From 1625143974 at qq.com  Wed Apr  4 06:37:13 2018
From: 1625143974 at qq.com (=?utf-8?B?5riF5769?=)
Date: Wed, 4 Apr 2018 14:37:13 +0800
Subject: [Haskell-beginners] I have a question in designing a 'show' function
Message-ID: <tencent_75099456B5378B55E7348165FAC596FFA408@qq.com>

here is the question:

Write a function showAreaOfCircle which, given the radius of a circle, calculates the area of the circle,
<code>showAreaOfCircle 12.3</code>  ⇒ <code>"The area of a circle with radius 12.3cm is about 475.2915525615999 cm^2"</code>
my solution is:
showAreaOfCircle :: Show a => a -> String showAreaOfCircle x = "The area of a circle with radius" ++ show x ++ "is about" ++ " " + show(pi * x * x) ++ "cm^2"
but there is an error

• Could not deduce (Num a) arising from a use of ‘*’
from the context: Show a
bound by the type signature for:
showAreaOfCircle :: forall a. Show a => a -> String
at FirstStep.hs:20:1-41
Possible fix:
add (Num a) to the context of 
the type signature for:
showAreaOfCircle :: forall a. Show a => a -> String
• In the first argument of ‘show’, namely ‘(pi * x * x)’
In the second argument of ‘(+)’, namely ‘show (pi * x * x)’
In the second argument of ‘(++)’, namely ‘" " + show (pi * x * x)’

I had tried to change the type declaration to 
Show a => Num a => a -> String
i guess there must be a problem with pi.

Can someone help me with the error message? Great thanks
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180404/d6bdacfa/attachment.html>

From mukeshtiwari.iiitm at gmail.com  Wed Apr  4 06:51:43 2018
From: mukeshtiwari.iiitm at gmail.com (mukesh tiwari)
Date: Wed, 4 Apr 2018 16:51:43 +1000
Subject: [Haskell-beginners] I have a question in designing a 'show'
	function
In-Reply-To: <tencent_75099456B5378B55E7348165FAC596FFA408@qq.com>
References: <tencent_75099456B5378B55E7348165FAC596FFA408@qq.com>
Message-ID: <CAFHZvE8avVDCNAbiSAPCK4e2=jH78XcJ-20uFGREFRigWsUtbQ@mail.gmail.com>

Add one more constraint that a is member of Floating type class and change
your + to ++

" " + show(pi * x * x



showAreaOfCircle :: (Floating a, Show a) => a -> String
showAreaOfCircle x = "The area of a circle with radius" ++ show x ++ "is
about" ++ " " ++ show (pi * x * x) ++ "cm^2"

*Tmp> showAreaOfCircle 10
"The area of a circle with radius10.0is about 314.1592653589793cm^2"
*Tmp> :t showAreaOfCircle 10
showAreaOfCircle 10 :: String
*Tmp> :t showAreaOfCircle
showAreaOfCircle :: (Show a, Floating a) => a -> String
*Tmp> :t showAreaOfCircle 10.12
showAreaOfCircle 10.12 :: String
*Tmp> showAreaOfCircle 10.12
"The area of a circle with radius10.12is about 321.74432666180644cm^2"
*Tmp>

Best,
Mukesh


On Wed, Apr 4, 2018 at 4:37 PM, 清羽 <1625143974 at qq.com> wrote:

>
> here is the question:
>
> Write a function showAreaOfCircle which, given the radius of a circle,
> calculates the area of the circle,
>
> <code>showAreaOfCircle 12.3</code>  ⇒ <code>"The area of a circle with radius 12.3cm is about 475.2915525615999 cm^2"</code>
>
> my solution is:
>
> showAreaOfCircle :: Show a => a -> String
> showAreaOfCircle x = "The area of a circle with radius" ++ show x ++ "is about" ++ " " + show(pi * x * x) ++ "cm^2"
>
> but there is an error
>
> • Could not deduce (Num a) arising from a use of ‘*’
> from the context: Show a
> bound by the type signature for:
> showAreaOfCircle :: forall a. Show a => a -> String
> at FirstStep.hs:20:1-41
> Possible fix:
> add (Num a) to the context of
> the type signature for:
> showAreaOfCircle :: forall a. Show a => a -> String
> • In the first argument of ‘show’, namely ‘(pi * x * x)’
> In the second argument of ‘(+)’, namely ‘show (pi * x * x)’
> In the second argument of ‘(++)’, namely ‘" " + show (pi * x * x)’
>
> I had tried to change the type declaration to
>
> Show a => Num a => a -> String
>
> i guess there must be a problem with pi.
>
> Can someone help me with the error message? Great thanks
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180404/ae18ab08/attachment.html>

From joel.neely at gmail.com  Wed Apr  4 11:19:11 2018
From: joel.neely at gmail.com (Joel Neely)
Date: Wed, 4 Apr 2018 06:19:11 -0500
Subject: [Haskell-beginners] I have a question in designing a 'show'
	function
In-Reply-To: <tencent_75099456B5378B55E7348165FAC596FFA408@qq.com>
References: <tencent_75099456B5378B55E7348165FAC596FFA408@qq.com>
Message-ID: <CAEEzXAjV7iTb8CNeJP6yPEMbYszN68H5d3tf_2Bv6+nFb7bzmg@mail.gmail.com>

You only have a single plus sign before the second invocation of show:

++ " " + show(pi * x * x) ++ "cm^2"

Don't you mean concatenation?

++ " " ++ show(pi * x * x) ++ "cm^2"


Good luck,
-jn-


On Wed, Apr 4, 2018 at 1:37 AM, 清羽 <1625143974 at qq.com> wrote:

>
> here is the question:
>
> Write a function showAreaOfCircle which, given the radius of a circle,
> calculates the area of the circle,
>
> <code>showAreaOfCircle 12.3</code>  ⇒ <code>"The area of a circle with radius 12.3cm is about 475.2915525615999 cm^2"</code>
>
> my solution is:
>
> showAreaOfCircle :: Show a => a -> String
> showAreaOfCircle x = "The area of a circle with radius" ++ show x ++ "is about" ++ " " + show(pi * x * x) ++ "cm^2"
>
> but there is an error
>
> • Could not deduce (Num a) arising from a use of ‘*’
> from the context: Show a
> bound by the type signature for:
> showAreaOfCircle :: forall a. Show a => a -> String
> at FirstStep.hs:20:1-41
> Possible fix:
> add (Num a) to the context of
> the type signature for:
> showAreaOfCircle :: forall a. Show a => a -> String
> • In the first argument of ‘show’, namely ‘(pi * x * x)’
> In the second argument of ‘(+)’, namely ‘show (pi * x * x)’
> In the second argument of ‘(++)’, namely ‘" " + show (pi * x * x)’
>
> I had tried to change the type declaration to
>
> Show a => Num a => a -> String
>
> i guess there must be a problem with pi.
>
> Can someone help me with the error message? Great thanks
>
> _______________________________________________
> 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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180404/17b9e8b3/attachment-0001.html>

From 1625143974 at qq.com  Wed Apr  4 12:14:57 2018
From: 1625143974 at qq.com (=?utf-8?B?5riF5769?=)
Date: Wed, 4 Apr 2018 20:14:57 +0800
Subject: [Haskell-beginners] =?utf-8?b?5Zue5aSN77yaICBJIGhhdmUgYSBxdWVz?=
 =?utf-8?q?tion_in_designing_a_=27show=27function?=
In-Reply-To: <CAEEzXAjV7iTb8CNeJP6yPEMbYszN68H5d3tf_2Bv6+nFb7bzmg@mail.gmail.com>
References: <tencent_75099456B5378B55E7348165FAC596FFA408@qq.com>
 <CAEEzXAjV7iTb8CNeJP6yPEMbYszN68H5d3tf_2Bv6+nFb7bzmg@mail.gmail.com>
Message-ID: <tencent_F9D317FDCE21032FF4DF2C33492D29496A09@qq.com>

Yes.And I finally make the function error-free.Thanks


------------------ 原始邮件 ------------------
发件人: "Joel Neely"<joel.neely at gmail.com>;
发送时间: 2018年4月4日(星期三) 晚上7:19
收件人: "The Haskell-Beginners Mailing List - Discussion of primarilybeginner-level topics related to Haskell"<beginners at haskell.org>;

主题: Re: [Haskell-beginners] I have a question in designing a 'show'function



You only have a single plus sign before the second invocation of show:


++ " " + show(pi * x * x) ++ "cm^2"Don't you mean concatenation?


++ " " ++ show(pi * x * x) ++ "cm^2"

Good luck,
-jn-




On Wed, Apr 4, 2018 at 1:37 AM, 清羽 <1625143974 at qq.com> wrote:


here is the question:

Write a function showAreaOfCircle which, given the radius of a circle, calculates the area of the circle,
<code>showAreaOfCircle 12.3</code>  ⇒ <code>"The area of a circle with radius 12.3cm is about 475.2915525615999 cm^2"</code>
my solution is:
showAreaOfCircle :: Show a => a -> String showAreaOfCircle x = "The area of a circle with radius" ++ show x ++ "is about" ++ " " + show(pi * x * x) ++ "cm^2"
but there is an error

• Could not deduce (Num a) arising from a use of ‘*’
from the context: Show a
bound by the type signature for:
showAreaOfCircle :: forall a. Show a => a -> String
at FirstStep.hs:20:1-41
Possible fix:
add (Num a) to the context of 
the type signature for:
showAreaOfCircle :: forall a. Show a => a -> String
• In the first argument of ‘show’, namely ‘(pi * x * x)’
In the second argument of ‘(+)’, namely ‘show (pi * x * x)’
In the second argument of ‘(++)’, namely ‘" " + show (pi * x * x)’

I had tried to change the type declaration to 
Show a => Num a => a -> String
i guess there must be a problem with pi.

Can someone help me with the error message? Great thanks


_______________________________________________
 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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180404/16f21435/attachment.html>

From 1625143974 at qq.com  Wed Apr  4 14:08:04 2018
From: 1625143974 at qq.com (=?utf-8?B?5riF5769?=)
Date: Wed, 4 Apr 2018 22:08:04 +0800
Subject: [Haskell-beginners] A question regarding the syntax behind the
	parenthesis and type class.
Message-ID: <tencent_7D0F03A1F41B2D2D487D299B6A0F17CBCF09@qq.com>

this is the original question:


Write a function showAreaOfCircle which, given the radius of a circle, calculates the area of the circle,

showAreaOfCircle 12.3  ⇒ "The area of a circle with radius 12.3cm is about  475.2915525615999 cm^2"

Use the show function, as well as the predefined value pi :: Floating a => a to write showAreaOfCircle.


the type signature works for this function is: showAreaOfCircle :: (Show a, Floating a) => a -> a -> String


my question is : I don't quite understand the syntax behind the parenthesis and the type class Floating a. Since pi is not a function, and hence the entire function only takes one argument which belongs to the type class Show, why should i put a Floating a in the type signature in the first place?(i come up with this type signature because of the error message and the suggestion stated above) Thanks
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180404/a9509ebf/attachment.html>

From toad3k at gmail.com  Wed Apr  4 14:20:40 2018
From: toad3k at gmail.com (David McBride)
Date: Wed, 4 Apr 2018 10:20:40 -0400
Subject: [Haskell-beginners] A question regarding the syntax behind the
 parenthesis and type class.
In-Reply-To: <tencent_7D0F03A1F41B2D2D487D299B6A0F17CBCF09@qq.com>
References: <tencent_7D0F03A1F41B2D2D487D299B6A0F17CBCF09@qq.com>
Message-ID: <CAN+Tr4277p43qzA9fiFLJYd1YVVj3KhjqinFERK2XM--GPDZrw@mail.gmail.com>

Because you are not showing x or pi, you are showing (pi * x * x).  And the
type of the (*) operator is (*) :: Num a => a -> a -> a.  So if pi is
Floating => a, then x must also be the same type as pi, and thus must be
have the same Floating constraint.

Keep in mind that Floating is a Fractional, and Fractional is a Num, so you
know that if you have a Floating it is already an instance of Num, so you
don't have to worry about that constraint.

On Wed, Apr 4, 2018 at 10:08 AM, 清羽 <1625143974 at qq.com> wrote:

> this is the original question:
>
>    1.
>
>    Write a function showAreaOfCircle which, given the radius of a circle,
>    calculates the area of the circle,
>
>    showAreaOfCircle 12.3  ⇒ "The area of a circle with radius 12.3cm is
>    about 475.2915525615999 cm^2"
>
>    Use the show function, as well as the predefined value pi :: Floating
>    a => a to write showAreaOfCircle.
>
> the type signature works for this function is: showAreaOfCircle :: (Show
> a, Floating a) => a -> a -> String
>
> my question is : I don't quite understand the syntax behind the
> parenthesis and the type class Floating a. Since pi is not a function, and
> hence the entire function only takes one argument which belongs to the type
> class Show, why should i put a Floating a in the type signature in the
> first place?(i come up with this type signature because of the error
> message and the suggestion stated above) Thanks
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180404/643206b5/attachment.html>

From 1625143974 at qq.com  Tue Apr 10 12:47:17 2018
From: 1625143974 at qq.com (=?utf-8?B?5riF5769?=)
Date: Tue, 10 Apr 2018 20:47:17 +0800
Subject: [Haskell-beginners] May I ask what is the meaning of ch <= 'z' and
	'a' <= ch.
Message-ID: <tencent_7D6BE0917E8BA523A90AA55DEDC522C57906@qq.com>

Hello everyone, I have a simple question here     Define a function isLower :: Char -> Bool which returns True if a given character is a lower case letter. You can use the fact that characters are ordered, and for all lower case letters ch we have ′a′ ≤ ch and ch ≤ ′z′. Alternatively, you can use the fact that ['a'..'z'] evaluates to a list containing all lower case letters.
    And May I ask what is the meaning of ch <= 'z' and 'a' <= ch? I copied this as a condition and solved the problem.here is my work.
    isLower :: Char -> Bool
    isLower x = if x ch <= 'z' && 'a' <= ch then True else False.
    Thanks in advance.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180410/c7dcbc1e/attachment.html>

From fa-ml at ariis.it  Tue Apr 10 13:38:58 2018
From: fa-ml at ariis.it (Francesco Ariis)
Date: Tue, 10 Apr 2018 15:38:58 +0200
Subject: [Haskell-beginners] May I ask what is the meaning of ch <= 'z'
 and 'a' <= ch.
In-Reply-To: <tencent_7D6BE0917E8BA523A90AA55DEDC522C57906@qq.com>
References: <tencent_7D6BE0917E8BA523A90AA55DEDC522C57906@qq.com>
Message-ID: <20180410133858.cl7yp2pepvvc4k7b@x60s.casa>

On Tue, Apr 10, 2018 at 08:47:17PM +0800, 清羽 wrote:
>     And May I ask what is the meaning of ch <= 'z' and 'a' <= ch? I copied this as a condition and solved the problem.here is my work.
>     isLower :: Char -> Bool
>     isLower x = if x ch <= 'z' && 'a' <= ch then True else False.
>     Thanks in advance.

Hello 清羽, Char is an instance of Ord, so you can use <, compare, etc.
with it.
<= and >= stand for "less or equal than" and "greater or equal than".

As with many enumeration types, the order is a bit arbitrary

    λ> minBound :: Char
    '\NUL'
    λ> maxBound :: Char
    '\1114111'
    λ> ['$'..'Z']
    "$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ"
            -- so * will be < than, say, A
    λ> '*' < 'A'
    True

Does that answer your question?


From karim.salah2048 at gmail.com  Tue Apr 10 15:35:56 2018
From: karim.salah2048 at gmail.com (Karim Salah)
Date: Tue, 10 Apr 2018 17:35:56 +0200
Subject: [Haskell-beginners] 2018 report
Message-ID: <CAAktkWxUnawo8AsGTm4OjfT8bVL+jNdJtkZCJ9zfXZNgBN4VYw@mail.gmail.com>

hi everyone,
I am wondering how can i submit a contribution in the Haskell report.
It is stated up here:
http://mail.haskell.org/pipermail/haskell-cafe/2018-March/128829.html
that i shall send my contribution hcar at haskell.org which i have no clue
how to this
i will appreciate any help
Thanks in advance
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180410/e1d5e72d/attachment.html>

From fa-ml at ariis.it  Tue Apr 10 15:56:45 2018
From: fa-ml at ariis.it (Francesco Ariis)
Date: Tue, 10 Apr 2018 17:56:45 +0200
Subject: [Haskell-beginners] 2018 report
In-Reply-To: <CAAktkWxUnawo8AsGTm4OjfT8bVL+jNdJtkZCJ9zfXZNgBN4VYw@mail.gmail.com>
References: <CAAktkWxUnawo8AsGTm4OjfT8bVL+jNdJtkZCJ9zfXZNgBN4VYw@mail.gmail.com>
Message-ID: <20180410155645.wqlml4w4sfczdpmm@x60s.casa>

On Tue, Apr 10, 2018 at 05:35:56PM +0200, Karim Salah wrote:
> hi everyone,
> I am wondering how can i submit a contribution in the Haskell report.
> It is stated up here:
> http://mail.haskell.org/pipermail/haskell-cafe/2018-March/128829.html
> that i shall send my contribution hcar at haskell.org which i have no clue
> how to this
> i will appreciate any help
> Thanks in advance

Hey Karim,
    it is "please send your contributions to *hcar* at haskell.org"
in the same (antispam) fashion as "karim.salah2048 at gmail.com"
-F



From karim.salah2048 at gmail.com  Tue Apr 10 17:06:21 2018
From: karim.salah2048 at gmail.com (Karim Salah)
Date: Tue, 10 Apr 2018 19:06:21 +0200
Subject: [Haskell-beginners] 2018 report
In-Reply-To: <20180410155645.wqlml4w4sfczdpmm@x60s.casa>
References: <CAAktkWxUnawo8AsGTm4OjfT8bVL+jNdJtkZCJ9zfXZNgBN4VYw@mail.gmail.com>
 <20180410155645.wqlml4w4sfczdpmm@x60s.casa>
Message-ID: <CAAktkWzfA7Tz38yD+BfFA2kaPYtTz5dd8FjmFAtGmcSQqey4Ng@mail.gmail.com>

Now i get it
thanks a lot

On Tue, Apr 10, 2018 at 5:56 PM, Francesco Ariis <fa-ml at ariis.it> wrote:

> On Tue, Apr 10, 2018 at 05:35:56PM +0200, Karim Salah wrote:
> > hi everyone,
> > I am wondering how can i submit a contribution in the Haskell report.
> > It is stated up here:
> > http://mail.haskell.org/pipermail/haskell-cafe/2018-March/128829.html
> > that i shall send my contribution hcar at haskell.org which i have no
> clue
> > how to this
> > i will appreciate any help
> > Thanks in advance
>
> Hey Karim,
>     it is "please send your contributions to *hcar* at haskell.org"
> in the same (antispam) fashion as "karim.salah2048 at gmail.com"
> -F
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180410/4a7dde24/attachment.html>

From tanuki at gmail.com  Tue Apr 10 17:51:17 2018
From: tanuki at gmail.com (Theodore Lief Gannon)
Date: Tue, 10 Apr 2018 17:51:17 +0000
Subject: [Haskell-beginners] May I ask what is the meaning of ch <= 'z'
 and 'a' <= ch.
In-Reply-To: <tencent_7D6BE0917E8BA523A90AA55DEDC522C57906@qq.com>
References: <tencent_7D6BE0917E8BA523A90AA55DEDC522C57906@qq.com>
Message-ID: <CAJoPsuBYndP7z6=ahNSdxO5AQwQK5SdSEEn4aUa=ZM65otQfsA@mail.gmail.com>

'ch' here is a variable, standing for 'character.' It takes the place of
'x'. Your solution is almost correct, you just need to pick one name or the
other for your variable.

On Tue, Apr 10, 2018, 5:51 AM 清羽 <1625143974 at qq.com> wrote:

> Hello everyone, I have a simple question here
>     Define a function isLower :: Char -> Bool which returns True if a
> given character is a lower case letter. You can use the fact that
> characters are ordered, and for all lower case letters *ch* we have ′*a*′
> ≤ *c**h* and *c**h* ≤ ′*z*′. Alternatively, you can use the fact that ['a'
> ..'z'] evaluates to a list containing all lower case letters.
>     And May I ask what is the meaning of ch <= 'z' and 'a' <= ch? I
> copied this as a condition and solved the problem.here is my work.
>     isLower :: Char -> Bool
>     isLower x = if x ch <= 'z' && 'a' <= ch then True else False.
>     Thanks in advance.
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180410/0f9db2c8/attachment-0001.html>

From mike_k_houghton at yahoo.co.uk  Thu Apr 12 10:25:57 2018
From: mike_k_houghton at yahoo.co.uk (mike h)
Date: Thu, 12 Apr 2018 11:25:57 +0100
Subject: [Haskell-beginners] EitherT
Message-ID: <57AFC9F5-3601-4D18-B860-41352D3F2204@yahoo.co.uk>

Hi,
I’m trying to write EitherT from first principles and I’m stuck at the first hurdle - Functor.  I’m looking for a hint rather than a complete answer :) 


This is what I have

newtype EitherT m a b = EitherT {runEitherT :: m (Either a b)}
instance Monad m => Functor (EitherT m a) where
    ---- fmap :: (a -> b) -> f a -> f b
    fmap f m = EitherT $ do
        mv <- runEitherT m
        case mv of
            Left _   -> return mv
            Right rv -> return $ Right (f rv)



and here is the compilers view
Phrase.hs:31:25: error:
    • Couldn't match type ‘b’ with ‘a1’
      ‘b’ is a rigid type variable bound by
        the type signature for:
          fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b
        at Phrase.hs:27:5-8
      ‘a1’ is a rigid type variable bound by
        the type signature for:
          fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b
        at Phrase.hs:27:5-8
      Expected type: m (Either a a1)
        Actual type: m (Either a b)
    • In the expression: return $ Right (f rv)
      In a case alternative: Right rv -> return $ Right (f rv)
      In a stmt of a 'do' block:
        case mv of
          Left _ -> return mv
          Right rv -> return $ Right (f rv)
    • Relevant bindings include
        rv :: a1 (bound at Phrase.hs:31:19)
        mv :: Either a a1 (bound at Phrase.hs:28:9)
        m :: EitherT m a a1 (bound at Phrase.hs:27:12)
        f :: a1 -> b (bound at Phrase.hs:27:10)
        fmap :: (a1 -> b) -> EitherT m a a1 -> EitherT m a b
          (bound at Phrase.hs:27:5)
   

what I think I need to do is fmap over the right value after pulling if out of the monad m by doing   mv <- runEitherT m

these lines from the compiler are particularly confusing 
      Expected type: m (Either a a1)
        Actual type: m (Either a b)

as I believe f is    f:: a1->b 

So just hints please and I expect I’ll have another duh moment. 

Thanks

Mike




From fa-ml at ariis.it  Thu Apr 12 15:59:32 2018
From: fa-ml at ariis.it (Francesco Ariis)
Date: Thu, 12 Apr 2018 17:59:32 +0200
Subject: [Haskell-beginners] EitherT
In-Reply-To: <57AFC9F5-3601-4D18-B860-41352D3F2204@yahoo.co.uk>
References: <57AFC9F5-3601-4D18-B860-41352D3F2204@yahoo.co.uk>
Message-ID: <20180412155932.r5fbeh5wn52k42sp@x60s.casa>

On Thu, Apr 12, 2018 at 11:25:57AM +0100, mike h wrote:
> This is what I have
> 
> newtype EitherT m a b = EitherT {runEitherT :: m (Either a b)}
> instance Monad m => Functor (EitherT m a) where
>     ---- fmap :: (a -> b) -> f a -> f b
>     fmap f m = EitherT $ do
>         mv <- runEitherT m
>         case mv of
>             Left _   -> return mv
>             Right rv -> return $ Right (f rv)

Tricky error! The signature for this fmap is:

    fmap :: (b -> c) -> EitherT m a b -> EitherT m a c

The offending line is:

    Left _   -> return mv

You *think* you are returning `Either a c`, but are you really?

Type HINTS for more :P
-F

From toad3k at gmail.com  Thu Apr 12 16:27:30 2018
From: toad3k at gmail.com (David McBride)
Date: Thu, 12 Apr 2018 12:27:30 -0400
Subject: [Haskell-beginners] EitherT
In-Reply-To: <57AFC9F5-3601-4D18-B860-41352D3F2204@yahoo.co.uk>
References: <57AFC9F5-3601-4D18-B860-41352D3F2204@yahoo.co.uk>
Message-ID: <CAN+Tr41enxTVB8dyVRxtCwc+3KoJbYfuS6DfOGBTg4f0K7WYRQ@mail.gmail.com>

You will feel like this was obvious in hindsight.

fmap :: (a -> b) -> f a -> f b
fmap (a -> b) -> EitherT m c a -> EitherT m c b

Now follow the types in your code.
m :: EitherT m c a
mv :: Either c a
return mv :: EitherT m c a -- <- you are back to the wrong type.

How can you instead return EitherT m c b?


On Thu, Apr 12, 2018 at 6:25 AM, mike h <mike_k_houghton at yahoo.co.uk> wrote:

> Hi,
> I’m trying to write EitherT from first principles and I’m stuck at the
> first hurdle - Functor.  I’m looking for a hint rather than a complete
> answer :)
>
>
> This is what I have
>
> newtype EitherT m a b = EitherT {runEitherT :: m (Either a b)}
> instance Monad m => Functor (EitherT m a) where
>     ---- fmap :: (a -> b) -> f a -> f b
>     fmap f m = EitherT $ do
>         mv <- runEitherT m
>         case mv of
>             Left _   -> return mv
>             Right rv -> return $ Right (f rv)
>
>
>
> and here is the compilers view
> Phrase.hs:31:25: error:
>     • Couldn't match type ‘b’ with ‘a1’
>       ‘b’ is a rigid type variable bound by
>         the type signature for:
>           fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b
>         at Phrase.hs:27:5-8
>       ‘a1’ is a rigid type variable bound by
>         the type signature for:
>           fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b
>         at Phrase.hs:27:5-8
>       Expected type: m (Either a a1)
>         Actual type: m (Either a b)
>     • In the expression: return $ Right (f rv)
>       In a case alternative: Right rv -> return $ Right (f rv)
>       In a stmt of a 'do' block:
>         case mv of
>           Left _ -> return mv
>           Right rv -> return $ Right (f rv)
>     • Relevant bindings include
>         rv :: a1 (bound at Phrase.hs:31:19)
>         mv :: Either a a1 (bound at Phrase.hs:28:9)
>         m :: EitherT m a a1 (bound at Phrase.hs:27:12)
>         f :: a1 -> b (bound at Phrase.hs:27:10)
>         fmap :: (a1 -> b) -> EitherT m a a1 -> EitherT m a b
>           (bound at Phrase.hs:27:5)
>
>
> what I think I need to do is fmap over the right value after pulling if
> out of the monad m by doing   mv <- runEitherT m
>
> these lines from the compiler are particularly confusing
>       Expected type: m (Either a a1)
>         Actual type: m (Either a b)
>
> as I believe f is    f:: a1->b
>
> So just hints please and I expect I’ll have another duh moment.
>
> Thanks
>
> Mike
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180412/05fe8014/attachment.html>

From traqueofziche at gmail.com  Thu Apr 12 16:40:53 2018
From: traqueofziche at gmail.com (=?UTF-8?B?6bKN5Yev5paH?=)
Date: Thu, 12 Apr 2018 16:40:53 +0000
Subject: [Haskell-beginners] EitherT
In-Reply-To: <mailman.11.1523534402.27594.beginners@haskell.org>
References: <mailman.11.1523534402.27594.beginners@haskell.org>
Message-ID: <CAMjcG+Go=Uk==Ur+NXyxOPsCsNYwZgg15NngTnnFiOiFeX3oYQ@mail.gmail.com>

Hi,

Try to unify the types returned by each of the case alternatives, starting
from what the compiler has inferred so far.

I’ve made this mistake many times lol.

Best,

toz

On Thu, Apr 12, 2018 at 5:38 AM <beginners-request at haskell.org> wrote:

> Send Beginners mailing list submissions to
>         beginners at haskell.org
>
> To subscribe or unsubscribe via the World Wide Web, visit
>         http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> or, via email, send a message with subject or body 'help' to
>         beginners-request at haskell.org
>
> You can reach the person managing the list at
>         beginners-owner at haskell.org
>
> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of Beginners digest..."
>
>
> Today's Topics:
>
>    1.  EitherT (mike h)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Thu, 12 Apr 2018 11:25:57 +0100
> From: mike h <mike_k_houghton at yahoo.co.uk>
> To: The Haskell-Beginners Mailing List - Discussion of primarily
>         beginner-level topics related to Haskell <beginners at haskell.org>
> Subject: [Haskell-beginners] EitherT
> Message-ID: <57AFC9F5-3601-4D18-B860-41352D3F2204 at yahoo.co.uk>
> Content-Type: text/plain; charset=utf-8
>
> Hi,
> I’m trying to write EitherT from first principles and I’m stuck at the
> first hurdle - Functor.  I’m looking for a hint rather than a complete
> answer :)
>
>
> This is what I have
>
> newtype EitherT m a b = EitherT {runEitherT :: m (Either a b)}
> instance Monad m => Functor (EitherT m a) where
>     ---- fmap :: (a -> b) -> f a -> f b
>     fmap f m = EitherT $ do
>         mv <- runEitherT m
>         case mv of
>             Left _   -> return mv
>             Right rv -> return $ Right (f rv)
>
>
>
> and here is the compilers view
> Phrase.hs:31:25: error:
>     • Couldn't match type ‘b’ with ‘a1’
>       ‘b’ is a rigid type variable bound by
>         the type signature for:
>           fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b
>         at Phrase.hs:27:5-8
>       ‘a1’ is a rigid type variable bound by
>         the type signature for:
>           fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b
>         at Phrase.hs:27:5-8
>       Expected type: m (Either a a1)
>         Actual type: m (Either a b)
>     • In the expression: return $ Right (f rv)
>       In a case alternative: Right rv -> return $ Right (f rv)
>       In a stmt of a 'do' block:
>         case mv of
>           Left _ -> return mv
>           Right rv -> return $ Right (f rv)
>     • Relevant bindings include
>         rv :: a1 (bound at Phrase.hs:31:19)
>         mv :: Either a a1 (bound at Phrase.hs:28:9)
>         m :: EitherT m a a1 (bound at Phrase.hs:27:12)
>         f :: a1 -> b (bound at Phrase.hs:27:10)
>         fmap :: (a1 -> b) -> EitherT m a a1 -> EitherT m a b
>           (bound at Phrase.hs:27:5)
>
>
> what I think I need to do is fmap over the right value after pulling if
> out of the monad m by doing   mv <- runEitherT m
>
> these lines from the compiler are particularly confusing
>       Expected type: m (Either a a1)
>         Actual type: m (Either a b)
>
> as I believe f is    f:: a1->b
>
> So just hints please and I expect I’ll have another duh moment.
>
> Thanks
>
> Mike
>
>
>
>
>
> ------------------------------
>
> Subject: Digest Footer
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
> ------------------------------
>
> End of Beginners Digest, Vol 118, Issue 6
> *****************************************
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180412/e78207cb/attachment.html>

From mike_k_houghton at yahoo.co.uk  Thu Apr 12 20:32:05 2018
From: mike_k_houghton at yahoo.co.uk (mike h)
Date: Thu, 12 Apr 2018 21:32:05 +0100
Subject: [Haskell-beginners] EitherT
In-Reply-To: <CAN+Tr41enxTVB8dyVRxtCwc+3KoJbYfuS6DfOGBTg4f0K7WYRQ@mail.gmail.com>
References: <57AFC9F5-3601-4D18-B860-41352D3F2204@yahoo.co.uk>
 <CAN+Tr41enxTVB8dyVRxtCwc+3KoJbYfuS6DfOGBTg4f0K7WYRQ@mail.gmail.com>
Message-ID: <7EDD529B-500B-4138-A4FD-FBDD14D5D474@yahoo.co.uk>

instance Monad m => Functor (EitherT m a) where
   fmap f m = EitherT $ do
       mv <- runEitherT m
       case mv of
           Left  lv -> return $ Left lv
           Right rv -> return $ Right (f rv)


Thanks all :) I think its correct. The compiler does! 
Mike


> On 12 Apr 2018, at 17:27, David McBride <toad3k at gmail.com> wrote:
> 
> You will feel like this was obvious in hindsight.
> 
> fmap :: (a -> b) -> f a -> f b
> fmap (a -> b) -> EitherT m c a -> EitherT m c b
> 
> Now follow the types in your code.
> m :: EitherT m c a
> mv :: Either c a
> return mv :: EitherT m c a -- <- you are back to the wrong type.
> 
> How can you instead return EitherT m c b?
> 
> 
> On Thu, Apr 12, 2018 at 6:25 AM, mike h <mike_k_houghton at yahoo.co.uk <mailto:mike_k_houghton at yahoo.co.uk>> wrote:
> Hi,
> I’m trying to write EitherT from first principles and I’m stuck at the first hurdle - Functor.  I’m looking for a hint rather than a complete answer :)
> 
> 
> This is what I have
> 
> newtype EitherT m a b = EitherT {runEitherT :: m (Either a b)}
> instance Monad m => Functor (EitherT m a) where
>     ---- fmap :: (a -> b) -> f a -> f b
>     fmap f m = EitherT $ do
>         mv <- runEitherT m
>         case mv of
>             Left _   -> return mv
>             Right rv -> return $ Right (f rv)
> 
> 
> 
> and here is the compilers view
> Phrase.hs:31:25: error:
>     • Couldn't match type ‘b’ with ‘a1’
>       ‘b’ is a rigid type variable bound by
>         the type signature for:
>           fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b
>         at Phrase.hs:27:5-8
>       ‘a1’ is a rigid type variable bound by
>         the type signature for:
>           fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b
>         at Phrase.hs:27:5-8
>       Expected type: m (Either a a1)
>         Actual type: m (Either a b)
>     • In the expression: return $ Right (f rv)
>       In a case alternative: Right rv -> return $ Right (f rv)
>       In a stmt of a 'do' block:
>         case mv of
>           Left _ -> return mv
>           Right rv -> return $ Right (f rv)
>     • Relevant bindings include
>         rv :: a1 (bound at Phrase.hs:31:19)
>         mv :: Either a a1 (bound at Phrase.hs:28:9)
>         m :: EitherT m a a1 (bound at Phrase.hs:27:12)
>         f :: a1 -> b (bound at Phrase.hs:27:10)
>         fmap :: (a1 -> b) -> EitherT m a a1 -> EitherT m a b
>           (bound at Phrase.hs:27:5)
> 
> 
> what I think I need to do is fmap over the right value after pulling if out of the monad m by doing   mv <- runEitherT m
> 
> these lines from the compiler are particularly confusing
>       Expected type: m (Either a a1)
>         Actual type: m (Either a b)
> 
> as I believe f is    f:: a1->b
> 
> So just hints please and I expect I’ll have another duh moment.
> 
> Thanks
> 
> Mike
> 
> 
> 
> _______________________________________________
> 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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180412/6d8f6b7d/attachment-0001.html>

From mike_k_houghton at yahoo.co.uk  Fri Apr 13 08:35:01 2018
From: mike_k_houghton at yahoo.co.uk (mike h)
Date: Fri, 13 Apr 2018 09:35:01 +0100
Subject: [Haskell-beginners] EitherT
In-Reply-To: <7EDD529B-500B-4138-A4FD-FBDD14D5D474@yahoo.co.uk>
References: <57AFC9F5-3601-4D18-B860-41352D3F2204@yahoo.co.uk>
 <CAN+Tr41enxTVB8dyVRxtCwc+3KoJbYfuS6DfOGBTg4f0K7WYRQ@mail.gmail.com>
 <7EDD529B-500B-4138-A4FD-FBDD14D5D474@yahoo.co.uk>
Message-ID: <0709C844-FD6E-4130-B959-59EA459F798F@yahoo.co.uk>

Even ‘better’ 
instance Monad m => Functor (EitherT m a) where
    fmap f m = EitherT $ runEitherT m >>= \mv -> return $ fmap f mv

:)


> On 12 Apr 2018, at 21:32, mike h <mike_k_houghton at yahoo.co.uk> wrote:
> 
> instance Monad m => Functor (EitherT m a) where
>    fmap f m = EitherT $ do
>        mv <- runEitherT m
>        case mv of
>            Left  lv -> return $ Left lv
>            Right rv -> return $ Right (f rv)
> 
> 
> Thanks all :) I think its correct. The compiler does! 
> Mike
> 
> 
>> On 12 Apr 2018, at 17:27, David McBride <toad3k at gmail.com <mailto:toad3k at gmail.com>> wrote:
>> 
>> You will feel like this was obvious in hindsight.
>> 
>> fmap :: (a -> b) -> f a -> f b
>> fmap (a -> b) -> EitherT m c a -> EitherT m c b
>> 
>> Now follow the types in your code.
>> m :: EitherT m c a
>> mv :: Either c a
>> return mv :: EitherT m c a -- <- you are back to the wrong type.
>> 
>> How can you instead return EitherT m c b?
>> 
>> 
>> On Thu, Apr 12, 2018 at 6:25 AM, mike h <mike_k_houghton at yahoo.co.uk <mailto:mike_k_houghton at yahoo.co.uk>> wrote:
>> Hi,
>> I’m trying to write EitherT from first principles and I’m stuck at the first hurdle - Functor.  I’m looking for a hint rather than a complete answer :)
>> 
>> 
>> This is what I have
>> 
>> newtype EitherT m a b = EitherT {runEitherT :: m (Either a b)}
>> instance Monad m => Functor (EitherT m a) where
>>     ---- fmap :: (a -> b) -> f a -> f b
>>     fmap f m = EitherT $ do
>>         mv <- runEitherT m
>>         case mv of
>>             Left _   -> return mv
>>             Right rv -> return $ Right (f rv)
>> 
>> 
>> 
>> and here is the compilers view
>> Phrase.hs:31:25: error:
>>     • Couldn't match type ‘b’ with ‘a1’
>>       ‘b’ is a rigid type variable bound by
>>         the type signature for:
>>           fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b
>>         at Phrase.hs:27:5-8
>>       ‘a1’ is a rigid type variable bound by
>>         the type signature for:
>>           fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b
>>         at Phrase.hs:27:5-8
>>       Expected type: m (Either a a1)
>>         Actual type: m (Either a b)
>>     • In the expression: return $ Right (f rv)
>>       In a case alternative: Right rv -> return $ Right (f rv)
>>       In a stmt of a 'do' block:
>>         case mv of
>>           Left _ -> return mv
>>           Right rv -> return $ Right (f rv)
>>     • Relevant bindings include
>>         rv :: a1 (bound at Phrase.hs:31:19)
>>         mv :: Either a a1 (bound at Phrase.hs:28:9)
>>         m :: EitherT m a a1 (bound at Phrase.hs:27:12)
>>         f :: a1 -> b (bound at Phrase.hs:27:10)
>>         fmap :: (a1 -> b) -> EitherT m a a1 -> EitherT m a b
>>           (bound at Phrase.hs:27:5)
>> 
>> 
>> what I think I need to do is fmap over the right value after pulling if out of the monad m by doing   mv <- runEitherT m
>> 
>> these lines from the compiler are particularly confusing
>>       Expected type: m (Either a a1)
>>         Actual type: m (Either a b)
>> 
>> as I believe f is    f:: a1->b
>> 
>> So just hints please and I expect I’ll have another duh moment.
>> 
>> Thanks
>> 
>> Mike
>> 
>> 
>> 
>> _______________________________________________
>> 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 <mailto:Beginners at haskell.org>
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180413/b774bbe0/attachment.html>

From marc at lamarciana.com  Fri Apr 13 09:42:20 2018
From: marc at lamarciana.com (=?ISO-8859-15?Q?Marc_Busqu=E9?=)
Date: Fri, 13 Apr 2018 11:42:20 +0200 (CEST)
Subject: [Haskell-beginners] Lazyness and forM_
Message-ID: <alpine.LNX.2.21.1804131127290.6944@localhost>

Hi there!

I'm using [selda](https://github.com/valderman/selda) package to work
with databases.

I'm trying to write a function to generate several tables at once. In
the following examples, `categories` and `expenses` are two selda
`Table`. The other functions in use and their respective imports should
be self-evident:

If I do:

```
migrate :: IO ()
migrate = do
     dir <- dBDir
     createDirectoryIfMissing True dir
     forM_ (categories, expenses)
        $ withDB . createTable
```

tables are not actually created.

However, if I do:

```
migrate :: IO ()
migrate = do
     dir <- dBDir
     createDirectoryIfMissing True dir
     withDB . createTable $ categories
     withDB . createTable $ expenses
```

Both tables are actually created.

So it seems like the thugs created with `formM_` are actually never
executed. Is it so? I'm new with Haskell and it seems strange for me. If
it is so, doesn't make it useless `forM_` for IO?

Furthermore, I can't reproduce it in ghci. Doing this:

```
["a", "b"] `forM_` print
```

Actually prints both `"a"` and `"b"`.

Thanks for any enlightment.

Marc Busqué
http://waiting-for-dev.github.io/about/

From ky3 at atamo.com  Fri Apr 13 10:53:28 2018
From: ky3 at atamo.com (Kim-Ee Yeoh)
Date: Fri, 13 Apr 2018 17:53:28 +0700
Subject: [Haskell-beginners] Lazyness and forM_
In-Reply-To: <alpine.LNX.2.21.1804131127290.6944@localhost>
References: <alpine.LNX.2.21.1804131127290.6944@localhost>
Message-ID: <CAPY+ZdRr72uyTju7wduJd=HAtrswFpRsUmGKc1NZ-uEafTb1mw@mail.gmail.com>

Hi Marc, and welcome to the haskell beginners list.

```
> migrate :: IO ()
> migrate = do
>     dir <- dBDir
>     createDirectoryIfMissing True dir
>     forM_ (categories, expenses)
>        $ withDB . createTable
> ```
>
> tables are not actually created.
>
>
Well,

forM_ (categories, expenses) $ withDB . createTable

is equivalent to

withDB . createTable $ expenses.

So exactly one table is created.



> ```
> ["a", "b"] `forM_` print
> ```
>
> Actually prints both `"a"` and `"b"`.
>
>
Here the code uses square brackets--and so we have honest-to-goodness
lists--whereas the previous used parentheses.

See what happens with: ("a","b") `forM_` print.

Marc: Feel free to write to the haskell-cafe mailing list for questions
such as this. Fortuitously in this case, it turns out that your query
needed knowledge only about the forM* combinators and--ever since their ilk
was generalized--the known instances declared for the Traversable
constraint. As you explore the domain-specific package "selda" further, you
will find more people acquainted with the package over at the cafe than
here in beginners. Suffice to say, everyone here in this list is also in
cafe, which is also open to beginners questions.

p.s. Veterans would recognize this as ye olde controversy on the Foldable
instance for pairs introduced in GHC 7.10.x. The controversy still simmers
apparently because there isn't an instance for triples and higher tuples in
the latest and greatest GHC 8.4.1. We are at the mercy of this potentially
toe-stubbing absence of uniformity.

-- Kim-Ee
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180413/8addc080/attachment-0001.html>

From marc at lamarciana.com  Fri Apr 13 13:32:27 2018
From: marc at lamarciana.com (=?ISO-8859-15?Q?Marc_Busqu=E9?=)
Date: Fri, 13 Apr 2018 15:32:27 +0200 (CEST)
Subject: [Haskell-beginners] Lazyness and forM_
In-Reply-To: <CAPY+ZdRr72uyTju7wduJd=HAtrswFpRsUmGKc1NZ-uEafTb1mw@mail.gmail.com>
References: <alpine.LNX.2.21.1804131127290.6944@localhost>
 <CAPY+ZdRr72uyTju7wduJd=HAtrswFpRsUmGKc1NZ-uEafTb1mw@mail.gmail.com>
Message-ID: <alpine.LNX.2.21.1804131520060.7309@localhost>

On Fri, 13 Apr 2018, Kim-Ee Yeoh wrote:

> Well, 
> 
> forM_ (categories, expenses) $ withDB . createTable
> 
> is equivalent to 
> 
> withDB . createTable $ expenses.
> 
> So exactly one table is created.

Oops, you are right.

>       ```
>       ["a", "b"] `forM_` print
>       ```
>
>       Actually prints both `"a"` and `"b"`.
> 
> 
> Here the code uses square brackets--and so we have honest-to-goodness lists--whereas the previous used parentheses.
> 
> See what happens with: ("a","b") `forM_` print.

Now I feel embarrassed I overlooked that :)

I used a tuple instead of a list because in selda `Table` are type-safe.
So `categories` and `expenses` are different types and they can't go in
the same list.

So I guess, once the `forM_` mistery is gone, I can boil down my problem
to something very different. As it has nothing to do with the current
subject, I'll submit another question.

> Marc: Feel free to write to the haskell-cafe mailing list for questions such as this. Fortuitously in this case, it turns out that your query
> needed knowledge only about the forM* combinators and--ever since their ilk was generalized--the known instances declared for the Traversable
> constraint. As you explore the domain-specific package "selda" further, you will find more people acquainted with the package over at the cafe
> than here in beginners. Suffice to say, everyone here in this list is also in cafe, which is also open to beginners questions.

And following your recommendation I'll submit that question in the
haskell-cafe mailing list :)

Thank you very much for your help :)

Marc Busqué
http://waiting-for-dev.github.io/about/

From quentin.liu.0415 at gmail.com  Sat Apr 14 20:28:55 2018
From: quentin.liu.0415 at gmail.com (Quentin Liu)
Date: Sat, 14 Apr 2018 16:28:55 -0400
Subject: [Haskell-beginners] Efficient Binary Multiplication
Message-ID: <2cd382a0-b5be-4eed-a0c0-e25d2045031d@Spark>

Hi all,

Suppose I want to multiply two binary numbers whose representation uses lists (e.g. 14 is represented as [1, 1, 1, 0]). Is there any efficient way to do binary multiplication? The way I could come up with involves a lot of intermediate lists that will be discarded eventually and is extremely inefficient. I know one fast algorithm that uses Array but would it be possible to do the multiplication with only lists?

Regards,
Qingbo Liu
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180414/257fb37f/attachment.html>

From trupill at gmail.com  Mon Apr 16 13:26:20 2018
From: trupill at gmail.com (Alejandro Serrano Mena)
Date: Mon, 16 Apr 2018 15:26:20 +0200
Subject: [Haskell-beginners] Call for Participation: Summer School on
	Advanced Functional Programming
Message-ID: <CAHnFXOtM7v5tdEfr7c53wXDh+9FZRO5sAXH9fbUdJO8gnn3GeA@mail.gmail.com>

# Call for Participation

 SUMMER SCHOOL ON ADVANCED FUNCTIONAL PROGRAMMING

 Utrecht, the Netherlands, 27-31 August 2018

 http://www.afp.school

## ABOUT

The Advanced Functional Programming summer school has been running for
more than ten years. We aim to educate aspiring Haskell programmers
beyond the basic material covered by many textbooks.

The lectures will cover several more advanced topics regarding the
theory and practice of Haskell programming, including topics such as:

  * lambda calculus;
  * monads and monad transformers;
  * lazy evaluation;
  * generalized algebraic data types;
  * type families and type-level programming;
  * concurrency and parallelism.

The summer school consists of a mix of lectures, labs, and a busy
social program.

## LECTURERS

Utrecht staff:
  * Johan Jeuring
  * Alejandro Serrano Mena
  * Doaitse Swierstra
  * Wouter Swierstra

Guest lectures:
  * Manuel Chakravarty
  * Koen Claessen
  * Gabriele Keller


## PREREQUISITES

We expect students to have a basic familiarity with Haskell
already. You should be able to write recursive functions over
algebraic data types, such as lists and trees. There is a great deal
of material readily available that covers this material. If you’ve
already started learning Haskell and are looking to take your
functional programming skills to the next level, this is the course
for you.

## DATES

Registration deadline: 1 August, 2017
School:                27-31 August

## COSTS

€1700 - Housing and registration
€1500 - Registration only

We offer a €1000 discount for students and staff members affiliated with
a university.

## SCHOLARSHIPS

If you’re struggling to finance your trip to Utrecht, please let us
know. We have a limited number of scholarships or discounts available
for students that would not be able to attend otherwise, especially
for women and under represented minorities.

## FURTHER INFORMATION

Further information, including instructions on how to register, is
available on our website:

                        http://www.afp.school
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180416/b6a88a6f/attachment.html>

From leiva.steven at gmail.com  Tue Apr 17 00:48:40 2018
From: leiva.steven at gmail.com (Steven Leiva)
Date: Mon, 16 Apr 2018 20:48:40 -0400
Subject: [Haskell-beginners] Efficient Binary Multiplication
In-Reply-To: <2cd382a0-b5be-4eed-a0c0-e25d2045031d@Spark>
References: <2cd382a0-b5be-4eed-a0c0-e25d2045031d@Spark>
Message-ID: <CA+=woS7=H4mc4Bj-r6B51Z_S37xwpndjeDrZKdd9+uyzQXAk3A@mail.gmail.com>

Interesting question.

So there is a concept in computer science called *fusion*.

My understanding (I don't have a CS degree) is that when data is subject to
fusion, we don't create the intermediary lists as you are talking about.
This is why, in Haskell, we have the *ByteString* and *Text* datatypes.
Unlike *String*, they (ByteString / Text) are subject to fusion. *Text* is
useful when we are dealing with unicode data, while *ByteString* is the
correct tool when we are dealing with binary data.

All of this is to say that you should look at the *ByteString* data type.
(Take a look at this description, for example:
https://hackage.haskell.org/package/bytestring-0.10.8.2/docs/Data-ByteString.html#t:ByteString).


The only potential problem I see is that *ByteString* internally uses
*Word8*, and since *Word8* is *unsigned*, negatives are problematic. (Now
that I think about it, how the heck do you represent negative numbers in
binary?).

I hope this is useful.

On Sat, Apr 14, 2018 at 4:28 PM, Quentin Liu <quentin.liu.0415 at gmail.com>
wrote:

> Hi all,
>
> Suppose I want to multiply two binary numbers whose representation uses
> lists (e.g. 14 is represented as [1, 1, 1, 0]). Is there any efficient way
> to do binary multiplication? The way I could come up with involves a lot of
> intermediate lists that will be discarded eventually and is extremely
> inefficient. I know one fast algorithm that uses Array but would it be
> possible to do the multiplication with only lists?
>
> Regards,
> Qingbo Liu
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>


-- 
Steven Leiva
305.528.6038
leiva.steven at gmail.com
http://www.linkedin.com/in/stevenleiva
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180416/79305639/attachment.html>

From frederic-emmanuel.picca at synchrotron-soleil.fr  Wed Apr 18 14:39:58 2018
From: frederic-emmanuel.picca at synchrotron-soleil.fr (PICCA Frederic-Emmanuel)
Date: Wed, 18 Apr 2018 14:39:58 +0000
Subject: [Haskell-beginners] phantom type
In-Reply-To: <CAJoPsuCxADbprC+JcCG54P1_-H9tSqcHS-2vzSL4ey_BswcJ2g@mail.gmail.com>
References: <A2A20EC3B8560D408356CAC2FC148E530107DD506A@SUN-DAG3.synchrotron-soleil.fr>
 <20180331084011.5ixszt3pboaovr4i@x60s.casa>,
 <CAJoPsuCxADbprC+JcCG54P1_-H9tSqcHS-2vzSL4ey_BswcJ2g@mail.gmail.com>
Message-ID: <A2A20EC3B8560D408356CAC2FC148E530107DED50A@SUN-DAG4.synchrotron-soleil.fr>

> Directly relevant blog: https://www.benjamin.pizza/posts/2017-12-15-functor-functors.html

thanks a lot very interesting post :)

From frederic-emmanuel.picca at synchrotron-soleil.fr  Wed Apr 18 14:41:39 2018
From: frederic-emmanuel.picca at synchrotron-soleil.fr (PICCA Frederic-Emmanuel)
Date: Wed, 18 Apr 2018 14:41:39 +0000
Subject: [Haskell-beginners] phantom type
In-Reply-To: <20180331084011.5ixszt3pboaovr4i@x60s.casa>
References: <A2A20EC3B8560D408356CAC2FC148E530107DD506A@SUN-DAG3.synchrotron-soleil.fr>,
 <20180331084011.5ixszt3pboaovr4i@x60s.casa>
Message-ID: <A2A20EC3B8560D408356CAC2FC148E530107DED52A@SUN-DAG4.synchrotron-soleil.fr>

Hello

>     I am in a rush so I cant write a minimal example, but wouldn't
> a typeclass + make Collect and Cara instances of that typeclass do?

Yes I can do this in my case and it is not thaht difficult :).

thanks for your help.

cheers

Frederic

From stephen.tetley at gmail.com  Wed Apr 18 17:33:12 2018
From: stephen.tetley at gmail.com (Stephen Tetley)
Date: Wed, 18 Apr 2018 18:33:12 +0100
Subject: [Haskell-beginners] Efficient Binary Multiplication
In-Reply-To: <CA+=woS7=H4mc4Bj-r6B51Z_S37xwpndjeDrZKdd9+uyzQXAk3A@mail.gmail.com>
References: <2cd382a0-b5be-4eed-a0c0-e25d2045031d@Spark>
 <CA+=woS7=H4mc4Bj-r6B51Z_S37xwpndjeDrZKdd9+uyzQXAk3A@mail.gmail.com>
Message-ID: <CAB2TPRB_VrjCThcvSyVE-toFdUhi_tspTMpzN=TB3jE=Ss4STQ@mail.gmail.com>

List fusion has existed for a long time (based on a change to the
internal representation of lists to make them streams).

IIRC the actual implementation was not considered a satisfactory
replacement for GHCs existing list implementation as performance
improvements were not uniform, the stream representation is good for
some things worse for others. I don't know whether this situation has
now changed.

Byte String and Text exist because implementing strings or byte
sequences as a cons list of Char or Byte is very inefficient both for
storage / data locality and for many operations. Fusion for Byte
String and Text is a bonus rather than a motivation.


[*] Stream Fusion: From Lists to Streams to Nothing at All
Duncan Coutts , Roman Leshchinskiy , Don Stewart
http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.104.7401

On 17 April 2018 at 01:48, Steven Leiva <leiva.steven at gmail.com> wrote:
> Interesting question.
>
> So there is a concept in computer science called fusion.
>
> My understanding (I don't have a CS degree) is that when data is subject to
> fusion, we don't create the intermediary lists as you are talking about.
> This is why, in Haskell, we have the ByteString and Text datatypes. Unlike
> String, they (ByteString / Text) are subject to fusion. Text is useful when
> we are dealing with unicode data, while ByteString is the correct tool when
> we are dealing with binary data.
>
> All of this is to say that you should look at the ByteString data type.
> (Take a look at this description, for example:
> https://hackage.haskell.org/package/bytestring-0.10.8.2/docs/Data-ByteString.html#t:ByteString).
>
> The only potential problem I see is that ByteString internally uses Word8,
> and since Word8 is unsigned, negatives are problematic. (Now that I think
> about it, how the heck do you represent negative numbers in binary?).
>
> I hope this is useful.
>
> On Sat, Apr 14, 2018 at 4:28 PM, Quentin Liu <quentin.liu.0415 at gmail.com>
> wrote:
>>
>> Hi all,
>>
>> Suppose I want to multiply two binary numbers whose representation uses
>> lists (e.g. 14 is represented as [1, 1, 1, 0]). Is there any efficient way
>> to do binary multiplication? The way I could come up with involves a lot of
>> intermediate lists that will be discarded eventually and is extremely
>> inefficient. I know one fast algorithm that uses Array but would it be
>> possible to do the multiplication with only lists?
>>
>> Regards,
>> Qingbo Liu
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
>
>
> --
> Steven Leiva
> 305.528.6038
> leiva.steven at gmail.com
> http://www.linkedin.com/in/stevenleiva
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>

From kc1956 at gmail.com  Wed Apr 18 19:07:06 2018
From: kc1956 at gmail.com (KC)
Date: Wed, 18 Apr 2018 19:07:06 +0000
Subject: [Haskell-beginners] Efficient Binary Multiplication
In-Reply-To: <2cd382a0-b5be-4eed-a0c0-e25d2045031d@Spark>
References: <2cd382a0-b5be-4eed-a0c0-e25d2045031d@Spark>
Message-ID: <CAMLKXy=A-4LONGfoYFrOgGszyhk2m5rpOibWeYUQLAWfBUcBbw@mail.gmail.com>

Hi

Must you use only lists?

Must each node only contain one digit?

--
Sent from an expensive device which will be obsolete in a few months
Casey

On Sat, Apr 14, 2018, 1:30 PM Quentin Liu, <quentin.liu.0415 at gmail.com>
wrote:

> Hi all,
>
> Suppose I want to multiply two binary numbers whose representation uses
> lists (e.g. 14 is represented as [1, 1, 1, 0]). Is there any efficient way
> to do binary multiplication? The way I could come up with involves a lot of
> intermediate lists that will be discarded eventually and is extremely
> inefficient. I know one fast algorithm that uses Array but would it be
> possible to do the multiplication with only lists?
>
> Regards,
> Qingbo Liu
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180418/a05313fd/attachment.html>

From leiva.steven at gmail.com  Sat Apr 28 16:08:22 2018
From: leiva.steven at gmail.com (Steven Leiva)
Date: Sat, 28 Apr 2018 11:08:22 -0500
Subject: [Haskell-beginners] Efficient Binary Multiplication
In-Reply-To: <CAB2TPRB_VrjCThcvSyVE-toFdUhi_tspTMpzN=TB3jE=Ss4STQ@mail.gmail.com>
References: <2cd382a0-b5be-4eed-a0c0-e25d2045031d@Spark>
 <CA+=woS7=H4mc4Bj-r6B51Z_S37xwpndjeDrZKdd9+uyzQXAk3A@mail.gmail.com>
 <CAB2TPRB_VrjCThcvSyVE-toFdUhi_tspTMpzN=TB3jE=Ss4STQ@mail.gmail.com>
Message-ID: <CA+=woS5xTmROZj1a=awAWSv3Ka_J3q92R=KnrS5ig8KKWMtAdg@mail.gmail.com>

Thank you Stephen for the clarification!

On Wed, Apr 18, 2018 at 12:33 PM, Stephen Tetley <stephen.tetley at gmail.com>
wrote:

> List fusion has existed for a long time (based on a change to the
> internal representation of lists to make them streams).
>
> IIRC the actual implementation was not considered a satisfactory
> replacement for GHCs existing list implementation as performance
> improvements were not uniform, the stream representation is good for
> some things worse for others. I don't know whether this situation has
> now changed.
>
> Byte String and Text exist because implementing strings or byte
> sequences as a cons list of Char or Byte is very inefficient both for
> storage / data locality and for many operations. Fusion for Byte
> String and Text is a bonus rather than a motivation.
>
>
> [*] Stream Fusion: From Lists to Streams to Nothing at All
> Duncan Coutts , Roman Leshchinskiy , Don Stewart
> http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.104.7401
>
> On 17 April 2018 at 01:48, Steven Leiva <leiva.steven at gmail.com> wrote:
> > Interesting question.
> >
> > So there is a concept in computer science called fusion.
> >
> > My understanding (I don't have a CS degree) is that when data is subject
> to
> > fusion, we don't create the intermediary lists as you are talking about.
> > This is why, in Haskell, we have the ByteString and Text datatypes.
> Unlike
> > String, they (ByteString / Text) are subject to fusion. Text is useful
> when
> > we are dealing with unicode data, while ByteString is the correct tool
> when
> > we are dealing with binary data.
> >
> > All of this is to say that you should look at the ByteString data type.
> > (Take a look at this description, for example:
> > https://hackage.haskell.org/package/bytestring-0.10.8.2/
> docs/Data-ByteString.html#t:ByteString).
> >
> > The only potential problem I see is that ByteString internally uses
> Word8,
> > and since Word8 is unsigned, negatives are problematic. (Now that I think
> > about it, how the heck do you represent negative numbers in binary?).
> >
> > I hope this is useful.
> >
> > On Sat, Apr 14, 2018 at 4:28 PM, Quentin Liu <quentin.liu.0415 at gmail.com
> >
> > wrote:
> >>
> >> Hi all,
> >>
> >> Suppose I want to multiply two binary numbers whose representation uses
> >> lists (e.g. 14 is represented as [1, 1, 1, 0]). Is there any efficient
> way
> >> to do binary multiplication? The way I could come up with involves a
> lot of
> >> intermediate lists that will be discarded eventually and is extremely
> >> inefficient. I know one fast algorithm that uses Array but would it be
> >> possible to do the multiplication with only lists?
> >>
> >> Regards,
> >> Qingbo Liu
> >>
> >> _______________________________________________
> >> Beginners mailing list
> >> Beginners at haskell.org
> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >>
> >
> >
> >
> > --
> > Steven Leiva
> > 305.528.6038
> > leiva.steven at gmail.com
> > http://www.linkedin.com/in/stevenleiva
> >
> > _______________________________________________
> > 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
>



-- 
Steven Leiva
305.528.6038
leiva.steven at gmail.com
http://www.linkedin.com/in/stevenleiva
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20180428/f5ecf601/attachment.html>