In Haskell, you declare that a type is an instance of a typeclass, and provide implementations for the required functions. You can do this anywhere for any type. Then, you have your functions accept any type that is an instance of you typeclass you need. Go has a vaguely similar concept. This is incompatible with subtyping, but not incompatible with static typing, and is one of the reasons fans of functional programming languages can so derisive of languages that allow subtyping.