ConstraintKinds and default associated empty constraints

[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:

{-# LANGUAGE ConstraintKinds, TypeFamilies, FlexibleInstances #-}

import GHC.Prim (Constraint)

import Prelude hiding (Functor, fmap)

import           Data.Set (Set)
import qualified Data.Set as S (map, fromList)

class Functor f where
    type C f :: * -> Constraint
    type C f = Empty

    fmap :: (C f a, C f b) => (a -> b) -> f a -> f b

class Empty a; instance Empty a

instance Functor Set where
    type C Set = Ord
    fmap = S.map

instance Functor [] where
    fmap = map

testList = fmap (+1) [1,2,3]
testSet  = fmap (+1) (S.fromList [1,2,3])

Cheers and thanks for a great new feature!

Bas