Agreed, except I would not call it "existential" C++ stores references to dictionaries from/with the things they describe, that's "existential" Haskell passes

Agreed, except I would not call it "existential" C++ stores references to dictionaries from/with the things they describe, that's "existential" Haskell passes in extra parmeters that are dictionaries and doesn't (except for user-written code that does) keep around extra references to them. That's more like "universal" than "existential".