Writing compilers is not a really a niche activity. If you think of it, every programmer is doing precisely that, defining a specific language for a given domain and then explaining its semantics to the computer. F#, ML and Haskell programmers know that union types are a life-saver for dealing with languages systematically. These types do, however, sometimes grow large, becoming less and less nice to work with. If a 30-case recursive union type, or 2-3 co-recursive 20-30 case union types, is something[...]