Swift is an open-source programming language developed by Apple that combines OOP and protocol-oriented programming (POP) with FP paradigms. Swift is not a pure FP language such as Haskell, Clojure, or F#, but it provides tools that facilitate FP. Swift can be used along with Objective-C to develop macOS, iOS, tvOS, and watchOS applications. Swift can also be used on Ubuntu Linux to develop web applications. This book explains Swift 3.1 and utilizes Xcode 8.3.2 compatible source code at a GitHub (https://github.com/PacktPublishing/Swift-Functional-Programming) repository, which will be updated frequently to catch up with changes to Swift.