Leveraging Swift's Type System

One of the most remarkable qualities of Swift, compared to Objective-C, is its powerful type system. Swift developers have quickly accustomed themselves to the benefits of safer, more expressive APIs.

However, many of us don’t leverage the type system as much as we could. Types can help us eliminate duplicate code, build APIs that are easy to comprehend and even verify a significant portion of our program’s behavior.

This talk will explore how we can leverage the power of Swift’s type system in three broad categories:

1. Code Sharing with Types, via Protocols & Generics
2. Program Verification with Types, including a brief discussion on Types vs. Tests
3. Type First Development

We will discuss how these ideas can be applied in practice by including examples from production apps with large code bases.
Throughout the talk we will also discuss concepts from languages with other type systems and see how they apply to Swift.
Hopefully this talk will inspire the audience to unleash the superpowers of types in their own programs.

Transcript

1. Benjamin Encz @benjaminencz 1 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
2. Leveraging Swift's Type System 2 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
3. 1. Why Types? 2. Correctness & Documentation via Types 3. Code Design with Types 3 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
4. ! Margaret Hamilton with the Apollo 11 code listings / NASA P63SPOT3 CA BIT6 # IS THE LR ANTENNA IN POSITION 1 YET EXTEND RAND CHAN33 EXTEND BZF P63SPOT4 # BRANCH IF ANTENNA ALREADY IN POSITION 1 CAF CODE500 # ASTRONAUT: PLEASE CRANK THE TC BANKCALL # SILLY THING AROUND CADR GOPERF1 TCF GOTOP00H # TERMINATE TCF P63SPOT3 # PROCEED SEE IF HE'S LYING P63SPOT4 TC BANKCALL # ENTER INITIALIZE LANDING RADAR CADR SETPOS1 TC POSTJUMP # OFF TO SEE THE WIZARD ... CADR BURNBABY 4 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
5. 5 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
6. int a = 2; int b = a; // Copy n bytes into a new stack variable ✅ 6 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
7. NSNumber *a = @(2); NSString *c = a; // Incompatible pointer types ⚠ [c viewDidLoad]; // No visibile interface declares selector ❌ 7 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
8. NSString *name = nil; NSDictionary *broken = @{@"test": name}; // Nil crash! ! 8 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
9. For Each Type a Type System Defines... 1. ... a valid set of values 2. ... a valid set of operations 3. ... a representation 9 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
10. Correctness & Documentation 10 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
11. Asynchronous Error Handling 11 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
12. (result: NSData?, error: NSError?) 12 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
13. (result: NSData?, error: NSError?) // 4 possible combinations ! 13 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
14. (result: NSData?, error: NSError?) // 4 possible combinations ! (result: Result<Data, NetworkError>) // 2 possible combinations 14 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
15. switch result { case let .success(value): // use success value case let .failure(error): // handle error } 15 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
16. public enum Optional<Wrapped> { case none case some(Wrapped) } public enum Result<T, Error: Swift.Error> { case success(T) case failure(Error) } 16 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
17. Enums with Associated Values1 ! 1 Formally known as tagged unions 17 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
18. Typealiasing 18 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
19. 19 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
20. // Document semantics of types typealias Address = String typealias Email = String struct Person { // ... let email: Email let address: Address } 20 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
21. // Document & validate semantics struct Address { let value: String init(addressString: String) throws { // ... } } 21 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
22. github.com/Ben-G/Validated typealias Email = Validated<String, EmailValidator> let maybeEmail = Email("test@test.com") 22 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
23. Typealias ! Wrapper Types !! 23 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
24. Phantom Types 24 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
25. struct ObjectUid<!> { let value: String } 25 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
26. func currentUserUid() -> ObjectUid<User> func annotationByUid(annotationUid: ObjectUid<Annotation>) -> Annotation func annotationsForUser(userUid: ObjectUid<User>) -> [Annotation] 26 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
27. Type Case Study: Modelling JSON Encodable? 27 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
28. -(NSString *)toJsonString:(id)json; 28 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
29. -(NSString *)toJsonString:(id)json; // ! 29 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
30. enum JsonRoot { case dictionary(JsonDictionary) case array(JsonArray) } 30 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
31. protocol JsonValue {} enum JsonRoot: JsonValue { case dictionary(JsonDictionary) case array(JsonArray) } extension NSString : JsonValue { } extension NSNumber : JsonValue { } 31 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
32. typealias JsonDictionary = [String : JsonValue?] typealias JsonArray = [JsonValue] 32 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
33. 33 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
34. protocol JsonValue {} typealias JsonDictionary = [String : JsonValue?] typealias JsonArray = [JsonValue] enum JsonRoot: JsonValue { case dictionary(JsonDictionary) case array(JsonArray) } extension NSString : JsonValue { } extension NSNumber : JsonValue { } func toJsonString(json: JsonRoot) -> String 34 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
35. Code Design 35 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
36. Protocols ! Generics 36 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
37. Subtype vs. Parametric Polymorphism 37 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
38. Type Explosion 38 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
39. 39 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
40. protocol Downloadable { static var endpoint: URL { get } } protocol Decodable { init(data: Data) throws } protocol Cachable { static var cachingDuration: TimeInterval { get } } 40 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
41. final class Downloader<T: Downloadable> { func download(identifier: String) throws -> Data { let targetUrl = T.self.endpoint.appendingPathComponent(identifier) // .. return data } } 41 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
42. final class Downloader<T: Downloadable> { func download(identifier: String) throws -> Data { let targetUrl = T.self.endpoint.appendingPathComponent(identifier) // ... // Check if this type is cachable if let type = T.self as? Cachable.Type, let cachedResult = cache[targetUrl] { // Attempt to fetch data from cache } // Otherwise download & cache result if type is cachable if T.self is Cachable.Type { cache[targetUrl] = (Date(), data) } return data } } 42 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
43. extension Downloader where T: Decodable { func download(identifier: String) throws -> T { let result: Data = try self.download(identifier: identifier) return try T(data: result) } } 43 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
44. extension Photo: Downloadable { static let endpoint = URL(string: "https://photos.example.com")! } extension Photo: Cachable { static let cachingDuration: TimeInterval = 3600 } extension Photo: Decodable { static func decode(data: Data) throws -> Photo { // ... } } 44 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
45. let downloader: Downloader<Photo> = Downloader() let photo: Photo = try downloader.download(identifier: "hawaii2") 45 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
46. Protocols ! Generics " Safe Dynamic Type Checking ! Constrained Extensions ! 46 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
47. Types Scale 47 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
48. ! Types Document ! Types Enable Correctness Checks ! Generics Offer Alternative to Subtyping ! Concepts: Optional, Result, Enums with Associated Values, Typealias, Wrapper Types, Phantom Types, Safe Dynamic Type Checking, Constrained Extensions 48 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
49. Thank you! bit.ly/types360 @benjaminencz Thanks a lot to Jaden Geller, Russ Bishop and Morgan Chen for feedback on this talk. 49 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016
50. Related Talks: → Correct Behavior Through Type Safety (Justin Spahr-Summers) → Type Driven Development in Swift (Johannas Weiß) → Protocol Oriented Programming in Swift (WWDC 2015) 50 — Leveraging Swift's Type System | @benjaminencz | 360iDev 2016, August 2016