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

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#a8330The 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 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 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 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 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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