[Haskell-cafe] Instances of constrained datatypes

This is a contrived example, but contains the essence of what I'd like to do. Suppose I have this datatype:

data (Eq v) => EqList v = EqList [v]

I'd like to make it an instance of Functor. However, fmap takes an arbitrary function of type a -> b. I need an Eq constraint on a and b. Is there any way to do this without creating my own `EqFunctor' class with explicitly-kinded quantification:

class (Eq a) => EqFunctor (f :: * -> *) a where eqfmap:: (Eq b) => (a -> b) -> f a -> f b

Thanks. -Arjun