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
I wrote a paper proposing an extension to allow this, published at the Haskell Workshop in 1999. Here's the link: http://www.cs.chalmers.se/~rjmh/Papers/restricted-datatypes.ps Getting the right dictionaries to the right place involves adding a concept of well-formed types, which perhaps is why it hasn't been taken up by the Simons... John Hughes