Not the parent, but I have used Standard ML, OCaml, Scala and Swift, and I like union types as well. They are strictly more powerful than sum types. If I want t

Not the parent, but I have used Standard ML, OCaml, Scala and Swift, and I like union types as well. They are strictly more powerful than sum types. If I want to create an Option type such that Option<Option<T>> = Option<T>, then that's possible. With sum types only, it is not. Yes, this gives you Option<T> = T in situations where T = Option<S>. If I want to have a sum type, then I can just create a distinguishing "kind" field, as you do in TypeScript. The argument against union types would be, that they are an advanced feature compared to sum types, and sum types are simpler to use right. The argument against sum types is that they are moving the role of types from descriptive to prescriptive, and that is what leads to the problem Hickey is pointing out.