[Haskell-cafe] Re: Where do I put the seq?

Wed Aug 19 12:28:16 EDT 2009

Expect more bugs with this though :-) Just found out that looping does not
work, it hangs, e.g.

test = do  out "Enter your first name:"  fstName <- inp  out "Enter
your second name:"  sndName <- inp  out ("Welcome "++fstName++"
"++sndName)  out "Goodbye!"*  **test*

Doesn't seem to work :-) Back to the drawing board.

On Wed, Aug 19, 2009 at 5:55 PM, Peter Verswyvelen <bugfact at gmail.com>wrote:

> Not at all, use it for whatever you want to :-)
> I'm writing this code because I'm preparing to write a bunch of tutorials
> on FRP, and I first wanted to start with simple console based FRP, e.g.
> making a little text adventure game, where the input/choices of the user
> might be parsed ala parsec, using monadic style, applicative style, and
> arrows, and then doing the same with FRP frameworks like Yampa, Elera,
> Reactive, etc...
>
> After that I would start writing tutorials that use OpenGL, making some
> very simple games, again with the above approaches, and ending with a
> conversion of a very old game of mine (Zarathrusta written in assembler from
> 1991, which was based on Thrust<http://www.bytecellar.com/archives/000106.php>from 1986, converted by myself in C++ to PocketPC as
> G-Pod<http://www.handango.com/catalog/ProductDetails.jsp?productId=16574&platformId=30>,
> and so I would like to make a version in Haskell that runs on the iPhone :-)
>
> This of course is a lot of work, and I would like to put this on the
> Haskell wiki or a blog or something, so others can contribute and comment. I
> would like to show real examples that explain the shortcomings of the FRP
> approaches, because now this is still a bit blurry to me.
>
>
>
> On Wed, Aug 19, 2009 at 5:43 PM, David Leimbach <leimy2k at gmail.com> wrote:
>
>> This Monad you've created is quite excellent.  I was trying to do
>> something like this about a year ago, to make the input and output handling
>> of an interactive bowling score card work nicely.  I kept running into
>> issues, and did not believe that seq was going to do the trick.  Nice work!
>> This is a very useful monad I think, it could be called "Prompter" or
>> something to that effect.
>>
>> Do you mind if I use it in some of my code?
>>
>> Dave
>>
>>
>> On Wed, Aug 19, 2009 at 8:42 AM, Peter Verswyvelen <bugfact at gmail.com>wrote:
>>
>>> LOL. Maybe we should have that coffee together ;-) at least virtually!
>>>
>>> On Wed, Aug 19, 2009 at 5:39 PM, David Leimbach <leimy2k at gmail.com>wrote:
>>>
>>>> Argh... I too have been up too late :-).  I edited THE WRONG FILE!  No
>>>> wonder your change didn't take effect!  :-/
>>>> Time for coffee I suppose.
>>>>
>>>>
>>>> On Wed, Aug 19, 2009 at 8:38 AM, David Leimbach <leimy2k at gmail.com>wrote:
>>>>
>>>>> This doesn't seem to be working for me interactively though on a Mac.
>>>>>  I still get "Welcome" before I've entered text.
>>>>>
>>>>>
>>>>> On Wed, Aug 19, 2009 at 8:25 AM, Peter Verswyvelen <bugfact at gmail.com>wrote:
>>>>>
>>>>>> I fixed it myself but it's really tricky :-)
>>>>>> http://hpaste.org/fastcgi/hpaste.fcgi/view?id=8316#a8330
>>>>>>
>>>>>> <http://hpaste.org/fastcgi/hpaste.fcgi/view?id=8316#a8330>The idea
>>>>>> is, that when the input is requested, the output that is then generated must
>>>>>> be in sync with the input.
>>>>>>
>>>>>> inp = S $ \s i -> let r = (*s** **`**D**.**append**`** **(**i** **`**seq**`** **D**.**empty**)*, head i) in (tail i, r)
>>>>>>
>>>>>>
>>>>>>
>>>>>> I first had
>>>>>>
>>>>>> inp = S $ \s i -> let r = (i `seq` *s*, head i) in (tail i, r)
>>>>>>
>>>>>>
>>>>>> But that was too eager, since i syncs the input not with the output,
>>>>>> but with the function that will generate the output.
>>>>>>
>>>>>> Okay, now I can sleep again :-)
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Wed, Aug 19, 2009 at 5:12 PM, Peter Verswyvelen <bugfact at gmail.com
>>>>>> > wrote:
>>>>>>
>>>>>>> Thanks, but that doesn't really matter in my example, my code is just
>>>>>>> buggy, and I'm not sure why. For example if I change my test function so
>>>>>>> that it outputs lines only, then it still prints Welcome first before asking
>>>>>>> for input.
>>>>>>> See e.g. http://hpaste.org/fastcgi/hpaste.fcgi/view?id=8316#a8328
>>>>>>>
>>>>>>> On Wed, Aug 19, 2009 at 5:00 PM, David Leimbach <leimy2k at gmail.com>wrote:
>>>>>>>
>>>>>>>> Try LineBuffering.
>>>>>>>> I do linewise stuff with interact a lot.  You'll find stuff like
>>>>>>>>
>>>>>>>> unlines . lines
>>>>>>>>
>>>>>>>> may help too.  In fact I just wrote a blog post about this.
>>>>>>>>
>>>>>>>> http://leimy9.blogspot.com
>>>>>>>>
>>>>>>>> I'm trying to write some interactive code to automate working with
>>>>>>>> serial console controlled power strips, so I need to either use Expect
>>>>>>>> (yuck) or do my own thing.
>>>>>>>>
>>>>>>>> Dave
>>>>>>>>
>>>>>>>> On Wed, Aug 19, 2009 at 7:35 AM, Peter Verswyvelen <
>>>>>>>> bugfact at gmail.com> wrote:
>>>>>>>>
>>>>>>>>> Apparently this particular example happens to work on Mac and Linux
>>>>>>>>> because of different buffering (thanks Martijn for the help!)
>>>>>>>>> To make sure we have no buffering at all, the main function should
>>>>>>>>> be:
>>>>>>>>>
>>>>>>>>> main = do  hSetBuffering stdout NoBuffering  hSetBuffering stdin NoBuffering  test
>>>>>>>>>
>>>>>>>>> Now I think it should also be *incorrect* on Unix systems.
>>>>>>>>>
>>>>>>>>> I guess the way I'm concatenating the strings is not correct, not
>>>>>>>>> sure.
>>>>>>>>>
>>>>>>>>> I would like to use a graphical tool to show the graph reduction
>>>>>>>>> step by step, to get a better understanding of the laziness & strictness.
>>>>>>>>> Does such a tool exist? I know people often say this is not usable because
>>>>>>>>> the amount of information is too much, but I used to be an assembly language
>>>>>>>>> programmer so I still would like to give it a try :-)
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Wed, Aug 19, 2009 at 1:07 PM, Peter Verswyvelen <
>>>>>>>>> bugfact at gmail.com> wrote:
>>>>>>>>>
>>>>>>>>>> In an attempt to get a deeper understanding of several monads
>>>>>>>>>> (State, ST, IO, ...) I skimmed over some of the research papers (but didn't
>>>>>>>>>> understand all of it, I lack the required education) and decided to write a
>>>>>>>>>> little program myself without using any prefab monad instances that should
>>>>>>>>>> mimic the following:
>>>>>>>>>> main = do
>>>>>>>>>>   putStrLn "Enter your name:"
>>>>>>>>>>   x <- getLine
>>>>>>>>>>   putStr "Welcome "
>>>>>>>>>>   putStrLn x
>>>>>>>>>>   putStrLn "Goodbye!"
>>>>>>>>>>
>>>>>>>>>> But instead of using IO, I wanted to make my own pure monad that
>>>>>>>>>> gets evaluated with interact, and does the same.
>>>>>>>>>>
>>>>>>>>>> However, I get the following output:
>>>>>>>>>>
>>>>>>>>>> Enter your name:
>>>>>>>>>> Welcome ......
>>>>>>>>>>
>>>>>>>>>> So the Welcome is printed too soon.
>>>>>>>>>>
>>>>>>>>>> This is obvious since my monad is lazy, so I tried to put a seq at
>>>>>>>>>> some strategic places to get the same behavior as IO. But I completely
>>>>>>>>>> failed doing so, either the program doesn't print anything and asks input
>>>>>>>>>> first, or it still prints too much output.
>>>>>>>>>>
>>>>>>>>>> Of course I could just use ST, State, transformers, etc, but this
>>>>>>>>>> is purely an exercise I'm doing.
>>>>>>>>>>
>>>>>>>>>> So, I could re-read all papers and look in detail at all the code,
>>>>>>>>>> but maybe someone could help me out where to put the seq or what to do :-)
>>>>>>>>>>
>>>>>>>>>> The code is at http://hpaste.org/fastcgi/hpaste.fcgi/view?id=8316
>>>>>>>>>>
>>>>>>>>>> Oh btw, the usage of DList here might not be needed; intuitively
>>>>>>>>>> it felt like the correct thing to do, but when it comes to Haskell, my
>>>>>>>>>> intuition is usually wrong ;-)
>>>>>>>>>>
>>>>>>>>>> Thanks a lot,
>>>>>>>>>> Peter Verswyvelen
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> Haskell-Cafe mailing list
>>>>>>>>> Haskell-Cafe at haskell.org
>>>>>>>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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