[Haskell-cafe] Re: GHC 6.7 and Associated Types

17 Apr 2007

      Maxime Henrion wrote:
...
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'
Ah, oh, I didn't even check whether the types in the class are good. I'm
not sure, but don't you want

  class MonadState m where
     type StateType m :: *
     get              :: m (StateType m)
     put              :: StateType m -> m ()

? Then, the substitutions m = State s and StateType (State s) = s yields
the expected types for put and get:

  get :: (State s) s
  put :: s -> (State s) ()

Regards,
apfelmus