Hi, I'm having a problem using Typeable with STRefs. Basically, I want to store STRefs (among other things) in a universal type. STRef is an instance of Typeable2, which means that STRef s a is Typeable if s and a are both Typeable. The problem is that the state type s is opaque and I can see no way to make it Typeable (other than making it RealWorld, and I don't want to use IO for this). If this is the case, then AFAICT there is no point in having STRefs be instances of Typeable2. Am I missing something? Here's the code I'd like to write: import Data.Typeable import Data.STRef import Control.Monad.ST data Value = forall a . Typeable a => V a deriving Typeable getValue :: Typeable a => Value -> Maybe a getValue (V v) = cast v -- I need the Typeable s constraint for the code to compile, but I'd rather leave it out. test :: Typeable s => ST s Integer test = do ref <- newSTRef (10 :: Integer) let refVal = V ref case getValue refVal of Nothing -> error "BAD" Just r -> readSTRef r -- This doesn't compile, because s is not Typeable. test2 :: Integer test2 = runST test Thanks in advance, Mike