Back to the Futures

Presentation given at Swift Summit 2015 in London on the problems with traditional asynchronous, callback-based APIs in Objective-C, NSError-based error handling, and how we can do both a lot better in Swift, for example, by implementing a simple Future API.

1. Back to the Futures — @Javi "Back to the Futures" - Javier Soto. March 2015 1
2. Agenda 4 Traditional Asynchronous Code 4 Problems with Traditional Error Handling 4 Future 4 Future.map 4 Future.andThen "Back to the Futures" - Javier Soto. March 2015 2
3. Traditional asynchronous code "Back to the Futures" - Javier Soto. March 2015 3
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!) } } } "Back to the Futures" - Javier Soto. March 2015 4
5. Traditional asynchronous code "Back to the Futures" - Javier Soto. March 2015 5
6. Traditional asynchronous code func downloadImage(URL: NSURL, completion: (UIImage?, NSError?) -> ()) 4 (.Some, .None) 4 (.None, .Some) 4 (.Some, .Some) 4 (.None, .None) "Back to the Futures" - Javier Soto. March 2015 6
7. 4 (.Some, .Some) 4 (.None, .None) "Back to the Futures" - Javier Soto. March 2015 7
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 { } } "Back to the Futures" - Javier Soto. March 2015 8
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 ... } "Back to the Futures" - Javier Soto. March 2015 9
10. "Back to the Futures" - Javier Soto. March 2015 10
11. "Back to the Futures" - Javier Soto. March 2015 11
12. "Back to the Futures" - Javier Soto. March 2015 12
13. NSError Alternative protocol ErrorType { } enum UserInfoErrorDomain: ErrorType { case UserDoesNotExist case UserRequestFailure(reason: String) case NetworkRequestFailure(reason: String) } extension NSError: ErrorType { } "Back to the Futures" - Javier Soto. March 2015 13
14. NSError Alternative enum NoError: ErrorType { } let error = NoError(?) "Back to the Futures" - Javier Soto. March 2015 14
15. Result enum Result<T, E: ErrorType> { case Success(T) case Error(E) } "Back to the Futures" - Javier Soto. March 2015 15
16. Say hello to Future<T, E> "Back to the Futures" - Javier Soto. March 2015 16
17. Futures 4 Encapsulate a deferred computation. 4 Treat values that incur a delay to be retrieved as if they were regular values. 4 Allow us to treat errors as first class citizens. 4 Easily composable. "Back to the Futures" - Javier Soto. March 2015 17
18. Future struct Future<T, E: ErrorType> { typealias ResultType = Result<T, E> typealias Completion = ResultType -> () typealias AsyncOperation = Completion -> () private let operation: AsyncOperation } "Back to the Futures" - Javier Soto. March 2015 18
19. Future struct Future<T, E: ErrorType> { init(operation: AsyncOperation) { self.operation = operation } func start(completion: Completion) { self.operation() { result in completion(result) } } } "Back to the Futures" - Javier Soto. March 2015 19
20. Future.map(): transforming the computed value "Back to the Futures" - Javier Soto. March 2015 20
21. 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 } } "Back to the Futures" - Javier Soto. March 2015 21
22. Future.map(): transforming the computed value struct Future<T, E: ErrorType> { func map<U>(f: T -> U) -> Future<U, E> } "Back to the Futures" - Javier Soto. March 2015 22
23. 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> } "Back to the Futures" - Javier Soto. March 2015 23
24. 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 } } "Back to the Futures" - Javier Soto. March 2015 24
25. 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 } } } "Back to the Futures" - Javier Soto. March 2015 25
26. 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 .Error... switch result { } } }) } "Back to the Futures" - Javier Soto. March 2015 26
27. Future.map(): transforming the computed value case .Success(let value): // Call completion with the transformed value completion(Result.Success(f(value))) "Back to the Futures" - Javier Soto. March 2015 27
28. Future.map(): transforming the computed value case .Error(let error): // We didn't get a value: no transformation completion(Result.Error(error)) "Back to the Futures" - Javier Soto. March 2015 28
29. 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 .Success(let value): completion(Result.Success(f(value))) case .Error(let error): completion(Result.Error(error)) } } }) } "Back to the Futures" - Javier Soto. March 2015 29
30. Future.andThen(): concatenating async work "Back to the Futures" - Javier Soto. March 2015 30
31. 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) } "Back to the Futures" - Javier Soto. March 2015 31
32. Future.andThen(): concatenating async work struct Future<T, E: ErrorType> { func andThen<U>(f: T -> Future<U, E>) -> Future<U, E> } "Back to the Futures" - Javier Soto. March 2015 32
33. Future.andThen(): concatenating async work func andThen<U>(f: T -> Future<U, E>) -> Future<U, E> { return Future<U, E>(operation: { completion in } } "Back to the Futures" - Javier Soto. March 2015 33
34. 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 } } } "Back to the Futures" - Javier Soto. March 2015 34
35. 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 .Success(let value): // ... case .Error(let error): // ... } } } } "Back to the Futures" - Javier Soto. March 2015 35
36. Future.andThen(): concatenating async work case .Success(let value): let nextFuture = f(value) "Back to the Futures" - Javier Soto. March 2015 36
37. Future.andThen(): concatenating async work case .Success(let value): let nextFuture = f(value) nextFuture.start { finalResult in completion(finalResult) } "Back to the Futures" - Javier Soto. March 2015 37
38. Future.andThen(): concatenating async work case .Error(let error): completion(Result.Error(error)) "Back to the Futures" - Javier Soto. March 2015 38
39. 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 .Success(let value): f(value).start(completion) case .Error(let error): completion(Result.Error(error)) } } }) } "Back to the Futures" - Javier Soto. March 2015 39
40. Recap "Back to the Futures" - Javier Soto. March 2015 40
41. We went 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) } } } "Back to the Futures" - Javier Soto. March 2015 41
42. ... 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) } "Back to the Futures" - Javier Soto. March 2015 42
43. Limitations of Futures 4 Only represent computation to retrieve one value. 4 Can't encapsulate streams of values. 4 Only consumer (pull) driven, not producer (push) driven. "Back to the Futures" - Javier Soto. March 2015 43
44. ReactiveCocoa Signals > Futures "Back to the Futures" - Javier Soto. March 2015 44
45. Follow-up Resources 4 Playground: https://github.com/JaviSoto/Talks 4 ReactiveCocoa: https://github.com/ReactiveCocoa/ ReactiveCocoa 4 Railway Oriented Programming: http:// fsharpforfunandprofit.com/posts/recipe-part2/ "Back to the Futures" - Javier Soto. March 2015 45
46. Thanks! Questions? "Back to the Futures" - Javier Soto. March 2015 46