Swift Generics - The 5 Stages of PATs

The Swift generics system is extremely powerful. Protocols with associated types are everywhere and allow us to abstract over types with hidden type information. But they come with their complexity tax. Join David Hart in a journey through Swift's generics to better understand PATs, discover ways to overcome them, and how future Swift might resolve most of those issues.

Transcript

1. Swift Generics The 5 Stages of PATs David Hart
2. SwiftPM Contributor by " iOS Developer at Atipik by ! David Hart
5. Redux State is read-only Single source of truth Changes are made with pure functions
6. Redux struct AppState { var todos: [UUID: Todo] } struct Todo { let uuid: UUID var text: String } protocol Action { associatedtype State func reduce(_ state: inout State) }
7. Redux struct SetTodoText: Action { let uuid: UUID let text: String func reduce(_ state: inout AppState) { state.todos[uuid]!.text = text } } struct CreateTodo: Action { let uuid: UUID func reduce(_ state: inout AppState) { state.todos[uuid] = Todo(uuid: uuid, text: "New Todo") } }
8. Redux func duplicate(todo: Todo) -> [Action] { let uuid = UUID() return [ CreateTodo(uuid: uuid), SetTodoText(uuid: uuid, text: todo.text) ] }
9. Redux func duplicate(todo: Todo) -> [Action] { let uuid = UUID() return [ CreateTodo(uuid: uuid), SetTodoText(uuid: uuid, text: todo.text) ] } Protocol 'Action' can only be used as… Protocol 'Action' can only be used as a generic constraint because it has Self or associated type requirements
10. Anger Denial 5 stages of PATs Bargaining Depression Acceptance
11. Denial #
12. "I must be doing something wrong."
15. Anger $
16. “I’m never using protocols ever again!”
17. If all you have is a hammer Protocol-Oriented Programming, everything looks like a nail protocol.
18. Classes open class Action<State> { open func reduce(_ state: inout State) { // Override in subclass. } }
19. Classes final class SetTodoText: Action<AppState> { let uuid: UUID let text: String override func reduce(_ state: inout AppState) { state.todos[uuid]!.text = text } } init(uuid: UUID, text: String) { self.uuid = uuid self.text = text }
21. Bargaining %
22. “I wish protocols were generic instead of having associated types.” — Dave DeLong
24. Generic Protocols // from func sort<C: Collection>(_ todos: C) where C.Element == Todo { … } protocol Action<State> { func reduce(_ state: inout State) } // to func sort(_ todos: Collection<Todo>) { … } // actually func sort<I>(_ todos: Collection<Todo, I>) { … }
25. Generic Protocols protocol ConstructibleFrom<T> { init(_ value: T) } struct Real { ... } extension Real: ConstructibleFrom<Float> { init(_ value: Float) { ... } } extension Real: ConstructibleFrom<Double> { init(_ value: Double) { ... } }
26. Depression &
27. "They're these parasitic things that end up infesting your code in ways you really don't want." — Dave DeLong
28. Type-Erased Wrappers AnyKeyPath AnyIterator AnySequence AnyCollection AnyIndex AnyHashable AnyRandomAccessCollection AnyBidirectionalCollection
29. Type-Erased Wrappers protocol Action { associatedtype State func reduce(_ state: inout State) } private let reduceClosure: (inout State) -> Void } struct AnyAction<State>: Action { init<A: Action>(_ action: A) where A.State == State { reduceClosure = { action.reduce(&$0) } } func reduce(_ state: inout State) { reduceClosure(&state) }
30. Type-Erased Wrappers func duplicate(todo: Todo) -> [ AnyAction( ) AnyAction<AppState>] { let uuid = UUID() return [ ] } AnyAction( ) CreateTodo(uuid: uuid) , SetTodoText(uuid: uuid, text: todo.text)
31. Acceptance '
32. “I wish the compiler generated type erasers for me.” — Dave DeLong
34. Generalized Existentials typealias Codable = Encodable & Decodable typealias TableViewController = UIViewController & UITableViewDelegate typealias AnyCollection<T> = Collection where .Element == T typealias AnyAction<S> = Action where .State == S
35. Type-Erased Wrappers func duplicate(todo: Todo) -> [AnyAction<AppState>] { let uuid = UUID() return [ CreateTodo(uuid: uuid), SetTodoText(uuid: uuid, text: todo.text) ] }
36. The 5 Stages of PATs PATs are a PITA OOP is not dead Generic Protocols Type-erasers FTW Generalized Existentials
37. ( ?
38. Follow up + Come talk to me ) Generics Manifesto https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md * Advanced Swift https://www.objc.io/books/advanced-swift/ Rust - Advanced Traits https://doc.rust-lang.org/book/second-edition/ch19-03-advanced-traits.html Generalized Existentials Proposal https://github.com/austinzheng/swift-evolution/blob/az-existentials/proposals/XXXX-enhanced-existentials.md
39. Thanks! dhartbit hartbit @icanzilb AppBuilders