[seanmcdirmid]

You mean CLOS? This is exactly the context RPG coined "worse is better".

Languages are not so much a collection of features but mindsets. So polymorphism, dynamic dispatch, subtyping, etc...do not define OOP so much as they are leveraged by those languages to enable reasoning with names and metaphors. Calling them just domain specific features misses the point like talking about some dish only as the sum of its ingredients.

Tagged unions and GADTs are quite different in expressiveness and modularity, I still remember the mega case matches used in scalac. Should one function really be given so much functionality when a layered design with several virtual method implementations would be much more amenable to change and modular reasoning? Well, I guess it's a matter of how you view code.

[l_dopa]

  Languages are not so much a collection of features ...

If you want to define objects precisely, even just to have a language spec, they are absolutely made up of sums, products, recursive types, etc. Whatever useful metaphors one might have to work with objects doesn't change what they actually are. If you give the programmer access to these building blocks, you get ML.

  Calling them just domain specific ...

I meant that the particular way they're combined to get OOP is domain-specific.

Again, I'm just asking: why mess with the basics? Why require encoding simple, universal concepts in terms of an ad-hoc object system? You can still have an object system on top, if you find that helps with the really complicated cases, but why encode the parts of your code that are simple in terms of much more complicated, derived concepts?