Re: [Haskell-beginners] Iterating a monadic action with memoization

Brent Yorgey writes: [...]

How about this?

iterateM :: Monad m => (a -> m a) -> a -> m [a] iterateM f a = (a:) `liftM` (f a >>= iterateM f)

-Brent

The problem with this is that it is not "lazy" in the sense that inc :: Int -> IO Int inc x = (print x) >> return $! x + 1 main = do xs <- liftM (take 5) $ iterateM inc 0 print xs will never terminate. It will keep printing all natural numbers but it will never print the list xs. I don't quite understand why this is so, nor do I know how to rewrite iterateM to get the desired behavior. But I wish someone would enlighten me :-)