On Tue, May 31, 2011 at 2:49 PM, Scott Lawrence <byt...@gmail.com> wrote: > 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. >
You could use a different type: > type IOStream a = (a, IO (IOStream a)) > unfold :: ([a] -> IO a) -> IO (IOStream a) > unfold f = > let go prev = do > next <- f prev > return (next, go (next:prev)) > in do > z <- f [] > go [z] > toList :: Int -> IOStream a -> IO [a] > toList 0 _ = return [] > toList n (x,rest) = do > xs <- toList (n-1) rest > return (x:xs) Antoine _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
