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

[David Leimbach]

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
>>>>>
>>>>>
>>>>
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>>>>
>>>>
>>>
>>
>
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