On Fri, Mar 26, 2010 at 05:24:18AM -0700, Nicolas Pouillard wrote: > Could we have functors products, sums, fixpoints as well? It would really > avoid to redefine them each time. Do you mean data Product f g a = Product (f a) (g a) with Functor, Foldable, Traversable and Applicative instances? Not sure if the other two count as transformers.