I'm trying to modify Richard Bird's state transformer. The example in his book (_Introduction_to_Functional_Programming_using_Haskell_) has State defined as a explicit type. I.e. Here's the relevant snippet: -- State transformer definition newtype St a = MkSt (State -> (a, State)) type State = Int -- State transformer applied to state apply :: St a -> State -> (a, State) apply (MkSt f) s = f s -- State monad instance Monad St where return x = MkSt f where f s = (x,s) p >>= q = MkSt f where f s = apply (q x) s' where (x, s') = apply p s ----------------------------------------- What I want is something like this, so that the state transformer has a generic state type: newtype St a s = MkSt (s -> (a, s)) apply :: St a s -> s -> (a, s) apply (MkSt f) s = f s instance Monad St where return x = MkSt f where f s = (x,s) p >>= q = MkSt f where f s = apply (q x) s' where (x, s') = apply p s ----------------------------------------------------------- The trouble occurs on the instance line Couldn't match `*' against `* -> *' Expected kind: (* -> *) -> * Inferred kind: (* -> * -> *) -> * When checking kinds in `Monad St' In the instance declaration for `Monad St' Failed, modules loaded: none.