I mean, generally I would love (more) union types, unfortunately so few languages have them. For Ops requirement I think it's too specific for a language level

I mean, generally I would love (more) union types, unfortunately so few languages have them. For Ops requirement I think it's too specific for a language level feature. In Kotlin you'd just use a sealed type in the cases were you really need to know why function can't produce a value. Works like a charm.