GADTs and functional dependencies

[Wolfgang Jeltsch]

Am Dienstag, 23. September 2008 19:07 schrieben Sie:
> >> {-# LANGUAGE GADTs, MultiParamTypeClasses, FunctionalDependencies #-}
> >>
> >> data GADT a where
> >>
> >>     GADT :: GADT ()
> >>
> >> class Class a b | a -> b
> >>
> >> instance Class () ()
> >>
> >> fun :: (Class a b) => GADT a -> b
> >> fun GADT = ()
> >
> > I’d expect this to work but unfortunately, using GHC 6.8.2, it fails with
> > the
> >
> > following message:
> >> FDGADT.hs:12:11:
> >>     Couldn't match expected type `b' against inferred type `()'
> >>       `b' is a rigid type variable bound by
> >>           the type signature for `fun' at FDGADT.hs:11:16
> >>     In the expression: ()
> >>     In the definition of `fun': fun GADT = ()
> >
> > What’s the reason for this?  Is there a workaround?  Does this work in
> > 6.8.3 or 6.10.1?
>
> This similar code using type families compiles in 6.8.3 and 6.9:
>
> data GADT a where
>    GADT :: GADT ()
>
> type family F a
> type instance F () = ()
>
> fun :: GADT a -> F a
> fun GADT = ()

Exactly.  But this makes my code incompatible with GHC 6.6. :-(

I thought, someone said that with the new typing machinery in GHC 6.10, more 
functional dependency programs are accepted because functional dependencies 
are handled similarly to type families (or something like that).  Is this 
true?  Since the type family version is okay, why shouldn’t the functional 
dependency version be okay?

Best wishes,
Wolfgang