Generics and Inference

This talk will explore inference from the perspective of protocols and generics and is based off a series of blog posts I've written(foxinswift.com) on the topic. In the first part of my talk casting number types through inference. I'll then show you struct serialization example demonstrating inferring a type through a mapping function. My last example will take you through inferring an associatedtype on a barebones promise implementation and we'll use it to in the context of making a network request. To finish things off I'll briefly speak on what's new in swift generics and some limitations of those features.

Transcript

1. <Generics: Inference>
2. <Generics: Inference> → Rich Fox @RGfox → I work at Propeller → I am a contributer to
3. Generics and the Art of Inferences → What are generics → What is inference → Goal of Talk
4. Ex 1. Casting Number Types
5. protocol NumberConvertible { init(_ value: Int) init(_ value: Float) init(_ value: Double) } extension Double: NumberConvertible {} extension Float: NumberConvertible {} extension Int: NumberConvertible {} Int,Float,Double: NumberConvertible
6. extension NumberConvertible { func convert<T: NumberConvertible>() -> T { switch self { case let x as Float: return T(x) case let x as Int: return T(x) case let x as Double: return T(x) default: fatalError("NumberConvertible convert failed!") } } }
7. Cast by Inference! let number = 5.5 let a: Float = number.convert() let b: Int = number.convert() let c: Double = number.convert() let aa = number.convert() + Float(2) let bb = number.convert() + Int(2) let cc = number.convert() + Double(2)
8. Ex 2. Encoding/Decoding Structs (Map and Inferences)
9. protocol Serializable { init(construct: [String: Any]) func destruct() -> [String: Any] } extension Optional { func unbox<T>() -> T? { return self as? T } func unboxArray<T>() -> [T] { return unbox() ?? [] } }
10. struct SortItem { let name: String let subSorts: [SortItem] } extension SortItem: Serializable { func destruct() -> [String: Any] { var construct = [String: Any]() construct["name"] = name construct["subSorts"] = subSorts.map { $0.destruct() } return construct } init(construct: [String: Any]) { name = construct["name"].unbox() ?? "" let sorts = construct["subSorts"] .unboxArray() !.map(SortItem.init) } }
11. struct SortItem { let name: String let subSorts: [SortItem] } extension SortItem: Serializable { func destruct() -> [String: Any] { var construct = [String: Any]() construct["name"] = name construct["subSorts"] = subSorts.map { $0.destruct() } return construct } init(construct: [String: Any]) { name = construct["name"].unbox() ?? "" let sorts = construct["subSorts"] .unboxArray() !.map(SortItem.init(construct:)) } }
12. struct SortItem { let name: String let subSorts: [SortItem] //Default Value = Incognito init(subSorts: [SortItem] = [], name: String) { self.subSorts = subSorts self.name = name } } extension SortItem: Serializable { func destruct() -> [String: Any] { var construct = [String: Any]() construct["name"] = name construct["subSorts"] = subSorts.map { $0.destruct() } return construct } init(construct: [String: Any]) { name = construct["name"].unbox() ?? "" let sorts = construct["subSorts"] .unboxArray() !.map(SortItem.init) } }
13. struct SortItem { let name: String let subSorts: [SortItem] //Default Value = Incognito init(subSorts: [SortItem] = [], name: String) { self.subSorts = subSorts self.name = name } } extension SortItem: Serializable { func destruct() -> [String: Any] { var construct = [String: Any]() construct["name"] = name construct["subSorts"] = subSorts.map { $0.destruct() } return construct } init(construct: [String: Any]) { name = construct["name"].unbox() ?? "" let sorts = construct["subSorts"] .unboxArray() .map(SortItem.init) subSorts = sorts } }
14. Return type of unboxArray() inferred through .map! let sorts = construct["subSorts"] .unboxArray() .map(SortItem.init(construct:))
15. Ex 3: Promises/Networking
16. Result/Promise → Concise implementation → Result Enum → Promise Class w/AssociateType
17. enum Result<T> { case error(BasicError), some(T) private func fire(target: Promise<T>) { switch self { case .error(let err): target.failedAction?(err) case .some(let val): target.completeAction?(val) } } }
18. final class Promise<T> { private var completeAction: (T -> Void)? private var failedAction: (BasicError -> Void)? func complete(action: T! -> Void) -> Promise<T> { completeAction = action return self } func failure(action: BasicError -> Void) -> Promise<T> { failedAction = action return self } var trigger: Result<T>? { didSet { trigger?.fire(self) } } } Closure keeps Promise alive - waiting for trigger
19. Usage Example: (Networking + Generics * Inference)
20. Promise on the Network: func getUser(url: String) -> Promise<User> { }
21. Promise on the Network: func getUser(url: String) -> Promise<User> { let promise = Promise<User>() return promise }
22. Promise on the Network: func getUser(url: String) -> Promise<User> { let promise = Promise<User>() //Async call keeping reference to ˆpromiseˆ makeGetRequest(url) { response in } return promise }
23. Promise on the Network: func getUser(url: String) -> Promise<User> { let promise = Promise<User>() //Async call keeping reference to ˆpromiseˆ makeGetRequest(url) { response in guard let json = response else { Promise.trigger = Result.error(.unknown) return } } return promise }
24. Promise on the Network: func getUser(url: String) -> Promise<User> { let promise = Promise<User>() //Async call keeping reference to ˆpromiseˆ makeGetRequest(url) { response in guard let json = response else { Promise.trigger = Result.error(.unknown) return } if let user = try? User(object: json) { Promise.trigger = Result.some(user) } } return promise }
25. Promise on the Network: func getUser(url: String) -> Promise<User> { let promise = Promise<User>() //Async call keeping reference to ˆpromiseˆ makeGetRequest(url) { response in guard let json = response else { Promise.trigger = Result.error(.unknown) return } if let user = try? User(object: json) { Promise.trigger = Result.some(user) } else { let error = BasicError(object: json) Promise.trigger = Result.error(error) } } return promise }
26. More Generics and Inferred Promises func getEncodableType<T: JSONDecodable>(url: String) -> Promise<T> { let promise = Promise<T>() //Async call keeping reference to ˆpromiseˆ makeGetRequest(url) { response in guard let json = response else { Promise.trigger = Result.error(.unknown) return } if let result = try? T(object: json) { Promise.trigger = Result.some(result) } else { let error = BasicError(object: json) Promise.trigger = Result.error(error) } } return promise }
27. More Generics and Inferred Promises func getEncodableType<T: JSONDecodable>(url: String) -> Promise<T> { let promise = Promise<T>() //Generic instead of User //Async call keeping reference to ˆpromiseˆ makeGetRequest(url) { response in guard let json = response else { Promise.trigger = Result.error(.unknown) return } if let result = try? T(object: json) { //JSONDecodable init Promise.trigger = Result.some(result) } else { let error = BasicError(object: json) Promise.trigger = Result.error(error) } } return promise }
28. var user:User? var guest:Guest? func fetchPeople() { let printError: BasicError -> Void = {"error: \($0.message)"} NetworkService.getEncodableType("/url/User") .complete { user = $0 } .failure(printError) NetworkService.getEncodableType("/url/Guest") .complete { guest = $0 } .failure(printError) }
29. Promise<T> inferred by complete:(T)->Void NetworkService.getEncodableType("/url/User") .complete { user = $0 }
30. New in Swift 3 Generics Generic typealias typealias StringDictionary<T> = Dictionary<String, T> typealias BackwardTriple<T1,T2,T3> = (T3, T2, T1) Limited: Generic closure Crashes Playground typealias StringDictionaryValue<T> = (Dictionary<String, T>) -> T? let testValue: StringDictionaryValue = { return $0["test"] }
31. New in Swift 3 Generics Generic typealias typealias StringDictionary<T> = Dictionary<String, T> typealias BackwardTriple<T1,T2,T3> = (T3, T2, T1) Function Works as Expected typealias StringDictionary<T> = (Dictionary<String, T>) func valueForKey<T>(dict:StringDictionary<T>, key: String) -> T? { return dict[key] }
32. Inference Concluded
33. 1. Return Type Context func convert<T: NumberConvertible>() -> T 2. Though map Function .unboxArray() .map(SortItem.init(construct:)) 3. Through associatedtype Context func complete(action: T! -> Void) -> Promise<T>
34. Thank You Forward Swift @RGfox