Scala has a highly expressive type system, but it doesn’t include everything you might find yourself hankering after — at least, not as primitives. There are a few genuinely useful things which fall under this heading — higher-rank polymorphic function types and recursive structural types are two I’ll talk about more in later posts. Today I’m going to show how we can encode union types in Scala, in the course of which I’ll have an opportunity to shed a little light on the Curry-Howard isomorphism and show how it can be put to work for us.