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@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 :-)


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