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Swift?! - Alternatives to Optionals

Swift?! - Alternatives to Optionals

This talk explains the challenges and possibilities for designing more attractive APIs with Swift to escape building deep-nested branch hierarchies to unwrap optionals. It uncovers some alternatives to NSErrorPointers and function callbacks, not only for synchronous, but especially for expensive asynchronous operations.

Marius Rackwitz

March 02, 2015
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  1. 3

  2. enum Optional<T> : NilLiteralConvertible { case None case Some(T) ///

    Construct a `nil` instance. init() /// Construct a non-\ `nil` instance that stores `some`. init(_ some: T) /// Create an instance initialized with `nil`. init(nilLiteral: ()) /// If `self == nil`, returns `nil`. Otherwise, returns `f(self!)`. func map<U>(f: @noescape (T) -> U) -> U? } — Swift Standard Library 10
  3. /// An optional type that allows implicit member access (via

    compiler /// magic). /// /// The compiler has special knowledge of the existence of /// ImplicitlyUnwrappedOptional<T>, but always interacts with it using the /// library intrinsics below. enum ImplicitlyUnwrappedOptional<T> : Reflectable, NilLiteralConvertible { case None case Some(T) // … } — Swift Standard Library 11
  4. // Represents a StarWars film final class Film { //

    The url of this resource let url: String // The episode number of this film. let episodeId: Int // The title of this film. let title: String // The opening crawl text at the beginning of this film. let openingCrawl: String // The director of this film. let director: String //="George Lucas" // The producer(s) of this film. let producer: String } 13
  5. extension Film { static func decode(data: AnyObject) -> Film? {

    if let url = data["url"] as? String { if let episodeId = data["episode_id"] as? Int { if let title = data["title"] as? String { if let openingCrawl = data["opening_crawl"] as? String { if let director = data["director"] as? String { if let producer = data["producer"] as? String { return Film( url: url, episodeId: episodeId, title: title, openingCrawl: openingCrawl, director: director, producer: producer ) } } } } } } } } 14
  6. extension Film { static func decode(data: AnyObject) -> Film? {

    if let url = data["url"] as? String, let episodeId = data["episode_id"] as? Int, let title = data["title"] as? String, let openingCrawl = data["opening_crawl"] as? String, let director = data["director"] as? String, let producer = data["producer"] as? String { return Film(url: url, episodeId: episodeId, title: title, openingCrawl: openingCrawl, director: director, producer: producer) } } } 15
  7. import Argo extension Film { static func create(url: String)(episodeId: Int)(title:

    String)(openingCrawl: String) (director: String)(producer: String) -> Film { return Film(url: url, episodeId: episodeId, title: title, openingCrawl: openingCrawl, director: director, producer: producer) } static func decode(j: JSONValue) -> Film? { return Film.create <^> j <| "url" <*> j <| "episode_id" <*> j <| "title" <*> j <| "opening_crawl" <*> j <| "director" <*> j <| "producer" } } 16
  8. Optional Return Value + Error Pointer func request(urlString: String, error:

    NSErrorPointer) -> String? { … } var error: NSError? let result: NSString? = request("http://api.giphy.com/v1/gifs/search?q=doge", &error) if let e = error { println"(\(e)") } else { if let urlString = result { image.url = NSURL(urlString) } } 17
  9. Pairs of Optionals func request(urlString: String) -> (String?, NSError?) {

    … } let result: (String?, NSError?) = request("http://api.giphy.com/v1/gifs/search?q=doge") if let error = result.1 { println("No luck today: \(error)") } else { if let urlString = result.0 { image.url = NSURL(urlString) } } 18
  10. Named Optional Tuple Components func request(urlString: String) -> (String?, NSError?)

    { … } let result: (urlString: String?, error: NSError?) = request("http://api.giphy.com/v1/gifs/search?q=doge") if error = result.error { println("No luck today: \(error)") } else { if urlString = result.urlString { image.url = NSURL(urlString) } } 19
  11. Either Success or Failure final class Box<T> { let value:

    T public init(_ value: T) { self.value = value } } enum Result<T> { case Success(Box<T>) case Failure(NSError) } 23
  12. Either Success or Failure func request(urlString: String) -> Result<String> {

    … } let result: Result<String> = request("http://api.giphy.com/v1/gifs/search?q=doge") switch result { case Success(let box): image.url = NSURL(box.value) case Error(error: NSError): println("\(error)") } 24
  13. Using LlamaKit import LlamaKit func request(urlString: String, error: NSErrorPointer) ->

    String { … } let result = try { (error) in request("http://api.giphy.com/v1/gifs/search?q=doge", &error) } switch result { case Success(Box(urlString: String)): image.url = NSURL(urlString) case Error(error: NSError): println("\(error)") } 25
  14. Promises of Bright Futures import BrightFutures // or PromiseKit, or

    … func request(urlString: String) -> Future<String> { let promise = Promise<String>() Queue.global.async { var error: NSError? let result = request(urlString, error: &error) if let e = error { promise.failure(e) } else { promise.success(result!) } } return promise.future } request("http://api.giphy.com/v1/gifs/search?q=doge").onSuccess { urlString in image.url = NSURL(urlString) }.onFailure { error in println("\(error)") } 26
  15. Reactive Sneak Peek ⾠ Swift API not settled yet. import

    ReactiveCocoa let result: SignalProducer<String, NSError> = request("http://api.giphy.com/v1/gifs/search?q=doge") result.start(next: { urlString in image.url = NSURL(urlString) }, error: { error in println("\(error)") }, completed: { }) 27
  16. Summary: Swift allows to … • Design more meaningful interfaces

    • Restrict semantics • Increase Provability of Correctness • Reduce number of tests ! 28
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