Re: [Haskell-cafe] Lazy Evaluation in Monads

31 May 2011


      On Tue, May 31, 2011 at 6:10 PM, Antoine Latter  wrote:
...
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)
Let's pretend I did that right:
...
toList :: Int -> IOStream a -> IO [a]
toList 0 _ = return []
toList 1 (x,_) = return [x]
toList n (x,r) = do
  rest <- r
  xs <- toList (n-1) rest
  return (x:xs)
Antoine