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.
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