Re: ConstraintKinds and default associated empty constraints

Quoting Bas van Dijk :

I'm playing a bit with the new ConstraintKinds feature in GHC 7.4.1-rc1. I'm trying to give the Functor class an associated constraint so that we can make Set an instance of Functor. The following code works but I wonder if the trick with: class Empty a; instance Empty a, is the recommended way to do this:

Maybe something like this? class Functor f where type C f a :: Constraint type C f a = () instance Functor Set where type C Set a = Ord a ~d