[Haskell-cafe] IO in lists
gale at sefer.org
Tue Jan 16 07:06:08 EST 2007
Magnus Therning wrote:
> Not a very descriptive subject, I know, but here's what I'd like to do.
> I can take getChar and create an infinate list:
> listChars = getChar : listChars
> but how do I go about creating a finite list, e.g. a list that ends as
> soon as 'q' is pressed?
> I was thinking of something like
> listChars1 = do
> c <- lift getChar
> if c == 'q'
> then [return c]
> else [return c] ++ listChars1
> However, that triggers an interesting behaviour in ghci:
<GHC's impolite choking noises deleted>
You are trying to define something of type [IO Char].
But the list monad  is not a transformer, so you can't
lift in it, even if the contained type happens also to be
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.
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