Open variants (the sum-type version of extensible records) might be on that list too, but I’m not familiar with any prior uses of them so I guess there’s probably something that makes them difficult? Maybe they’re just not very ergonomic in actual use, in which case cool, they’ll fit right in.
OCaml has polymorphic variants which might be related to what you’re thinking of.
OCaml has polymorphic variants which might be related to what you’re thinking of.