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

17 Apr 2007

      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