apfelmus wrote:
Maxime Henrion wrote:
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
When defining the type function StateType, you have to give it the required argument m = State s:
type StateType (State s) = s
get = State $ \s -> (s, s) put s = State $ \_ -> ((), s)
I tried that too already, it gives: State.hs:19:39: Kind mis-match Expected kind `k -> *', but `()' has kind `*' In the type `m ()' In the type `m StateType -> m ()' In the class declaration for `MonadState' Line 19 being the definition of put in the class. Cheers, Maxime