Type classes in Swift

Type class design pattern in Swift

Transcript

1. Type classes in Swift
   ܕΫϥεษڧձ
   Katsumi Kishikawa
   kk@realm.io
   

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2. Katsumi Kishikawa
   Realm Inc.
   kk@realm.io
   github.com/kishikawakatsumi
   

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3. What’s type class?
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4. In computer science, a type class
   is a type system construct that
   supports ad hoc polymorphism.
   Wikipedia
   Type class
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5. In programming languages and type theory,
   polymorphism is the provision of a single interface
   to entities of different types. A polymorphic type
   is one whose operations can also be applied to
   values of some other type, or types.
   Wikipedia
   Polymorphism
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6. • Subtype Polymorphism
   • Parameteric Polymorphism
   • Ad-hoc Polymorphism
   Polymorphism
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7. • Subtype Polymorphism => Subclassing
   • Parameteric Polymorphism
   • Ad-hoc Polymorphism
   Polymorphism
   let button: UIControl = UIButton()
   let slider: UIControl = UISlider()
   button.addTarget(self, action: ..., forControlEvents: ...)
   slider.addTarget(self, action: ..., forControlEvents: ...)
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8. • Subtype Polymorphism
   • Parameteric Polymorphism => Generics
   • Ad-hoc Polymorphism
   Polymorphism
   let x: Int = 0, y: Int = 1
   let x: Double = 0.0, y: Double = 1.0
   min(x, y)
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9. • Subtype Polymorphism
   • Parameteric Polymorphism
   • Ad-hoc Polymorphism => Overloading, Type class
   Polymorphism
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10. Type class in Haskell
    class Eq a where
    (==), (/=) :: a -> a -> Bool
    x == y = not (x /= y)
    x /= y = not (x == y)
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11. Type class in Haskell
    class Eq a where
    (==), (/=) :: a -> a -> Bool
    x == y = not (x /= y)
    x /= y = not (x == y)
    instance Eq Integer class
    x == y = x `integerEq` y
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12. Type class in Scala
    trait Eq[A]
    def equal(x: A, y: A): Boolean
    def notEqual(x: A, y: A): Boolean = !equal(x, y)
    }
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13. Type class in Swift
    protocol Eq {
    associatedtype T
    func equalTo(other: T) -> Bool
    }
    extension Int : Eq {
    typealias T = Int
    func equalTo(other: T) -> Bool {
    return self == other
    }
    }
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14. Type class in Swift
    protocol Eq {
    func equalTo(other: Self) -> Bool
    }
    extension Int : Eq {
    func equalTo(other: Int) -> Bool {
    return self == other
    }
    }
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15. Type class in Swift
    protocol Eq {
    func equalTo(other: Self) -> Bool
    }
    extension Int : Eq {
    func equalTo(other: Int) -> Bool {
    return self == other
    }
    }
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16. Type class in Swift
    protocol Eq {
    func equalTo(other: Self) -> Bool
    }
    extension Int : Eq {
    func equalTo(other: Int) -> Bool {
    return self == other
    }
    }
    let i = 1
    let j = 1
    print(i.equalTo(j)) // => true
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17. Swift: Equtable
    protocol Equatable {
    func == (lhs: Self, rhs: Self) -> Bool
    }
    func != (lhs: T, rhs: T) -> Bool {
    return !(lhs == rhs)
    }
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18. Swift: Equtable
    extension CGPoint : Equatable {}
    public func == (lhs: CGPoint, rhs: CGPoint) -> Bool {
    return lhs.x == rhs.x && lhs.y == rhs.y
    }
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19. • ܕΫϥεͱ͸ҟͳΔܕʹରͯ͠ಛఆͷৼΔ෣͍ʹ
    ద߹ͤ͞ΔσβΠϯύλʔϯ
    • طଘͷܕʹରͯ͠ػೳΛ௥Ճ͢Δ͜ͱ͕Ͱ͖Δ
    • Կ͕࢖ΘΕΔ͔͸ίϯύΠϧ࣌ʹܾఆ͢Δ
    • Swiftඪ४ϥΠϒϥϦͰ΋࢖ΘΕ͍ͯΔύλʔϯ
    Short summary
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20. Example: Caesar cipher
    class Ceasarable c where
    toCeasar :: c -> String
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21. trait Ceasarable[T] {
    def toCeasar(input: T): String
    }
    Example: Caesar cipher
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22. protocol Ceasarable {
    associatedtype T
    static func toCeasar(input: T) -> String
    }
    Example: Caesar cipher
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23. extension Int : Ceasarable {
    typealias T = Int
    static func toCeasar(input: T) -> String {
    return "\(input)".unicodeScalars.reduce("") {
    (acc, char) in
    return acc + String(UnicodeScalar(char.value + 3))
    }
    }
    }
    extension String : Ceasarable {
    typealias T = String
    static func toCeasar(input: String) -> String {
    return input.unicodeScalars.reduce("") {
    (acc, char) in
    return acc + String(UnicodeScalar(char.value + 3))
    }
    }
    }
    Example: Caesar cipher
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24. func encodeInCeasar(c: C.T) -> String {
    return C.toCeasar(c)
    }
    print(encodeInCeasar(1234)) //=> 4567
    print(encodeInCeasar("abcd")) //=> defg
    print(encodeInCeasar(1.234))
    //=> Cannot invoke 'encodeInCeasar' with an argument list of type '(Double)'
    Example: Caesar cipher
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25. protocol Ceasarable {
    associatedtype T
    static func toCeasar(input: T) -> String
    }
    Example: Caesar cipher
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26. protocol Ceasarable {
    associatedtype T
    static func toCeasar(input: T) -> String
    }
    Example: Caesar cipher
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27. protocol Ceasarable {
    static func toCeasar(input: Self) -> String
    }
    Example: Caesar cipher
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28. protocol Ceasarable {
    static func toCeasar(input: Self) -> String
    }
    Example: Caesar cipher
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29. extension Int : Ceasarable {
    static func toCeasar(input: Int) -> String {
    return "\(input)".unicodeScalars.reduce("") {
    (acc, char) in
    return acc + String(UnicodeScalar(char.value + 3))
    }
    }
    }
    extension String : Ceasarable {
    static func toCeasar(input: String) -> String {
    return input.unicodeScalars.reduce("") {
    (acc, char) in
    return acc + String(UnicodeScalar(char.value + 3))
    }
    }
    }
    Example: Caesar cipher
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30. extension Int : Ceasarable {
    static func toCeasar(input: Int) -> String {
    return "\(input)".unicodeScalars.reduce("") {
    (acc, char) in
    return acc + String(UnicodeScalar(char.value + 3))
    }
    }
    }
    extension String : Ceasarable {
    static func toCeasar(input: String) -> String {
    return input.unicodeScalars.reduce("") {
    (acc, char) in
    return acc + String(UnicodeScalar(char.value + 3))
    }
    }
    }
    Example: Caesar cipher
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31. func encodeInCeasar(c: C) -> String {
    return C.toCeasar(c)
    }
    print(encodeInCeasar(1234)) //=> 4567
    print(encodeInCeasar("abcd")) //=> defg
    print(encodeInCeasar(1.234))
    //=> Cannot invoke 'encodeInCeasar' with an argument list of type '(Double)'
    Example: Caesar cipher
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32. protocol Ceasarable {
    static func toCeasar(input: Self) -> String
    }
    extension Int : Ceasarable {
    static func toCeasar(input: Int) -> String {
    ...
    }
    }
    extension String : Ceasarable {
    static func toCeasar(input: String) -> String {
    ...
    }
    }
    print(encodeInCeasar(1234)) //=> 4567
    print(encodeInCeasar("abcd")) //=> defg
    Example: Caesar cipher
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33. Recap
    • Swiftʹ͓͚ΔܕΫϥε͸ProtocolͱExtensionΛ࢖ͬͨσβΠϯύλʔϯ
    • ʢܕΫϥεͱ໌ࣔ͞Εͯ͸͍ͳ͍͕ʣඪ४ϥΠϒϥϦͳͲͰࣗવʹ࢖Θ
    Ε͍ͯΔ
    • ܕΫϥε͸ҟͳΔܕʹର͢Δڞ௨ͷৼΔ෣͍Λఏڙ͠ɺඞཁʹԠͯ͡ܕ
    ͕ͦΕΛ࢖͑ΔΑ͏ʹ͢Δ࢓૊Έ
    • Կ͕࢖ΘΕΔ͔͸ίϯύΠϧ࣌ʹܾఆ͢Δ
    • ܧঝΛ࢖ΘͣʹػೳΛ௥Ճͨ͠Γ૊Έ߹ΘͤΔ͜ͱ͕Ͱ͖Δ
    • ܧঝʹΑ࣮ͬͯݱ͢Δํ๏ʹൺ΂ͯΑΓॊೈʹܕΛ֦ுͰ͖Δ
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34. References
    • WWDC 2015 session “Protocol-Oriented Programming in Swift”
    https://developer.apple.com/videos/play/wwdc2015/408/
    • Typeclasses in Swift, Haskell and Scala
    https://touk.pl/blog/2015/09/14/typeclasses-in-swift/
    • Typeclass in Swift and Scala
    http://www.danishin.com/article/Typeclass_in_Swift_and_Scala
    • Associated types vs. type parameters - reason for the former?
    https://devforums.apple.com/thread/230611
    • Swift Generic Protocols
    https://milen.me/writings/swift-generic-protocols/
    • Swift: Associated Types
    http://www.russbishop.net/swift-associated-types
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35. Questions?
    Katsuma Kishikawa
    kk@realm.io
    www.realm.io/jp
    @k_katsumi
    

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