On Tue, May 11, 2010 at 2:06 PM, John Meacham
A better way might be
class (Functor f, Functor g) => FunctorPair f g where transformFunctor :: f a -> g a
though, I am not sure what your use is, there isn't an obvious instance to me, but I don't know what your motivating task is.
Furthermore, to select an instance at this point both Functors must be known in context of use, which exhausts pretty much the entire informational content of the instance--raising the question of what benefit is drawn from a type class at all, as opposed to simply passing around functions of type "f a -> g a" as needed. Consider that fmap doesn't require a multi-parameter type class for each pair a, b, it just takes a function (a -> b). The only way a type class would make sense here, I think, is with a fundep uniquely determining a particular natural transformation based on one of the Functors, at which point I'm not sure that the concept of "natural transformation" is what you're actually trying to model. - C.