Shawn P. Garbett writes: : | 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: (* -> * -> *) -> * Let's compare your declaration of St with the type signatures in class Monad. class Monad m where return :: a -> m a (>>=) :: m a -> (a -> m b) -> m b -- etc. If we instantiate m as St, we get a type of a -> St a for return, which lacks the state variable s. In turn, s corresponds to the third * in the inferred kind in the error message. Try partially applying St to its state variable, and declaring a Monad instance of that partial application, which will have the right kind *->*. Regards, Tom