Forgive my lack of C++ knowledge, but surely inheritance is a separate topic to polymophism?

If we think of a classic OOP example, we might say (in some made-up language):

    Mammals can breathe
    Mammals can move

    Dogs are Mammals
    Dolphins are Mammals

From this, we know that Dogs and Dolphins can breathe and move, so we can write code like:

    function checkStatus(Mammal m) {
      try {
        m.move();
        return "Free";
      } catch {
        try {
          m.breathe();
          return "Trapped";
        } catch {
          return "Dead";
        }
      }
    }

This code is subclass polymorphic, since we can pass in a Dog or a Dolphin, or some other type of Mammal, and it will work unchanged.

Yet this is completely orthogonal to inheritance! There are two ways we might actually implement these constructs:

- Mammal is an interface: Dog implements breathe by operating its lungs and move by operating its legs; Dolphin implements breathe by operating its lungs and move by operating its tail and fins.

- Mammal is an abstract class which implements breathe by operating its lungs. Dog inherits breathe and implements move by operating its legs; Dolphin inherits breathe and implements move by operating its tail and fins.

Both of these are valid approaches, but consider that:

- Only the second approach uses inheritance, so it seems strange to limit "polymorphism" to only this case.

- The checkStatus code doesn't actually care which approach we take. It's "polymorphic in our choice of polymorphism"!

Also, let's say that we did restrict the term "polymorphism" to these sorts of mechanisms. We might say that the above example is "polymorphic in the choice of Mammal". In which case, the "stage polymorphism" of this article is nothing other than "polymorphic in the choice of stage".

We could implement it in a language like C++ something like:

    Stages can call functions with arguments
    Stages can branch on booleans
    ...

    Interpreter is a Stage
    Compiler is a Stage

We achieve polymophism by passing around the currentStage, and writing code like `currentStage.call(myFunc, myArg)`. If `currentStage` is an `Interpreter`, the call will be performed immediately. If `currentStage` is a `Compiler`, code will be generated to perform the call.

The only difference between such an OO setup and the actual implementation in the article is that boilerplate like `currentStage` is all handled implicitly, rather than manually passed around, and we overload the normal language constructs rather than replacing them with methods, e.g. we can write `double(foo)` instead of `currentStage.call(double, foo)`, and `if foo then bar else baz` instead of `currentStage.branch(foo, bar, baz)` (or something even worse, to ensure that `bar` and `baz` get delayed!)