Hi all, I'm trying to figure out how to use STUArray. Is it possible to let it be polymorphic? Here is an excerpt from my program: class (IArray UArray k, Ord k, Fractional k) => Elt k class (Bounded a, Enum a, Ix a, Eq a, Show a) => IxB a -- |like Array but uses IxB instead of Ix newtype Vector k i = Vector (UArray.UArray i k) deriving (Eq) margin :: (IxB a, IxB b, Elt k, (MArray (STUArray s) k (ST s))) => Vector k a -> (a -> b) -> Vector k b margin (Vector v) f = Vector (runSTUArray (do ax <- newArray (minBound, maxBound) 0 mapM_ (\ (i,x) -> updateArray ax (f i) (+x)) (assocs v) return ax )) where updateArray ax i f = do e <- readArray ax i writeArray ax i (f e) When I try to run it, I get the following error. I think I understand what the error means, but not how to fix the problem: Vector2.hs:166:22: No instance for (MArray (STUArray s) k (ST s)) arising from use of `updateArray' at Vector2.hs:166:22-32 Probable fix: add (MArray (STUArray s) k (ST s)) to the expected type of an expression or add an instance declaration for (MArray (STUArray s) k (ST s)) In a lambda abstraction: \ (i, x) -> updateArray ax (f i) ((+ x)) In the first argument of `mapM_', namely `(\ (i, x) -> updateArray ax (f i) ((+ x)))' In a 'do' expression: mapM_ (\ (i, x) -> updateArray ax (f i) ((+ x))) (assocs v) Thanks in advance, Frederik -- http://ofb.net/~frederik/