Haskell 98 provides a simple and clean type system, which I feel I understand very well. GHC provides a vast zoo of strange and perplexing type system extensions, which I don't understand at all. (Well, some of it is simple enough - e.g., multiparameter type classes. But GADTs? FDs? ATs? Hmm...) Anyway, it seems there is a large set of such type system extensions that involve writing "forall" all over the place. I have by now more or less managed to comprehend the fact that data Thing = forall x. Thing x allows a type variable to appear on the RHS that is *not* present on the LHS, thus "hiding" the type of something inside the structure. And for some reason, they call this "existential quantification" [which I can't spell never mind pronounce]. Today I was reading a potentially interesting paper, and I stumbled across something referred to as a "rank-2 type". Specifically, class Typable x => Term x where gmapT :: (forall y. Term y => y -> y) -> x -> x At this point, I am at a complete loss as to how this is any different from gmapT :: Term y => (y -> y) -> x -> x Can anybody enlighten me? This is probably the first real use I've ever seen of so-called rank-2 types, and I'm curios to know why people think they need to exist. [Obviously when somebody vastly more intelligent than me says something is necessary, they probably know something I don't...] At this point, I don't think I even wanna *know* what the hell a rank-N type is... o_O