> Careful not to mix unions with sum types, though. The key distinction is that the latter are disjunct sets, even if you "sum" together the same type twice, you can always tell which "way" you went.

This is a really good point. I'd love to be able to have a sum type of two strings ("escaped" and "unescaped"); or any two kinds of the same type really, to model two kinds of the same type where one has already passed some sort of validation and the other one hasn't.

Edit to add: I figure what I want is for enums to be extended such that different branches are able to carry different properties.

Edit again (I should learn to think things through before posting. sorry): I suppose it can be faked using a union of different wrapper types, and in fact it might be the best way to do it so then methods can take just one of the types in arguments and maybe even provide different overloads.