การใช้งานเริ่มต้นในส่วนขยาย

protocol Greeter {
    func greet() -> String
}

protocol Greeter { func greet() -> String }

extension Greeter {
    func greet() -> String { "Hello!" }
}

struct Robot: Greeter {}
print(Robot().greet())

protocol Greeter { func greet() -> String }
extension Greeter { func greet() -> String { "Hello!" } }

struct French: Greeter {
    func greet() -> String { "Bonjour!" }
}
print(French().greet())

protocol Named { var name: String { get } }

extension Named {
    func loudName() -> String { name.uppercased() }
}

struct City: Named { let name: String }
print(City(name: "paris").loudName())

protocol Sized {
    var width: Double { get }
    var height: Double { get }
}

extension Sized {
    var area: Double { width * height }
}

struct Rect: Sized { let width: Double; let height: Double }
print(Rect(width: 2, height: 5).area)

protocol Summable {
    var values: [Int] { get }
}

extension Summable {
    var total: Int { values.reduce(0, +) }
}

struct Cart: Summable { let values: [Int] }
print(Cart(values: [10, 20, 30]).total)

protocol Container {
    associatedtype Item
    var items: [Item] { get }
}

extension Container where Item: Equatable {
    func contains(_ x: Item) -> Bool { items.contains(x) }
}

struct Bag: Container { let items: [Int] }
print(Bag(items: [1, 2, 3]).contains(2))

protocol Logger {
    func log() -> String
}
extension Logger {
    func log() -> String { "default log" }
}

struct A: Logger {}
struct B: Logger { func log() -> String { "B log" } }

let loggers: [Logger] = [A(), B()]
for l in loggers { print(l.log()) }

protocol P {}
extension P { func who() -> String { "P-default" } }
struct Q: P { func who() -> String { "Q" } }

let direct = Q()
let asP: P = Q()
print(direct.who())
print(asP.who())

protocol Identifiable { var id: Int { get } }
extension Identifiable {
    var tag: String { "ID-" + String(id) }
}
struct Order: Identifiable { let id: Int }
print(Order(id: 7).tag)

protocol Money { var cents: Int { get } }
extension Money { var dollars: Double { Double(cents) / 100 } }
extension Money { var label: String { "$" + String(format: "%.2f", dollars) } }

struct Price: Money { let cents: Int }
print(Price(cents: 1299).label)