More generally: the duck-typing approach could be seen as something like implicit declaration of all possible type classes and instances. So if we needed a function to be able to call show and +, we don't need to declare a new typeclass ShowAndPlus, and go around declaring hundreds of things to be members of this class (and recall that we can't simply say instance (Show a, Num a) => ShowAndPlus a).

Hmmmm. It ought to be possible to do a lot of that at compile-time...