Swift Sync and Async Error Handling NSMeetup San Francisco - July 2015 — @Javi "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 1

Agenda 4 Traditional Asynchronous Code 4 Problems with Traditional Error Handling 4 Result vs throws 4 Future 4 Future.map 4 Future.andThen 4 Making Result and throws play together "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 2

Traditional asynchronous code "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 3

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!) } } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 4

Traditional asynchronous code "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 5

Traditional asynchronous code func downloadImage(URL: NSURL, completion: (UIImage?, NSError?) -> ()) (.Some, .None) (.None, .Some) (.Some, .Some) (.None, .None) "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 6

(.Some, .Some) (.None, .None) "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 7

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 { } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 8

Error Handling var error: NSError? let string = NSString(contentsOfFile:path encoding:NSUTF8Encoding error:&error) if string == nil { // Oops: if error.code == NSFileReadNoSuchFileError { // ... } else if ... } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 9

"Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 10

"Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 11

"Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 12

NSError Alternative protocol ErrorType { } enum UserInfoErrorDomain: ErrorType { case UserDoesNotExist case UserRequestFailure(reason: String) case NetworkRequestFailure(reason: String) } extension NSError: ErrorType { } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 13

NSError Alternative enum NoError: ErrorType { } let error = NoError(?) "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 14

Result enum Result { case Success(T) case Failure(E) } Result by Rob Rix: https://github.com/antitypical/Result "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 15

Swift's throws "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 16

Swift's throws func readFile(path: String) throws -> NSData "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 17

Swift's throws func readFile(path: String) throws -> NSData // throws... what exactly? "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 18

Swift's throws func readFile(path: String) throws -> NSData Functionally equivalent to func readFile(path: String) -> Result "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 19

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. } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 20

Swift's throws func readFile(path: String) -> Result let file = readFile("path/to/my/file") switch file { case let .Succcess(value): // ... case let .Failure(error): // This `error` variable has type information! } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 21

Swift's throws Hopefully in a future beta... func readFile(path: String) throws FileIOError -> NSData "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 22

Error handling in Asynchronous APIs "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 23

Future "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 24

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. "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 25

Future struct Future { typealias ResultType = Result typealias Completion = ResultType -> () typealias AsyncOperation = Completion -> () private let operation: AsyncOperation } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 26

Future struct Future { init(operation: AsyncOperation) { self.operation = operation } func start(completion: Completion) { self.operation() { result in completion(result) } } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 27

Future.map(): transforming the computed value "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 28

Future.map(): transforming the computed value struct User { let avatarURL: NSURL } func requestUserInfo(userID: String) -> Future func requestUserAvatarURL(userID: String) -> Future { return requestUserInfo(userID) .map { $0.avatarURL } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 29

Future.map(): transforming the computed value struct Future { func map(f: T -> U) -> Future } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 30

map() in other types struct Array { func map(f: T -> U) -> [U] } enum Optional { func map(f: T -> U) -> U? } struct Future { func map(f: T -> U) -> Future } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 31

Future.map(): transforming the computed value func map(f: T -> U) -> Future { // Return a new Future w/ a new operation... return Future(operation: { completion in } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 32

Future.map(): transforming the computed value func map(f: T -> U) -> Future { return Future(operation: { completion in // Retrieve the value from self... self.start { result in } } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 33

Future.map(): transforming the computed value func map(f: T -> U) -> Future { return Future(operation: { completion in self.start { result in // Consider .Success and .Failure... switch result { } } }) } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 34

Future.map(): transforming the computed value case let .Success(value): // Call completion with the transformed value completion(Result.Success(f(value))) "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 35

Future.map(): transforming the computed value case let .Failure(error): // We didn't get a value: no transformation completion(Result.Failure(error)) "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 36

Future.map(): transforming the computed value func map(f: T -> U) -> Future { return Future(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)) } } }) } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 37

Future.andThen(): concatenating async work "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 38

Future.andThen(): concatenating async work func requestUserAvatarURL(userID: String) -> Future func downloadImage(URL: NSURL) -> Future func downloadUserAvatar(userID: String) -> Future { return requestUserAvatarURL(userID) .andThen(downloadImage) } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 39

Future.andThen(): concatenating async work struct Future { func andThen(f: T -> Future) -> Future } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 40

Future.andThen(): concatenating async work func andThen(f: T -> Future) -> Future { return Future(operation: { completion in } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 41

Future.andThen(): concatenating async work func andThen(f: T -> Future) -> Future { return Future(operation: { completion in self.start { firstFutureResult in } } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 42

Future.andThen(): concatenating async work func andThen(f: T -> Future) -> Future { return Future(operation: { completion in self.start { firstFutureResult in switch firstFutureResult { case let .Success(value): // ... case let .Failure(error): // ... } } } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 43

Future.andThen(): concatenating async work case let .Success(value): let nextFuture = f(value) "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 44

Future.andThen(): concatenating async work case let .Success(value): let nextFuture = f(value) nextFuture.start { finalResult in completion(finalResult) } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 45

Future.andThen(): concatenating async work case let .Failure(error): completion(Result.Failure(error)) "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 46

Future.andThen(): concatenating async work func andThen(f: T -> Future) -> Future { return Future(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)) } } }) } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 47

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) } } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 48

... To this func requestUserInfo(userID: String) -> Future func downloadImage(URL: NSURL) -> Future func loadAvatar(userID: String) -> Future { return requestUserInfo(userID) .map { $0.avatarURL } .andThen(downloadImage) } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 49

Mixing it all up Result and throws "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 50

Result and throws throws => Result extension Result { public init(@autoclosure _ f: () throws -> T) { do { self = .Success(try f()) } catch { self = .Failure(error as! Error) } } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 51

Result and throws Result => throws extension Result { public func dematerialize() throws -> T { switch self { case .Success(value): return value case let .Failure(error): throw error } } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 52

Result and throws Future => throws let avatarFuture = loadAvatar("4815162342") avatarFuture.start { result in do { let avatar = result.materialize() } catch { // handle `error` } } "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 53

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. "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 54

ReactiveCocoa Signals > Futures "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 55

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/ "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 56

Thanks! Questions? "Swift Sync and Async Error Handling" - Javier Soto. NSMeetup San Francisco - July 2015 57