I'm have some trouble using the ST monad, and I think I'm confused about its use of existential type.
{-# OPTIONS -XRankNTypes #-} import Control.Monad.ST import Data.Array.ST
I want to implement a map function that unfold all ST monads in a list:
mapST :: (a -> (forall s . ST s b)) -> [a] -> [b] mapST f (x:xs) = runST (f x) : mapST f xs mapST f [] = []
This is fine. However, it wouldn't typecheck if I had declared its type differently: mapST :: (a -> ST s b) -> [a] -> [b] I first thought the reason could be that runST has to take something of type (forall s . ST s b), but apparently this is not the case:
g :: (STArray s Int Int -> ST s a) -> ST s a g f = newArray (0,0) 0 >>= f
with this definition, runST (t (flip readArray 0)) happily returns me 0, despite the fact that the "s" in the type of "g" is not existentially quantified. Then when I try this one: mapST g [flip readArray 0] It fails to typecheck. Why? Is it possible, if at all, to implement a generic mapST? -- Regards, Paul Liu Yale Haskell Group http://www.haskell.org/yale