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

[Simon Peyton-Jones]

Associated *data* types should work in the HEAD (=6.7).  But associated *type synonyms* do not, I'm afraid. We are actively working on it, but it'll be a couple of months at least I guess.

You can see the state of play, and description of where we are up to here
        http://hackage.haskell.org/trac/ghc/wiki/TypeFunctions

Simon

| -----Original Message-----
| From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Maxime
| Henrion
| Sent: 17 April 2007 13:27
| To: apfelmus
| Cc: haskell-cafe at haskell.org
| Subject: Re: [Haskell-cafe] Re: GHC 6.7 and Associated Types
|
| 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
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