I think there is a difference. Sum types allows you to choose between values, union types allows you to choose between types. The values of the sum types can co

I think there is a difference. Sum types allows you to choose between values, union types allows you to choose between types. The values of the sum types can contain themselves types, but they don't have to. On the other hand, you have to declare sum types beforehand, and you don't have that with union types. The equivalent of sum types in Ceylon seems to be enumerated types, which from what I understand you also have to declare beforehand. The difference with Rust is that Ceylon also has union types (Scala is the same, Typescript has unions but lacks proper sum types). As an interesting aside, Rust has what seem to be intersection types for trait bounds (fn toto<T: Trait1 + Trait2)(a: T)). I'm still not sure about the tradeoffs between union/intersection types and sum/product types. I think the difference is that "set types" are structural, while algebraic types are nominal, which comes with the usual tradeoffs, but I'm not sure.