Jon Pretty

Corollary: "class hierarchies" become "class dynasties".https://twitter.com/propensive/status/1244903422643572736 …

I’m trying very hard to not jump in and say that actually, you’re not talking about inheritance but subclassing- one implies the other, but they’re not the same. Clearly I failed.

I think I never considered anything other than subclass inheritance to be inheritance... though I've been encouraging careful distinction between subtyping and inheritance for years...

But subclass inheritance implies subtyping, doesn’t it? I agree that inheritance and subtyping are distinct concepts, but if you consider that only subclass inheritance exists, how is it distinct from subtyping?

Subtyping concerns a superset relationship on the set of provable properties of a type (e.g. method signatures), and inheritance concerns the sharing of the runtime implementations of none, some, most or all those properties, and is conferred (only, I thought...) by subclassing.

Here’s my current understanding: subtyping is telling you “A is a B”, inheritance “A has access to a known subset of B’s code”. I believe modules can inherit from one another and they’re not classes, the same argument goes for typeclasses.

There is, of course, clearly an ancestor / descendant relationship, but it needs not be semantic - subclassing mixes the two, at least in languages like scala and java, but it’s not a necessity. C++ has inheritance without subtyping, java subtyping without inheritance

/rant