Tobias Haeberlein writes: | | > What is wrong with: | > | > class Functor ConstInt where | > map f (Const n) = Const n | | Ups - absolutely nothing. | I actually tried it with | | data ConstInt a = Int This version doesn't refer to the type Int. Instead, it introduces a new nullary data constructor called Int, which you could use like this: instance Functor ConstInt where fmap f Int = Int ...but I don't think that's what you were after. If you use a type alias (type ConstInt a = Int) instead of declaring a data type, it will refer to the type Int. However, then you'd need to use ConstInt partially applied in the instance declaration, which Haskell doesn't allow. I think Gofer has a more liberal type system which does allow it. | and didnt manage to make this an instance of | class Functor. Sorry for giving the wrong | example.