> With polymorphism and inheritance in the mainstream languages, the method signature still doesn't exactly tell you what you are going to get, so you have all

> With polymorphism and inheritance in the mainstream languages, the method signature still doesn't exactly tell you what you are going to get, so you have all of the pain of micromanaging type annotations and you still don't really get an invariant. This is exactly what is addressed by the Liskov substitution principle [0] -- the "L" in SOLID [1]. If your subtype does not have the same semantics as the type it extends, then it fails this criteria. It is violating the (implicit) contract of the type it is extending. [0] https://en.wikipedia.org/wiki/Liskov_substitution_principle https://en.wikipedia.org/wiki/Liskov_substitution_principle [1] https://en.wikipedia.org/wiki/SOLID_%28object-oriented_design%29 https://en.wikipedia.org/wiki/SOLID_%28object-oriented_desig...