Did it have an ergonomic way to exhaustively match on all the variants? Since the 70s?
How does the ABI work? If a library adds a new constructor, but I am still linking against the old version, I imagine that it could be reading the wrong fields, since the constructor it's reading is now at a different index?
You don't really need `String|Integer`, for most usecases an isomorphic type that you can exhaustively pattern match on is more than enough, and sealed classes (along with the support in `switch` expressions) does exactly that.
Scala 3 has had union types for 4 years now. Scala can be used to do Haskell style pure FP, but with much better tooling. And it has the power of the JVM, you can fall back to Java libraries if you want.
Java has recently added sealed classes/interfaces which offer the same features as sum types, and I would argue that Java is definitely mainstream.
Kotlin has a similar feature. It might be used less than Java, but it's the default language for Android.
Swift has `enum` for sum types and is the default language for iOS and MacOS.
Likewise for Rust, which is gaining traction recently.
Typescript also has union/sum types and is gaining lot of traction.