I would like to build a class that includes a method to move data from one arbitrary functor to another, a natural transformation. The structures might be more than just functors, but I can start with that. I ran into some practical issues with resolving the type variables for my multiparameter type class, which I can resolve with functional dependencies. I can also not isolate the natural transformation from my overall operation, muddling it with the element transformation. I was wondering if anyone had any words of advice, example or warning about this kind of function or method in general? Class (Functor f, Functor g) => Foo f g a where foo :: f a -> g a bar :: (a->b) -> g a -> g b Thanks, Gordon J. Uszkay, McMaster University uszkaygj@mcmaster.ca