Hello guys, I've been documenting myself on associated types, which look like a very nice way to deal with the problems that arise with multi-parameter type classes. As an exercise, I am trying to rewrite the MonadState type class from the mtl package without functional dependencies. Here is my (probably very naive) approach : class MonadState m where type StateType m :: * get :: m StateType put :: m StateType -> m () As for instances: instance MonadState (State s) where type StateType = s -- this is line 22 get = State $ \s -> (s, s) put s = State $ \_ -> ((), s) I think I'm probably doing some very stupid thing and missing important points. In any case, here's the error I get : State.hs:22:19: Not in scope: type variable `s' Failed, modules loaded: none. I'd be happy to be explained why this doesn't make sense, and how I should proceed to implement this correctly. I have tried various other approaches with no luck yet. Thanks, Maxime