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 _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
