Either.swift contains basic Either functionality such as map(), bind(), either(). EitherSpec.swift can help you understand how things are working.
For all who don't know Either: Either is one of the basic Functional programming abstraction. It denotes either of the value can be present at a time. This is very useful abstraction and it is present in all the leading Functional programming languages. Either documentation in Scala can be found at https://www.scala-lang.org/api/current/scala/util/Either.html
Copy Either.swift to your project and enjoy Functional programming in Swift.