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

[Maxime Henrion]

apfelmus wrote:
> 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) ()

Ah, I tried something like that too, and then I get errors in the
definition of the instance :

State.hs:23:19:
    Couldn't match expected type `StateType (State s)'
           against inferred type `s' (a rigid variable)
      `s' is bound by the instance declaration at State.hs:21:27
      Expected type: State s (StateType (State s))
      Inferred type: State s s
    In the expression: State $ (\ s -> (s, s))
    In the definition of `get': get = State $ (\ s -> (s, s))

State.hs:24:19:
    Couldn't match expected type `s' (a rigid variable)
           against inferred type `StateType (State s)'
      `s' is bound by the instance declaration at State.hs:21:27
      Expected type: State s ()
      Inferred type: State (StateType (State s)) ()
    In the expression: State $ (\ _ -> ((), s))
    In the definition of `put': put s = State $ (\ _ -> ((), s))

I would expect GHC to see that 'State s (StateType (State s))' is the
same as 'State s s', per the definition of StateType.  I'm not sure how
to express get differently so that it matches, and similarly for put.

If I write:

  get            = State $ \s -> (StateType (State s), s)

I get:

State.hs:23:34: Not in scope: data constructor `StateType'

Thanks,
Maxime