This is not an error in the compilers! In this example, it is the
context of the derived Eq operator that is the problem. In the
expression NilSet == NilSet, the typing of == is
(==) :: Eq a => a -> a -> Bool
This `a' is instantiated by Set b (to choose a different type
variable), not Eq b => Set b, resulting in a typing of
Eq(Set b) => Set b -> Set b -> Bool
for your expression. Since the instance for Eq derived by the
compiler for Set is:
instance Eq a => Eq (Set a) where ...
context reduction will yield a final type of
Eq b => Set b -> Set b -> Bool
The ambiguous Eq b comes from the instance for Eq, not the data
constructor NilSet. This sort of ambiguity problem is not solved by
the restriction in the report in constructor contexts.
Moral: If all three compilers disagree with you, you must be wrong :-)
John Peterson
Yale Haskell Project
