Zero-Overhead Union Types in Scala

I like this solution to union types. It uses sealed traits in a way I hadn't thought of before -- essentially inverted the usual strategy for using sealed traits to establish a sum type. Other strategies I've seen for union types involve implicit evidence, which this approach does not need.

I just wrote a blog post about error handling which shows where union types can be really useful (http://underscore.io/blog/posts/2015/02/23/designing-fail-fa...). I didn't really go into the use of union types in that post because the pre-existing techniques I knew of were all rather involved. This is simple enough that I think it's viable to use even in relatively unsophisticated code bases.

This is really neat. As a professional Scala hacker who's played a bit with SML/OCaml, I always seem to find problems that would be best solved using discriminated unions like this, but I've never loved the traditional Scala idioms for achieving that sort of thing. I'm interested in Shapeless, but my coworkers are suspicious and I'm too inexpert in it to really argue strongly for it. This looks like something useful that I can implement when a union type is really what I need, without having to get Shapeless in the door.