On Mon, Feb 02, 2009 at 06:03:15PM +0100, Josef Svenningsson wrote:
Hi Tyson,
I also needed something like this a while ago so I knocked up a really simple module and put it on hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/STMonadTrans
Warning! The STMonadTrans package uses State# nonlinearly, and as a result, can violate referential transparency:
import Control.Monad import Control.Monad.Trans import Control.Monad.ST.Trans
data Tree a = Leaf a | Branch (Tree a) (Tree a) deriving Show
instance Monad Tree where return = Leaf Leaf a >>= k = k a Branch l r >>= k = Branch (l >>= k) (r >>= k)
foo :: STT s Tree Integer foo = do x <- newSTRef 0 y <- lift (Branch (Leaf 1) (Leaf 2)) when (odd y) (writeSTRef x y) readSTRef x
main = do print $ runST foo let Branch _ (Leaf x) = runST foo print x
prints Branch (Leaf 1) (Leaf 1) 0 Evaluating the thunk in the left branch affects the value seen in the right branch. In general a monad transformer version of ST would need to duplicate its state for each branch when used in conjunction with a nondeterminism monad like Tree, which would make it not really different from State, I think. Regards, Reid