[Haskell-cafe] Lazy Evaluation in Monads
Scott Lawrence
bytbox at gmail.com
Tue May 31 21:49:02 CEST 2011
I was under the impression that operations performed in monads (in this
case, the IO monad) were lazy. (Certainly, every time I make the
opposite assumption, my code fails :P .) Which doesn't explain why the
following code fails to terminate:
iRecurse :: (Num a) => IO a
iRecurse = do
recurse <- iRecurse
return 1
main = (putStrLn . show) =<< iRecurse
Any pointers to a good explanation of when the IO monad is lazy?
=== The long story ===
I wrote a function unfold with type signature (([a] -> a) -> [a]), for
generating a list in which each element can be calculated from all of
the previous elements.
unfold :: ([a] -> a) -> [a]
unfold f = unfold1 f []
unfold1 :: ([a] -> a) -> [a] -> [a]
unfold1 f l = f l : unfold1 f (f l : l)
Now I'm attempting to do the same thing, except where f returns a monad.
(My use case is when f randomly selects the next element, i.e. text
generation from markov chains.) So I want
unfoldM1 :: (Monad m) => ([a] -> m a) -> [a] -> m [a]
My instinct, then, would be to do something like:
unfoldM1 f l = do
next <- f l
rest <- unfoldM1 f (next : l)
return (next : rest)
But that, like iRecurse above, doesn't work.
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