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
runListT listChars2
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 do runListT listChars2 >>= (mapM_ putChar) because it first reads the input until 'q' is pressed. I.e. this produces the interaction: abcqabcq What I want is the interaction: aabbccqq Hope you understand what I mean. /M 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@therning.org Jabber: magnus.therning@gmail.com http://therning.org/magnus