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.

Transcript

1. Back to the Futures
   — @Javi
   "Back to the Futures" - Javier Soto. March 2015 1
   

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2. Agenda
   4 Traditional Asynchronous Code
   4 Problems with Traditional Error Handling
   4 Future
   4 Future.map
   4 Future.andThen
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3. Traditional asynchronous code
   "Back to the Futures" - Javier Soto. March 2015 3
   

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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
   

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5. Traditional asynchronous code
   "Back to the Futures" - Javier Soto. March 2015 5
   

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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
   

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7. 4 (.Some, .Some)
   4 (.None, .None)
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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
   

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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
   

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10. "Back to the Futures" - Javier Soto. March 2015 10
    

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11. "Back to the Futures" - Javier Soto. March 2015 11
    

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12. "Back to the Futures" - Javier Soto. March 2015 12
    

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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
    

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14. NSError Alternative
    enum NoError: ErrorType { }
    let error = NoError(?)
    "Back to the Futures" - Javier Soto. March 2015 14
    

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15. Result
    enum Result {
    case Success(T)
    case Error(E)
    }
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16. Say hello to
    Future
    "Back to the Futures" - Javier Soto. March 2015 16
    

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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
    

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18. Future
    struct Future {
    typealias ResultType = Result
    typealias Completion = ResultType -> ()
    typealias AsyncOperation = Completion -> ()
    private let operation: AsyncOperation
    }
    "Back to the Futures" - Javier Soto. March 2015 18
    

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19. Future
    struct Future {
    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
    

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20. Future.map(): transforming the computed value
    "Back to the Futures" - Javier Soto. March 2015 20
    

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21. 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 }
    }
    "Back to the Futures" - Javier Soto. March 2015 21
    

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22. Future.map(): transforming the computed value
    struct Future {
    func map(f: T -> U) -> Future
    }
    "Back to the Futures" - Javier Soto. March 2015 22
    

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23. 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
    }
    "Back to the Futures" - Javier Soto. March 2015 23
    

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24. 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
    }
    }
    "Back to the Futures" - Javier Soto. March 2015 24
    

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25. 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
    }
    }
    }
    "Back to the Futures" - Javier Soto. March 2015 25
    

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26. Future.map(): transforming the computed value
    func map(f: T -> U) -> Future {
    return Future(operation: { completion in
    self.start { result in
    // Consider .Success and .Error...
    switch result {
    }
    }
    })
    }
    "Back to the Futures" - Javier Soto. March 2015 26
    

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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
    

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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
    

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29. Future.map(): transforming the computed value
    func map(f: T -> U) -> Future {
    return Future(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
    

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30. Future.andThen(): concatenating async work
    "Back to the Futures" - Javier Soto. March 2015 30
    

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31. 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)
    }
    "Back to the Futures" - Javier Soto. March 2015 31
    

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32. Future.andThen(): concatenating async work
    struct Future {
    func andThen(f: T -> Future) -> Future
    }
    "Back to the Futures" - Javier Soto. March 2015 32
    

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33. Future.andThen(): concatenating async work
    func andThen(f: T -> Future) -> Future {
    return Future(operation: { completion in
    }
    }
    "Back to the Futures" - Javier Soto. March 2015 33
    

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34. Future.andThen(): concatenating async work
    func andThen(f: T -> Future) -> Future {
    return Future(operation: { completion in
    self.start { firstFutureResult in
    }
    }
    }
    "Back to the Futures" - Javier Soto. March 2015 34
    

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35. Future.andThen(): concatenating async work
    func andThen(f: T -> Future) -> Future {
    return Future(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
    

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36. Future.andThen(): concatenating async work
    case .Success(let value):
    let nextFuture = f(value)
    "Back to the Futures" - Javier Soto. March 2015 36
    

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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
    

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38. Future.andThen(): concatenating async work
    case .Error(let error):
    completion(Result.Error(error))
    "Back to the Futures" - Javier Soto. March 2015 38
    

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39. Future.andThen(): concatenating async work
    func andThen(f: T -> Future) -> Future {
    return Future(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
    

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40. Recap
    "Back to the Futures" - Javier Soto. March 2015 40
    

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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
    

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42. ... 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)
    }
    "Back to the Futures" - Javier Soto. March 2015 42
    

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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
    

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44. ReactiveCocoa
    Signals > Futures
    "Back to the Futures" - Javier Soto. March 2015 44
    

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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
    

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46. Thanks!
    Questions?
    "Back to the Futures" - Javier Soto. March 2015 46
    

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