Query regarding GHC handling of overlapping instances.

[Keean Schupke]

I have been doing some work recently which involves classes with 
overlapping instances... for example

class Test x y where
    test :: x -> y

instance Test (a b) (c b) where
    test =

instance Test (a b) (a b) where
    test =

This gives an overlapping instance error - which cannot be avoided with 
-fallow-overlapping-instances.
However - it is fairly obvious that the first case 'a' cannot be unified 
with 'c' or it would be a type error, therefore
the cases do not overlap... Is this a bug in ghc, is it easily fixable - 
or am I confused?


This leads on to my second point: A request for a new feature (if it 
cannot be done with existing features). I would like
to be able to specify class non-membership. Say we have orthogonal 
classes A and B, I can see situations where definitions like the 
following would be useful:

instance (A a,B a) => C a ...

instance (A a, (not B) a) => C a ...

perhaps the complete set of boolean operators would make sense for 
completeness allowing:

instance ((A a) and ((B a) or (C a))) => D a ...

Obviously functions defined in classes B and C would not be usable in 
the function definitions of D because
we are uncertain of membership of B and C individually.


Regards,
    Keean Schupke.