Example of a simple lens in Swift 3

simpleLens.swift

// Concepts based on an excellent article by Maciej Konieczny.
// http://narf.pl/posts/lenses-in-swift

struct ComicBook {
    let title: String
    let issueNumber: Int
    let publisher: Publisher
}

struct Publisher {
    let name: String
    let address: String
}

// A simple Lens - “functional getters and setters”
struct Lens<Whole, Part> {
    let get: (Whole) -> Part
    let set: (Part, Whole) -> Whole
}

// Publisher Lens
let comicPublisherLens: Lens<ComicBook, Publisher> =
    Lens(get: { $0.publisher },
         set: { ComicBook(title: $1.title, issueNumber: $1.issueNumber, publisher: $0) })

// Publisher Address Lens
let publisherAddressLens: Lens<Publisher, String> =
    Lens(get: { $0.address },
         set: { Publisher(name: $1.name, address: $0) })

let publisher = Publisher(name: "DC", address: "123 Main St., New York, NY")
let comicBook = ComicBook(title: "Action Comics", issueNumber: 1, publisher: publisher)

// Lens Composition
infix operator >>> { associativity right precedence 100 }

func >>><A, B, C>(lhs: Lens<A, B>, rhs: Lens<B, C>) -> Lens<A, C> {
    return Lens(get: { rhs.get(lhs.get($0)) },
                set: { (c, a) in
                    lhs.set(rhs.set(c, lhs.get(a)), a)
    })
}

// The ComicBook's Publisher moved!
let comicBookPublisherAddress = comicPublisherLens >>> publisherAddressLens
let currentComicBookPublisherAddress = comicBookPublisherAddress.get(comicBook)
let newComicBookPublisherAddress = comicBookPublisherAddress.set("456 Main Ave, Los Angeles, C.A", comicBook)

print(newComicBookPublisherAddress) // ComicBook(title: "Action Comics", issueNumber: 1, publisher: Publisher(name: "DC", address: "456 Main Ave, Los Angeles, C.A"))

Basic example of a Lens in Swift 3.