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

[David Leimbach]

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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