The diamond of Variances

try! Swift Tokyo 2018 presentation.
https://www.tryswift.co/events/2018/tokyo/en/#diamond

Transcript

1. Diamond of Variances Vu Nhat Minh / @orakaro try! Swift Tokyo 2018
2. Mercari / Souzoh iOS / Swift Engineer / @orakaro
3. What is Variance ? Subtyping relationship between type constructor Which relation of A and B make F<A> a subtype of F<B> ?
4. Contravariance Covariance Invariance Phantom Variance A: B then F<B> : F<A> A: B then F<A> : F<B> A: B but no relation between F<A> and F<B> No relation between A and B but F<A> : F<B> If A is a subtype of B, then F<A> is a subtype of F<B>
5. Swift Covariance: Array class Saiyan{} class SuperSaiyan: Saiyan{} let goku: Saiyan = SuperSaiyan() let saiyanArmy: [Saiyan] = [SuperSaiyan]() A: B [A] : [B]
6. Swift Covariance: Optional class Saiyan{} class SuperSaiyan: Saiyan{} let gohan: SuperSaiyan? = SuperSaiyan() let sonOfGoku: Saiyan? = gohan A: B A? : B?
7. Contravariance Covariance Invariance Phantom Variance A: B then F<B> : F<A> A: B then F<A> : F<B> A: B but no relation between F<A> and F<B> No relation between A and B but F<A> : F<B> If A is a subtype of B, then F<B> is a subtype of F<A>
8. Swift Contravariance: Function Parameter class Saiyan{} class SuperSaiyan: Saiyan{} typealias Transform<T> = (T) -> GreatApe let transformation: Transform<Saiyan> = { _ in GreatApe() } let superTransformation: Transform<SuperSaiyan> = transformation A: B F<B> : F<A>
9. Contravariance Covariance A: B then F<B> : F<A> A: B then F<A> : F<B> Covariance vs Contravariance typealias Transform1<T> = (T) -> Void // Contravariance typealias Transform2<T> = ((T) -> Void) -> Void // Covariance typealias Transform3<T> = (((T) -> Void) -> Void) -> Void // Contravariance vs
10. Contravariance Covariance Invariance Phantom Variance A: B then F<B> : F<A> A: B then F<A> : F<B> A: B but no relation between F<A> and F<B> No relation between A and B but F<A> : F<B>
11. Swift Invariance protocol Saiyan{} protocol SuperSaiyan: Saiyan{} class ZFighter<T> {} let goku: ZFighter<Saiyan> = ZFighter() let gokuSuperSaiyan: ZFighter<SuperSaiyan> = goku ❌ // cannot convert value of type 'ZFighter<Saiyan>' to specified type 'ZFighter<SuperSaiyan>' Any custom defined type in Swift is Invariance Custom type
12. Contravariance Covariance Invariance Phantom Variance A: B then F<B> : F<A> A: B then F<A> : F<B> A: B but no relation between F<A> and F<B> No relation between A and B but F<A> : F<B>
13. Phantom Variance protocol Saiyan {} protocol Namekian {} typealias ZFighter<> = Void let fighter: ZFighter<Saiyan> = ZFighter<Namekian>() A and B have no relation F<A> : F<B>
14. Recap
15. Contravariance Covariance Invariance Phantom Variance A: B then F<B> : F<A> A: B then F<A> : F<B> A: B but no relation between F<A> and F<B> No relation between A and B but F<A> : F<B>
16. Type of Variance • Covariance : Array and Optional • Contravariance : Function/Closure parameter • Invariance : Every Custom Type • Phantom Variance : Type parameter which is not used inside
17. Twitter / Github: @orakaro Thank you