[Haskell-cafe] can't figure out a type

Hello, I'm working on some code like the following:

class Chunkable c el | c -> el where cLength :: c -> Int cHead :: c -> Maybe el

I want to be able to map over this type, like this:

cMap :: Chunkable c' el' => (el -> el') -> c -> c'

but this isn't quite right. c' shouldn't be any instance of Chunkable, it should be the same instance except parameterized over a different type. Another approach would be something like: class (Functor c) => Chunkable c el ... except that's not right either. I think c has the wrong kind to be a Functor instance. I expect there's something very basic I'm missing. Could anyone point in the proper direction of how to do this? Can this be expressed with associated types, perhaps? Thanks, John Lato