Hello, I am experimenting with type-level HList based programming. The Apply class: class Apply f a r | f a -> r where apply :: f -> a -> r apply = undefined -- In case we use Apply for -- type-level computations only -- Normal function application instance Apply (x -> y) x y where apply f x = f x makes it seem like it should be possible to hMap: class HMap f l l' | f l -> l' where hMap :: f -> l -> l' instance HMap f HNil HNil where hMap f HNil = HNil instance ( Apply f x y, HMap f xs ys ) => HMap f (HCons x xs) (HCons y ys) where hMap f (HCons x xs) = HCons (apply f x) (hMap f xs) a (term-level) polymorphic function over a HList. As written it doesn't work at all. A (local) functional dependency to say that the function determines its argument type is missing. Towards this end I changed the Apply instance to: instance TypeCast a' a => Apply (a -> b) a' b where apply f x = f (typeCast x) This change allows me to apply a polymorphic function to elements of an HList by hand: (\(f :: forall a. a -> [a]) -> HCons (f 'a') (HCons (f "a") HNil)) (\x -> [x]) However, I need to explicitly write the polymorphic type of the argument f. Thus I cannot use hMap. So I have two questions: 1) Am I missing something? 2) Is there some way to pass an arbitrary (i.e. don't know explicit type at compile time) polymorphic function as an argument to another function? thanks, Jeff