[Haskell-cafe] Reflection functional dependency as an associated type?

I was trying to make a bridge like this class MonadStateTF m where   type StateOf m :: *instance (MonadState s m) => MonadStateTF m where  type StateOF m = s to allow writing constraints on state monads without a dangling type parameter type NumMonad m = (MonadStateTF m, Num (StateOf m)) Unfortunately, the declaration type StateOF m = s complains that "s" isn't mentioned on the left hand side rather than somehow making use of the fundep. Brandon