Diamond of Variances Vu Nhat Minh / @orakaro try! Swift Tokyo 2018

Mercari / Souzoh iOS / Swift Engineer / @orakaro

What is Variance ? Subtyping relationship between type constructor Which relation of A and B make F a subtype of F ?

Contravariance Covariance Invariance Phantom Variance A: B then F : F A: B then F : F A: B but no relation between F and F No relation between A and B but F : F If A is a subtype of B, then F is a subtype of F

Swift Covariance: Array class Saiyan{} class SuperSaiyan: Saiyan{} let goku: Saiyan = SuperSaiyan() let saiyanArmy: [Saiyan] = [SuperSaiyan]() A: B [A] : [B]

Swift Covariance: Optional class Saiyan{} class SuperSaiyan: Saiyan{} let gohan: SuperSaiyan? = SuperSaiyan() let sonOfGoku: Saiyan? = gohan A: B A? : B?

Contravariance Covariance Invariance Phantom Variance A: B then F : F A: B then F : F A: B but no relation between F and F No relation between A and B but F : F If A is a subtype of B, then F is a subtype of F

Swift Contravariance: Function Parameter class Saiyan{} class SuperSaiyan: Saiyan{} typealias Transform = (T) -> GreatApe let transformation: Transform = { _ in GreatApe() } let superTransformation: Transform = transformation A: B F : F

Contravariance Covariance A: B then F : F A: B then F : F Covariance vs Contravariance typealias Transform1 = (T) -> Void // Contravariance typealias Transform2 = ((T) -> Void) -> Void // Covariance typealias Transform3 = (((T) -> Void) -> Void) -> Void // Contravariance vs

Contravariance Covariance Invariance Phantom Variance A: B then F : F A: B then F : F A: B but no relation between F and F No relation between A and B but F : F

Swift Invariance protocol Saiyan{} protocol SuperSaiyan: Saiyan{} class ZFighter {} let goku: ZFighter = ZFighter() let gokuSuperSaiyan: ZFighter = goku ❌ // cannot convert value of type 'ZFighter' to specified type 'ZFighter' Any custom defined type in Swift is Invariance Custom type

Contravariance Covariance Invariance Phantom Variance A: B then F : F A: B then F : F A: B but no relation between F and F No relation between A and B but F : F

Phantom Variance protocol Saiyan {} protocol Namekian {} typealias ZFighter<> = Void let fighter: ZFighter = ZFighter() A and B have no relation F : F

Recap

Contravariance Covariance Invariance Phantom Variance A: B then F : F A: B then F : F A: B but no relation between F and F No relation between A and B but F : F

Type of Variance • Covariance : Array and Optional • Contravariance : Function/Closure parameter • Invariance : Every Custom Type • Phantom Variance : Type parameter which is not used inside

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