To follow-up, an additional feature that would be useful and resembles union
types is the variant where you could declare a union type first and then
separately other types could declare they are a member of the union. I'm
talking about loosely what mixins or type-roles or interfaces etc are in other
languages. The most trivial example would be declaring an ENUM-alike first and
then separately declaring the component values where the latter declare they are
part of the ENUM, and this could make it easier to add or change ENUM values.
But keep in mind that this is a distinct concept from what we're otherwise
talking about as being union types. -- Darren Duncan