nubie nubie writes: | So I want to have a polymorphic Collection type, just because. | | > class Collection c e | c -> e where | > empty :: c | > put :: c -> e -> c | | > data SomeCollection e = forall c . Collection c e => MakeSomeCollection c | | Hugs (February 2000) doesn't like it. It says | Variable "e" in constraint is not locally bound | | I feel that e *is* bound, sort of, because c is bound and there's a | fundep c->e. That line of reasoning establishes that e is constrained on the right hand side of the "=". However, it's still bound (by an implicit "forall e") on the left hand side of the "=". The problem is that e can leak details about c to parts of the program outside the "forall c". It's still a problem if you remove the "| c -> e" fundep. A more common use of a "Collection c e | c -> e" class is: data SomeCollection e = --some data structure involving e instance SomeContext e => Collection (SomeCollection e) e where --implementations of empty and put, for the aforementioned --data structure, and entitled to assume SomeContext Is that collection type polymorphic enough for your purposes? : | The following things work as expected: | | > data IntCollection = forall c . Collection c Int => MakeIntCollection c | > data AnyCollection = forall c e . Collection c e => MakeAnyCollection c Neither of them has a type variable tunnelling through the local quantifier. HTH. Tom