Simplify Enums by odersky · Pull Request #4003 · scala/scala3 (original) (raw)

I had a lingering unease that the rules for enums were too complex, and the syntax too cumbersome. I now think I have found a way to simplify things considerably. The key was to take away one capability: that cases can have bodies which can define members. Arguably, if we choose an ADT decomposition of a problem it's good style to write all methods using pattern matching instead of overriding individual cases. So this removes an unnecessary choice. What's more, once we have eliminated case bodies we also have eliminated scope confusion. All that remains are the case parameters and extends clause. Extends clauses of cases can be handled like super-calls in constructors: I.e. the enclosing enum class is not visible for them.

This means we can treat enums unequivocally as classes. They can have methods and other statements just like other classes can. Cases in enums are seen as a form of constructors. We do not need a distinction between enum class and enum object anymore. Enums can have companion objects just like normal classes can, of course.

This also means that type parameters of enums scope naturally over cases, just like they scope
over secondary constructors. We do not need to repeat them in cases anymore, which is a huge relief.

This first commit changes the syntax, docs and tests. It still needs to be implemented. So tests should fail right now.