Polymorphism in Programming Languages

If you have an interface that has to be implemented by the types, then it's sort of an upfront decision. Some types don't allow someone to some arbitrary person to extend the base because they're sealed. You can work with completely disjoint types that have nothing in common if you want. It depends on what we're probably trying to solve. And I as the program designer has to understand them well enough to know, oh, yeah, this is a better use or this will be an easier use.