I'm working through the interesting paper "Data type à la carte" and am confused by the Functor instance for Val. I think this stems from some confusion of mine regarding the Functor class in general. The Functor instance I'm confused about is: instance Functor Val where fmap f (Val x ) = Val x where Val is defined as: data Val e = Val Int Is this the only valid Functor instance for the Val type? Even though I'd, naively, expect the Functor instance to look like: instance Functor Val where fmap f (Val x) = Val (f x) I suspect that would not work out due to the type of the Val constructor. Is this correct? The reason I find all this odd is because I'm not sure how the type class Functor relates to the category theory concept of a functor. How does declaring a type constructor to be an instance of the Functor class relate to a functor? Is the type constructor considered a functor? Any help would be, of course, greatly appreciated. -Corey O'Connor