Hi,
you'll need to loosen up your data dependencies a little.
Consider the following expression,
let action = a1 >> a2 >> ... >> an in runI action
'runI' forces the evaluation of an IState, which according
to your (>>=) defn, action's IState will be the one that 'an'
evaluates to/returns.
Hence, in your particular example, you won't see any output
until 'an' has been evaluated ==> no output will appear until
the end of the input has been seen.
One way out is to define (>>=) as follows:
(I c1) >>= fc2 = I $ \s0 ->
let (r,s1) = c1 s0
(I c2) = fc2 r
(r2, s2) = c2 (s1{io=return ()})
in
(r2, IState { io = io s1 >> io s2})
or, redefine your monad to distinguish between output &
the (threaded) interaction state.
hth
--sigbjorn
> -----Original Message-----
> From: Sengan [mailto:[EMAIL PROTECTED]]
> Sent: Thursday, August 10, 2000 16:54
> To: [EMAIL PROTECTED]
> Subject: Why is the following not lazy?
>
>
> I'm writing a program that involves the use of an interaction
> Monad. However it seems to be insufficiently lazy to provide
> any interaction. The following code is derived from my original
> but highly simplified. Why is "works" lazy, but "main" is not?
>
> Thanks for any help
>
> Sengan
>
> > module Main(main) where
> > import IO
>
> --------------------------------------------------------------
> ------------------
>
> > works
> > = hGetContents stdin >>= \input ->
> > foldr (>>) (return ()) $ map putStr $ lines input
>
> --------------------------------------------------------------
> ------------------
>
> > main
> > = hGetContents stdin >>= \input ->
> > runI $ foldr (>>) (return "") $ map output $ lines input
>
> > data IState = IState { io :: IO () }
>
> > data I a = I (IState -> (a, IState))
>
> > instance Monad I where
> > return k = I $ \s -> (k,s)
> > (I c1) >>= fc2 = I $ \s0 -> let (r,s1) = c1 s0
> > I c2 = fc2 r in
> > c2 s1
>
> > runI (I c) = let (result, state) = c (IState $ return ()) in
> > io state >> return ()
>
> > output string
> > = I $ \s -> (string, s { io = (io s) >> putStr (string ++ "\n") })
>
