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

[Maxime Henrion]

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