iOS Conf Singapore October 2015

Swift Sync and Async Error Handling. Showing some of the problems with traditional synchronous and asynchronous error handling in Objective-C, and how we can do better in Swift with Result, some kind of Future API, and comparing it with the new Swift 2 throws syntax.

1. Swift Sync and Async Error Handling iOS Conf Singapore - October 2015 — @Javi 1 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
2. Agenda → Traditional Asynchronous Code → Problems with Traditional Error Handling → Result vs throws → Modeling asynchronous computation: Future → Making Result and throws play together 2 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
3. Traditional asynchronous code 3 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
4. Traditional asynchronous code struct User { let avatarURL: NSURL } func requestUserInfo(userID: String, completion: (User?, NSError?) -> ()) func downloadImage(URL: NSURL, completion: (UIImage?, NSError?) -> ()) func loadAvatar(userID: String, completion: (UIImage?, NSError?) -> ()) { requestUserInfo(userID) { user, error in if let user = user { downloadImage(user.avatarURL) { avatar, error in if let avatar = avatar { completion(avatar, nil) } else { completion(nil, error!) } } } else { completion(nil, error!) } } } 4 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
5. Traditional asynchronous code 5 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
6. Traditional asynchronous code func downloadImage(URL: NSURL, completion: (UIImage?, NSError?) -> ()) (.Some, .None) (.None, .Some) (.Some, .Some) (.None, .None) 6 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
7. (.Some, .Some) (.None, .None) 7 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
8. Error Handling func downloadImage(URL: NSURL, completion: (UIImage?, NSError?) -> ()) downloadImage(url) { imageOrNil, errorOrNil in if error = errorOrNil { // What if image is also not nil?? } else if image = imageOrNil { } } 8 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
9. Error Handling var error: NSError? let string = NSString(contentsOfFile:path encoding:NSUTF8Encoding error:&error) if string == nil { // Oops: if error.code == NSFileReadNoSuchFileError { // ... } else if ... } 9 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
10. 10 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
11. 11 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
12. 12 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
13. 13 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
14. NSError Alternative protocol ErrorType { } enum UserInfoErrorDomain: ErrorType { case UserDoesNotExist case UserRequestFailure(reason: String) case NetworkRequestFailure(underlyingError: NSError) } extension NSError: ErrorType { } 14 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
15. NSError Alternative enum NoError: ErrorType { } let error = NoError(?) 15 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
16. Result enum Result<T, E: ErrorType> { case Success(T) case Failure(E) } Result by Rob Rix: https://github.com/antitypical/Result 16 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
17. Swift's throws 17 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
18. Swift's throws func readFile(path: String) throws -> NSData 18 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
19. Swift's throws func readFile(path: String) throws -> NSData // throws... what exactly? 19 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
20. Swift's throws func readFile(path: String) throws -> NSData Functionally equivalent to func readFile(path: String) -> Result<NSData, ErrorType> 20 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
21. Swift's throws func readFile(path: String) throws -> NSData do { let contents = try readFile("path/to/my/file") } catch(SomeTypeOfError) { // How can I know which errors I should match..? } catch { // catch-all block. } 21 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
22. Swift's throws func readFile(path: String) throws -> NSData let contents: NSData? = try? readFile("path/to/my/file") 22 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
23. Swift's throws func readFile(path: String) -> Result<NSData, FileIOError> let file = readFile("path/to/my/file") switch file { case let .Succcess(value): // ... case let .Failure(error): // This `error` variable has type information! } 23 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
24. Swift's throws Hopefully in a future version of Swift ... func readFile(path: String) throws FileIOError -> NSData 24 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
25. Error handling in Asynchronous APIs 25 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
26. Future<T, E> 26 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
27. Futures → Encapsulate a deferred computation. → Treat values that incur a delay to be retrieved as if they were regular values. → Allow us to treat errors as first class citizens. → Easily composable. 27 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
28. Future struct Future<T, E: ErrorType> { typealias ResultType = Result<T, E> typealias Completion = ResultType -> () typealias AsyncOperation = Completion -> () private let operation: AsyncOperation } 28 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
29. Future struct Future<T, E: ErrorType> { init(operation: AsyncOperation) { self.operation = operation } func start(completion: Completion) { self.operation() { result in completion(result) } } } 29 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
30. Future.map(): transforming the computed value 30 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
31. Future.map(): transforming the computed value struct User { let avatarURL: NSURL } func requestUserInfo(userID: String) -> Future<User, ErrorDomain> func requestUserAvatarURL(userID: String) -> Future<NSURL, ErrorDomain> { return requestUserInfo(userID) .map { $0.avatarURL } } 31 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
32. Future.map(): transforming the computed value struct Future<T, E: ErrorType> { func map<U>(f: T -> U) -> Future<U, E> } 32 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
33. map() in other types struct Array<T> { func map<U>(f: T -> U) -> [U] } enum Optional<T> { func map<U>(f: T -> U) -> U? } struct Future<T, E: ErrorType> { func map<U>(f: T -> U) -> Future<U, E> } 33 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
34. Future.map(): transforming the computed value func map<U>(f: T -> U) -> Future<U, E> { // Return a new Future w/ a new operation... return Future<U, E>(operation: { completion in }) } 34 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
35. Future.map(): transforming the computed value func map<U>(f: T -> U) -> Future<U, E> { return Future<U, E>(operation: { completion in // Retrieve the value from self... self.start { result in } }) } 35 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
36. Future.map(): transforming the computed value func map<U>(f: T -> U) -> Future<U, E> { return Future<U, E>(operation: { completion in self.start { result in // Consider .Success and .Failure... switch result { } } }) } 36 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
37. Future.map(): transforming the computed value case let .Success(value): // Call completion with the transformed value completion(Result.Success(f(value))) 37 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
38. Future.map(): transforming the computed value case let .Failure(error): // We didn't get a value: no transformation completion(Result.Failure(error)) 38 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
39. Future.map(): transforming the computed value func map<U>(f: T -> U) -> Future<U, E> { return Future<U, E>(operation: { completion in self.start { result in switch result { case let .Success(value): completion(Result.Success(f(value))) case let .Failure(error): completion(Result.Failure(error)) } } }) } 39 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
40. Future.andThen(): concatenating async work 40 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
41. Future.andThen(): concatenating async work func requestUserAvatarURL(userID: String) -> Future<NSURL, ErrorDomain> func downloadImage(URL: NSURL) -> Future<UIImage, ErrorDomain> func downloadUserAvatar(userID: String) -> Future<UIImage, ErrorDomain> { return requestUserAvatarURL(userID) .andThen(downloadImage) } 41 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
42. Future.andThen(): concatenating async work struct Future<T, E: ErrorType> { func andThen<U>(f: T -> Future<U, E>) -> Future<U, E> } 42 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
43. Future.andThen(): concatenating async work func andThen<U>(f: T -> Future<U, E>) -> Future<U, E> { return Future<U, E>(operation: { completion in } } 43 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
44. Future.andThen(): concatenating async work func andThen<U>(f: T -> Future<U, E>) -> Future<U, E> { return Future<U, E>(operation: { completion in self.start { firstFutureResult in } }) } 44 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
45. Future.andThen(): concatenating async work func andThen<U>(f: T -> Future<U, E>) -> Future<U, E> { return Future<U, E>(operation: { completion in self.start { firstFutureResult in switch firstFutureResult { case let .Success(value): // ... case let .Failure(error): // ... } } }) } 45 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
46. Future.andThen(): concatenating async work case let .Success(value): let nextFuture = f(value) 46 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
47. Future.andThen(): concatenating async work case let .Success(value): let nextFuture = f(value) nextFuture.start { finalResult in completion(finalResult) } 47 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
48. Future.andThen(): concatenating async work case let .Failure(error): completion(Result.Failure(error)) 48 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
49. Future.andThen(): concatenating async work func andThen<U>(f: T -> Future<U, E>) -> Future<U, E> { return Future<U, E>(operation: { completion in self.start { firstFutureResult in switch firstFutureResult { case let .Success(value): f(value).start(completion) case let .Failure(error): completion(Result.Failure(error)) } } }) } 49 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
50. We can go from this... func loadAvatar(userID: String, completion: (UIImage?, NSError?) -> ()) { requestUserInfo(userID) { user, error in if let user = user { downloadImage(user.avatarURL) { avatar, error in if let avatar = avatar { completion(avatar, nil) } else { completion(nil, error) } } } else { completion(nil, error) } } } 50 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
51. ... To this func requestUserInfo(userID: String) -> Future<User, UserInfoErrorDomain> func downloadImage(URL: NSURL) -> Future<UIImage, UserInfoErrorDomain> func loadAvatar(userID: String) -> Future<UIImage, UserInfoErrorDomain> { return requestUserInfo(userID) .map { $0.avatarURL } .andThen(downloadImage) } 51 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
52. Mixing it all up Result<T, E> and throws 52 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
53. Result<T, E> and throws throws => Result extension Result { public init(@autoclosure _ f: () throws -> T) { do { self = .Success(try f()) } catch { self = .Failure(error as! Error) } } } 53 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
54. Result<T, E> and throws Result => throws extension Result { public func dematerialize() throws -> T { switch self { case .Success(value): return value case let .Failure(error): throw error } } } 54 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
55. Result<T, E> and throws Future => throws let avatarFuture = loadAvatar("4815162342") avatarFuture.start { result in do { let avatar = try result.materialize() } catch { // handle `error` } } 55 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
56. Limitations of Futures → Only represent computation to retrieve one value. → Can't encapsulate streams of values. → Only consumer (pull) driven, not producer (push) driven. 56 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
57. ReactiveCocoa Signals > Futures 57 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
58. Follow-up Resources → ReactiveCocoa: https://github.com/ ReactiveCocoa/ReactiveCocoa → Railway Oriented Programming: http:// fsharpforfunandprofit.com/posts/recipe-part2/ 58 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015
59. Thanks! Questions? 59 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015