[Haskell-cafe] IO in lists
magnus at therning.org
Tue Jan 16 09:13:41 EST 2007
On Tue, Jan 16, 2007 at 14:06:08 +0200, Yitzchak Gale wrote:
>But the list monad  is not a transformer, so you can't lift in it,
>even if the contained type happens also to be a monad.
Yeah, I had some vague thought of that being a problem, which lead me to
ListT. Your statement put some good words to that thought.
>Perhaps you are looking for something like this, using the monad
>transformer version of :
>listChars2 :: ListT IO Char
>listChars2 = do
> c <- lift getChar
> if c == 'q'
> then return [c]
> else return [c] `mplus` listChars2
>GHC finds this much more tasty, and then
>does what I think you may want.
Yes, that is fairly close to what I want to do. Now, it'd be really
good if I could apply a function to each item in the ListT, with the
constraint that it should be done lazily. I.e. the following will not
runListT listChars2 >>= (mapM_ putChar)
because it first reads the input until 'q' is pressed. I.e. this
produces the interaction:
What I want is the interaction:
Hope you understand what I mean.
P.S. I'm aware a reqursive solution is possible and I've already coded
one that works. I'm just curious if a "map solution" is possible.
Magnus Therning (OpenPGP: 0xAB4DFBA4)
magnus at therning.org Jabber: magnus.therning at gmail.com
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