Well, as my sibling comment says, you have just re-implemented dynamic dispatch in object programming ;). As a response to your edit, note that dynamic dispatch

Well, as my sibling comment says, you have just re-implemented dynamic dispatch in object programming ;). As a response to your edit, note that dynamic dispatch and immutability are not mutually exclusive; an object can have immutable state and can still have its behavior extended by creating an adapter object, for example.