Ambiguous uses of multi-param type classes are sometimes silently turned
into Void instances, instead of giving an error message.
Take this class declaration (please!)
> class Collection c a where
> empty :: c a
> add :: a -> c a -> c a
> isempty :: c a -> Bool
> singleton x = add x empty
If I try to compile the following ambiguous program, I get the expected
error message. (Yes I know it's a peculiar way to write `False' ;-)
> q = isempty . singleton
% ghc -fglasgow-exts -c t.hs
Cannot resolve the ambiguous context (Collection aun aum)
`Collection aun aum' arising from use of `isempty' at t.hs:10
However, if I try to compile the following, it works, much to my surprise.
> q' x = isempty (singleton x)
Here's the .hi line:
1 q' _:_ _forall_ [a] {Collection PrelGHC.Void a} => a -> PrelBase.Bool ;;
If I remove `a' as a parameter of the class, I get the error message
in both cases.
--Jeff
