Or if you are "Go", you just make up an interface with a "quack" method (and maybe "walk", "swim" and "fly", too), without the concrete types ever knowing / car

Or if you are "Go", you just make up an interface with a "quack" method (and maybe "walk", "swim" and "fly", too), without the concrete types ever knowing / caring that there was such an interface, all while still getting the needed type checking on the function/method calls.