But if contexts-on-datatypes worked correctly,
data Set a = Ord a => ....
then even the "real" map from Data.Set:
map :: (Ord a, Ord b) => (a -> b) -> Set a -> Set b
could be an instance method of Functor.
I'd love that. But I don't quite understand: do you think this is/should be possible with: current Haskell? Haskell-Prime? Current ghc (what extensions)?
It is not possible currently, because of the H'98 language definition. I do think it would be nice to fix this in Haskell-prime. However, although the idea is somewhat related to Polymorphic Components http://hackage.haskell.org/trac/haskell-prime/wiki/PolymorphicComponents there is no specific proposal about this issue on the wiki. (It was mentioned on some mailing list in the last couple of months, but I can't find the thread now.) By "working correctly" I mean that: it is a wart in Haskell'98 that you can declare a datatype to require some class constraints on contained elements, but that these extra constraints do not really buy you any expressive power. They just force you to repeat the same context decl on every function that uses such a type. Ideally, the data decl should be more like an "alias", capturing the constraints as part of the semantics associated with the type, so that you don't need to mention the constraints at every usage location of the type. Of course, there are some details to work out, about where you can validly omit the constraints, and where they are still required. Regards, Malcolm