Keep Calm and Type Erase On

Try!Swift 2016

Transcript

1. Keep Calm and Type Erase On Gwendolyn Weston @purpleyay
2. Why would you want to erase a type? !
3. What does it mean to erase a type? !
4. What is a type? !
5. A type is a classification that defines: • a set of values • and the legal operations on them
6. Example: Int Values: all integers in [-∞, ∞] Operations: ➕, ✖ , %
7. Compilers ! Types
8. Compilers ! (Some) Types
9. Two kinds of types ! Concrete types ! Abstract types "
10. Concrete types ! A type with unambiguous implementation; can be instantiated directly.
11. Concrete types are representations of data.
12. let concreteInt = 42 let concreteArray = ["much", "concrete", "wow"]
13. Abstract types ! A type with incomplete implementation; cannot be instantiated directly
14. Abstract types are representations of behaviour.
15. class GenericClass<T> { ... } let object: GenericClass<T> // ! struct GenericStruct<U> { ... } let object: GenericStruct<U> // !
16. How to make abstract types concrete?
17. Type reification Making an abstract type concrete by filling in its placeholder types
18. Type parameters <T>, <U>
19. ! + <T> = " class GenericClass<T> { ... } let StringClass: GenericClass<String> // ! struct GenericStruct<T> { ... } let IntStruct: GenericStruct<Int> // !
20. One exception
21. protocol + <T> = ! We cannot use type parameters for generic protocols
22. protocol Pokemon { typealias PokemonType func attack(move:PokemonType) } class Pikachu
    : Pokemon { func attack(move: Electric) { ⚡ } } class Charmander: Pokemon { func attack(move: Fire) { " } }
23. let pokemon: Pokemon pokemon.attack(selectedAttack)
24. ⾠! let pokemon: Pokemon Protocol Pokemon can only be used as a generic constraint because it has Self or associated type requirements
25. AnyPokemon class AnyPokemon <PokemonType>: Pokemon { required init<U:Pokemon where U.PokemonType == PokemonType>(_ pokemon: U) { } }
26. let p1 = AnyPokemon(Pikachu
    ()) let p2: AnyPokemon<Fire> p2 = AnyPokemon(Charmander()) let digimon = AnyPokemon(NotAPokemon()) //!
27. let pokemon = Pikachu
    () vs. let pokemon: AnyPokemon <Electric> pokemon = AnyPokemon(Pikachu
    ()) Pikachu
    is instantiated as type AnyPokemon <Electric>
28. Type erasure
29. Type erasure scaffolding?
30. let pokemon: AnyPokemon <AnyObject> pokemon = AnyPokemon(Pikachu
    ()) //!
31. No covariance Swift does not allow <Electric> to substitute <AnyObject>
32. Downsides • Boilerplate • Lost type information • No covariance
33. Recap • What is a type? • 2 kinds of types: concrete, abstract • Reification with type parameters • Type erasure
34. Thanks! @purpleyay
35. Example class AnyPokemon <PokemonType>: Pokemon { private let _attack: ((PokemonType) -> Void) required init<U:Pokemon where U.PokemonType == PokemonType>(_ pokemon: U) { _attack = pokemon.attack } func attack(type:PokemonType) { return _attack(type) } }