of text entry boxes. I shrank the program, mostly by
> removing features, until it was this:
>
>   main = start $ do
>     f      <- frame  [text := "Layout test"]
>     p      <- panel  f []
>     xinput <- textEntry p [text := "100"]
>     yinput <- textEntry p [text := "100"]
>     myVar <- varCreate [xinput,yinput]
>     set f [ layout := container p $ margin 10 $
>       column 5 [boxed "coordinates" (grid 5 5
>           [[hfill $ widget xinput], [hfill $ widget yinput]] -- replacing
>         ) ] ]
>     return ()
>
> I want to replace the line marked "replacing". Rather than hard-coding  
> the
> number of text entry boxes (2), I want it to deal with a mutable  
> collection
> of them, in myVar.
>
> I tried this:
>   [ fmap (\e -> hfill $ widget e) $ varGet myVar ]
:

The result of varGet is of type "IO something", you must convert that to  
"something", by using "<-". You can do this by adding the line:
   inputs <- map (\e -> [hfill $ widget e]) <$> varGet myVar
before the set command (note the square brackets). The <$> operator is  
 from module Data.Functor and is defined as
   (<$>) = fmap

The main function becomes this:
   main = start $ do
       f      <- frame  [text := "Layout test"]
       p      <- panel  f []
       xinput <- textEntry p [text := "100"]
       yinput <- textEntry p [text := "100"]
       myVar  <- varCreate [xinput, yinput]
       inputs <- map (\e -> [hfill $ widget e]) <$> varGet myVar
       set f [ layout := container p $ margin 10 $
               column 5 [boxed "coordinates" (grid 5 5 inputs)]
             ]
       return ()


Regards,
Henk-Jan van Tuyl


-- 
Folding at home
What if you could share your unused computer power to help find a cure? In  
just 5 minutes you can join the world's biggest networked computer and get  
us closer sooner. Watch the video.
http://folding.stanford.edu/


http://Van.Tuyl.eu/
http://members.chello.nl/hjgtuyl/tourdemonad.html
Haskell programming
--

From hjgtuyl at chello.nl  Tue Mar 31 07:52:31 2015
From: hjgtuyl at chello.nl (Henk-Jan van Tuyl)
Date: Tue, 31 Mar 2015 09:52:31 +0200
Subject: [Haskell-beginners] Mutable collection of gui objects
	(wxHaskell)
In-Reply-To: <CAEc4Ma2WbxWzUnYD4xooOD=vG_iTGcrDp-0upvOAxcgkNoq_MQ@mail.gmail.com>
References: <CAEc4Ma2WbxWzUnYD4xooOD=vG_iTGcrDp-0upvOAxcgkNoq_MQ@mail.gmail.com>
Message-ID: <op.xwcqttrxpz0j5l@alquantor>

On Tue, 31 Mar 2015 04:14:35 +0200, Jeffrey Brown  
<jeffbrown.the at gmail.com> wrote:

> On Github, Jeremy O'Donoghue provides wx example code. From his  
> collection

Note, that this repository is not actively maintained anymore, the  
official repository is at
   https://github.com/wxHaskell/wxHaskell

Regards,
Henk-Jan van Tuyl


-- 
Folding at home
What if you could share your unused computer power to help find a cure? In  
just 5 minutes you can join the world's biggest networked computer and get  
us closer sooner. Watch the video.
http://folding.stanford.edu/


http://Van.Tuyl.eu/
http://members.chello.nl/hjgtuyl/tourdemonad.html
Haskell programming
--

From tkoster at gmail.com  Tue Mar 31 09:54:40 2015
From: tkoster at gmail.com (Thomas Koster)
Date: Tue, 31 Mar 2015 20:54:40 +1100
Subject: [Haskell-beginners] Is there an aeson-like XML serialization
	library?
In-Reply-To: <CAKA2JgJzdchrUUboFBpY=+0VYYN-7BWs3PN1zc1c8rBkr_wfOw@mail.gmail.com>
References: <CAG1wH7DD-XBuhCnNnq=vc5yKGHMEc0QNyCmpp1N7QJ8Y5b4sJg@mail.gmail.com>
 <CAKA2JgJzdchrUUboFBpY=+0VYYN-7BWs3PN1zc1c8rBkr_wfOw@mail.gmail.com>
Message-ID: <CAG1wH7Bt4AAzgCODH_K4SzT34dQ5qZdfcSDY8B2-76P2Qw9K0g@mail.gmail.com>

Michael,

On 31 March 2015 at 14:26, Thomas Koster <tkoster at gmail.com> wrote:

> I want to write a simple XML web service in Haskell that services a .NET
> WCF client, but am struggling with choosing the right XML library.
> ...
> I need the library to understand XML namespaces and the mandatory
> predefined entities, but I do not need any other extensions like XML
> Schemas, XPath or XSLT. Preferably, "xmlns" attributes should be handled
> specially and namespace prefixes resolved by the parser. "xmlns" attributes
> should be placed automatically by the renderer (I don't care what prefixes
> it chooses). I do not want to have to keep track of seen namespace prefixes
> while I am traversing the document, or to manually place "xmlns" attributes
> on elements for rendering.


On 31 March 2015 at 16:39, Michael Snoyman <michael at snoyman.com> wrote:

> It doesn't have the typeclass stuff built in, but xml-conduit has proper
> support for XML namespaces.


That's OK. I can create the type classes myself.

class ToElement a where
  toElement :: a -> Element

class FromElement a where
  parseElement :: Element -> Either String a

The important thing is that namespaces are understood so that I am able
create instances that compose, e.g. (off the top of my head)

data SoapEnvelope h b = SoapEnvelope
  {
    soapEnvelopeHeaders :: [h],
    soapEnvelopeBody :: b
  }

instance (FromElement h, FromElement b) => FromElement (SoapEnvelope h b)
where
  parseElement e = SoapEnvelope <$> (...headers...) <*> (...body...)

Thanks,
--
Thomas Koster
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From jeffbrown.the at gmail.com  Tue Mar 31 20:11:47 2015
From: jeffbrown.the at gmail.com (Jeffrey Brown)
Date: Tue, 31 Mar 2015 13:11:47 -0700
Subject: [Haskell-beginners] Mutable collection of gui objects
	(wxHaskell)
In-Reply-To: <op.xwcqnxsppz0j5l@alquantor>
References: <CAEc4Ma2WbxWzUnYD4xooOD=vG_iTGcrDp-0upvOAxcgkNoq_MQ@mail.gmail.com>
 <op.xwcqnxsppz0j5l@alquantor>
Message-ID: <CAEc4Ma2ccaaFcc9Eynja7A3RnLcHDF74KC30tFLud4aAHeozMQ@mail.gmail.com>

It works! And I don't understand it! Particularly this line:

  inputs <- map (\e -> [hfill $ widget e]) <$> varGet myVar

I'm considering the type signatures:
  map :: (a -> b) -> [a] -> [b]
  (<$>) :: Functor f => (a -> b) -> f a -> f b
  varGet :: Var a -> IO a
map and <$> (fmap) are nearly synonymous; map seems like a special case of
fmap. I gather the fmap must be to map inside the IO type that varGet
returns, and the map must be mapping across some list. But what about their
function arguments? Is map using the lambda expression, or is fmap? What
function argument is the other one using? Are they both somehow sharing the
lambda expression?

On Tue, Mar 31, 2015 at 12:48 AM, Henk-Jan van Tuyl <hjgtuyl at chello.nl>
wrote:

> On Tue, 31 Mar 2015 04:14:35 +0200, Jeffrey Brown <jeffbrown.the at gmail.com>
> wrote:
>
> :
>
>> I am trying to modify Layout.hs [1] to permit a variable number, rather
>> than a fixed number, of text entry boxes. I shrank the program, mostly by
>> removing features, until it was this:
>>
>>   main = start $ do
>>     f      <- frame  [text := "Layout test"]
>>     p      <- panel  f []
>>     xinput <- textEntry p [text := "100"]
>>     yinput <- textEntry p [text := "100"]
>>     myVar <- varCreate [xinput,yinput]
>>     set f [ layout := container p $ margin 10 $
>>       column 5 [boxed "coordinates" (grid 5 5
>>           [[hfill $ widget xinput], [hfill $ widget yinput]] -- replacing
>>         ) ] ]
>>     return ()
>>
>> I want to replace the line marked "replacing". Rather than hard-coding the
>> number of text entry boxes (2), I want it to deal with a mutable
>> collection
>> of them, in myVar.
>>
>> I tried this:
>>   [ fmap (\e -> hfill $ widget e) $ varGet myVar ]
>>
> :
>
> The result of varGet is of type "IO something", you must convert that to
> "something", by using "<-". You can do this by adding the line:
>   inputs <- map (\e -> [hfill $ widget e]) <$> varGet myVar
> before the set command (note the square brackets). The <$> operator is
> from module Data.Functor and is defined as
>   (<$>) = fmap
>
> The main function becomes this:
>   main = start $ do
>       f      <- frame  [text := "Layout test"]
>       p      <- panel  f []
>       xinput <- textEntry p [text := "100"]
>       yinput <- textEntry p [text := "100"]
>       myVar  <- varCreate [xinput, yinput]
>       inputs <- map (\e -> [hfill $ widget e]) <$> varGet myVar
>       set f [ layout := container p $ margin 10 $
>               column 5 [boxed "coordinates" (grid 5 5 inputs)]
>             ]
>       return ()
>
>
> Regards,
> Henk-Jan van Tuyl
>
>
> --
> Folding at home
> What if you could share your unused computer power to help find a cure? In
> just 5 minutes you can join the world's biggest networked computer and get
> us closer sooner. Watch the video.
> http://folding.stanford.edu/
>
>
> http://Van.Tuyl.eu/
> http://members.chello.nl/hjgtuyl/tourdemonad.html
> Haskell programming
> --
>
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From hjgtuyl at chello.nl  Tue Mar 31 22:51:44 2015
From: hjgtuyl at chello.nl (Henk-Jan van Tuyl)
Date: Wed, 01 Apr 2015 00:51:44 +0200
Subject: [Haskell-beginners] Mutable collection of gui objects
	(wxHaskell)
In-Reply-To: <CAEc4Ma2ccaaFcc9Eynja7A3RnLcHDF74KC30tFLud4aAHeozMQ@mail.gmail.com>
References: <CAEc4Ma2WbxWzUnYD4xooOD=vG_iTGcrDp-0upvOAxcgkNoq_MQ@mail.gmail.com>
 <op.xwcqnxsppz0j5l@alquantor>
 <CAEc4Ma2ccaaFcc9Eynja7A3RnLcHDF74KC30tFLud4aAHeozMQ@mail.gmail.com>
Message-ID: <op.xwdwgiw7pz0j5l@alquantor>

On Tue, 31 Mar 2015 22:11:47 +0200, Jeffrey Brown  
<jeffbrown.the at gmail.com> wrote:

> It works! And I don't understand it! Particularly this line:
>
>   inputs <- map (\e -> [hfill $ widget e]) <$> varGet myVar
>
> I'm considering the type signatures:
>   map :: (a -> b) -> [a] -> [b]
>   (<$>) :: Functor f => (a -> b) -> f a -> f b
>   varGet :: Var a -> IO a
> map and <$> (fmap) are nearly synonymous; map seems like a special case  
> of
> fmap. I gather the fmap must be to map inside the IO type that varGet
> returns, and the map must be mapping across some list. But what about  
> their
> function arguments? Is map using the lambda expression, or is fmap? What
> function argument is the other one using? Are they both somehow sharing  
> the
> lambda expression?

The expression
   f <$> x
is equal to
   fmap f x
. If
   f :: a -> b
   x :: IO a
then
   fmap f :: IO a -> IO b
. You could say, fmap lifts f to the IO monad (you could also use liftM  
for this).

In your case, f is
   map (\e -> [hfill $ widget e])
and x is
   varGet myVar
You stored a list of textEntrys in myVar, the lambda expression is mapped  
over this list.

Another way to look at it: you can replace the line
   inputs <- map (\e -> [hfill $ widget e]) <$> varGet myVar
with
   xs <- varGet myVar
   let inputs = map (\e -> [hfill $ widget e]) xs

Regards,
Henk-Jan van Tuyl


-- 
Folding at home
What if you could share your unused computer power to help find a cure? In  
just 5 minutes you can join the world's biggest networked computer and get  
us closer sooner. Watch the video.
http://folding.stanford.edu/


http://Van.Tuyl.eu/
http://members.chello.nl/hjgtuyl/tourdemonad.html
Haskell programming
--