They are not the same feature. Enums allows you to create new values, while union types only allow you to combine values. For example, in Rust, enum Toto { Titi

They are not the same feature. Enums allows you to create new values, while union types only allow you to combine values. For example, in Rust, enum Toto { Titi, Tata } would create a new type, Toto, that contains two values, Toto::Titi and Toto::Tata. There's no way to express that with union types. On the other hand, Rust's enum have to be defined before use: I can't suddently decide to have a function that returns "Toto" | "Tata" without creating a type or returning string. OCaml has something called "polymorphic variants" that seem to be a middle-ground between the two: https://ocaml.org/manual/polyvariant.html https://ocaml.org/manual/polyvariant.html.