Going functional in Swift

Swift is so-called multi paradigm programming language, but iOS, on the other hand, is living in Object Oriented world and doesn't feel like going anywhere. From time to time we spot some functional stuff like map, filter and reduce, but are we actually using the on purpose, or is rather a coincidence? Where's the place for Protocol Oriented Programming? Let's learn something from functional programming and write better iOS applications.

#mobiconf2017

Transcript

1. Going functional in (But only a bit) #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
2. Hi, I'm Mateusz! @cojoj #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
3. #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
4. Agenda: OOP + iOS + (FP + Swift) = #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
5. Functions Inheritance Algebraic Data Types Surprise! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
6. Why Functional? · Functional Swi! by objc.io team · FRP frameworks (RxSwi!, ReactiveCocoa) · 3rd party libraries (Swi!z, Argo) · Tons of articles! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
7. They all can't be wrong, right? #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
8. What's so special about FP, that iOS dev should take a look at it? #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
9. It can help you expand your brain with some common iOS pitfalls! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
10. Ok, let's establish some common ground #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
11. Functional programming is a computer programming paradigm that relies on functions modeled on mathematical functions. — Haskell Programming from first principles #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
12. #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
13. OOP FP Single Responsibility principle Functions Open/Closed principle Functions Dependency Inversion principle Functions, also Interface Segregation principle Functions Factory pattern Yes, functions Strategy pattern Oh my, functions again! Decorator pattern Functions Visitor pattern Functions #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
14. However, it isn't about mathematics, but about abstraction and reducing complexity. #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
15. Functional programming qualities: 1. Modularity 2. A Careful Treatment of Mutable State 3. Types Taken from Functional Swi!
16. Swift is a general-purpose programming language... #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
17. It's main target are platforms, though... #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
18. ...and those still carry the burden of Objective-C... #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
19. ...which is object-oriented language! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
20. The Objective-C language was chosen for a variety of reasons. First and foremost, it’s an object-oriented language. The kind of functionality that’s packaged in the Cocoa frameworks can only be delivered through object-oriented techniques. — , Object-Oriented Programming with Objective-C #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
21. Does it make Swift OOP language as well? ! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
22. It kinda do... We're used to seeing it in OOP context, since iOS is in this context as well. #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
23. It doesn't mean we can't find any functional patterns in Swift! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
24. (...) we need to write a bit of extra code to achieve the same results that languages like Haskell have with built-in operators. — Swi! Functors, Applicatives, and Monads in Pictures #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
25. It's not perfect, but consider job done... #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
26. But sometimes misuse overgrows the purpose... #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
27. We just need to find this misuse and start using it on purpose. #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
28. Where do we start? #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
29. Functions #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
30. func viewDidLoad() { super.viewDidLoad // Setup UI navigationItem.title = "Details" // Setup delegates, data sources etc. tableView.delegate = self tableView.dataSource = self // Fetch initial data networking.fetchPosts { [weak self] json in // Serialize JSON let posts = self.decodeJSON(Post.self, from: json) // Handle response and present it self.dataSource = posts self.tableView.reloadData() // Persist fetched data for post in posts { PersistanceManager.shared.save(post) } } // Set some user defaults UserDefaults.standard.set("UserHasSeenEULA", forKey: "Permissions") } #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
31. Higher-order functions #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
32. 1. Takes one or more functions as arguments 2. Returns a function as its result #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
33. Swift higher order functions: map flatMap filter reduce forEach first drop contains sorted #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
34. Their power lies in chaining (And being super generic) #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
35. // Adding additional step to this chain is super simple. // We don't have any nested loops, if statements etc. ["1", "2", "7", "Eight", "Nine", "10"] .flatMap(Int.init) .map { $0 * 3 } .filter { $0 > 5 } .reduce(0, +) #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
36. Actually, that's what FRP frameworks do! self.provider .request(.login(email: self.username.value, password: self.password.value)) .filterSuccessfulStatusCodes() .mapJSON() .withLatestFrom(self.shouldRemeberUser) { (_, shouldRemeber) in let username = self.username.value let password = self.password.value return (username, password, shouldRemeber) } .subscribe { (username, _, _) in print("Hi, \(username)") } .disposed(by: disposeBag) #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
37. But, be careful... #wayTooComplex #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
38. Pure functions #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
39. 1. The function always evaluates the same result value given the same argument value 2. Evaluation of the result does not cause any semantically observable side effect or output #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
40. Translated into Swiftish 1. If you need something inside the function, pass it with arguments 2. Don't write functions without return statement (if you don't have to) #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
41. // Sample of nice, predictable, pure function // Imagine how easy it's to write a test cases for it! func decode<T: Decodable>(for model: T.Type, from json: [String: Any]) throws -> T // This is a function depending on some external state, not returning anything. // To test it we'd need to mock the whole context... func decode() #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
42. Function Composition #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
43. An act or mechanism to combine simple functions to build more complicated ones. — Wikipedia #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
44. Sounds super simple, right? #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
45. // It's totally unreadable... removeInvalidRows(fromRows: createRows(fromLines: splitLines(ofText: csv))) https://github.com/ijoshsmith/function-composition-in-swi!
46. It looks super UGLY! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
47. // That's why we tend to do it this way let lines = splitLines(ofText: csv) let rows = createRows(fromLines: lines) let validRows = removeInvalidRows(fromRows: rows) https://github.com/ijoshsmith/function-composition-in-swi!
48. -- Just to give you a grasp of how beautiful -- and simple it may look in Haskell splitLines . createRows . removeInvalidRows $ csv #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
49. // If we define a function like this one func ∘ <A, B, C> (aToB: @escaping (A) -> B, bToC: @escaping (B) -> C) -> (A) -> C { return { a in let b = aToB(a) let c = bToC(b) return c } } // We can get much better code let processCSV = splitLines() ∘ createRows() ∘ removeInvalidRows() let validRows = processCSV(csv) https://github.com/ijoshsmith/function-composition-in-swi!
50. Inheritance #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
51. open class NSArray: NSObject, CKRecordValue, CustomReflectable, CVarArg, Equatable, ExpressibleByArrayLiteral, Hashable, NSCopying, NSFastEnumeration, NSMutableCopying, NSSecureCoding, Sequence #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
52. Type classes #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
53. Way of creating the type composition in the world without inheritance. — Krzysztof Siejkowski #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
54. -- This is just a declaration of typeclass class Eq a where (==) :: a -> a -> Bool (/=) :: a -> a -> Bool x == y = not (x /= y) x /= y = not (x == y) #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
55. -- Just a sample data type data TrafficLight = Red | Yellow | Green -- Here we implement instance instance Eq TrafficLight where Red == Red = True Green == Green = True Yellow == Yellow = True _ == _ = False #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
56. Haskell's Typeclasses key to success is the amount of instances provided by basic libraries! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
57. 227 instances implements Eq! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
58. Protocols #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
59. protocol Eq { associatedtype T func equal(_ to: T) -> Bool } extension Eq { func notEqual(_ to: T) -> Bool { return !equal(to) } } extension Int: Eq { func equal(_ to: Int) -> Bool { return self == to } } 1.equal(2) // Prints false 1.notEqual(3) // Prints true #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
60. In Swift/CocoaTouch over 1000 elements implement Equatable! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
61. Typeclasses give us more abstraction, precision and flexibility #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
62. Next time you write some code, please think about this... #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
63. Do I need it only for X or should it work for all Ys as well? #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
64. Algebraic Data Types #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
65. URLSession.shared.dataTask(with: url, completionHandler: { (data, response, error) in guard let data = data else { if let error = error { print(error.localizedDescription) } return } let decoder = JSONDecoder() let result = try! decoder.decode([String: String].self, from: data) }).resume() #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
66. Product Types Sum Types #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
67. Type that can be one of multiple possible options. Each option can carry a different type inside of it and, crucially, each option has a unique label. — Tikhon Jelvis #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
68. // Really simple sum types data Bool = False | True data Digit = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 // Sum types with parameters data Maybe a = Just a | Nothing data Either a b = Left a | Right b #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
69. Enumerations #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
70. enum Optional<T> { case none case some(T) } #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
71. Optionality can sometimes mean more than just an existence. — paraphrased Soroush Khanlou #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
72. func safeDiv(x: Float, y: Float) -> Float? { guard y != 0 else { return nil } return x / y } #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
73. How do we inform our user that he needs to take math classes again? #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
74. enum Either<T1, T2> { case left(T1) case right(T2) } func safeDiv(x: Float, y: Float) -> Either<String, Float> { guard y != 0 else { return .left("Error: division by zero") } return .right(x / y) } #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
75. Either can be used as a generalization of optional types in which Left not only encodes failure, but is accompanied by an error message — Marcello Seri #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
76. Sum types give us exhaustiveness and more certainty! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
77. Go and read "That One Optional Property" by Soroush Khanlou #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
78. Pssst, Sum Types play pretty well with pattern matching! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
79. Last, but not least... #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
81. Functors Applicative Functors Monads #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
82. ? is a Monad, period. #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
83. if let beginsWithThe = john.residence?.address?.buildingIdentifier()?.hasPrefix("The") { if beginsWithThe { print("John's building identifier begins with \"The\".") } else { print("John's building identifier does not begin with \"The\".") } } #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
84. You take a wrapped value, you apply a function to it and you pack it back in the wrapper! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
85. // PromiseKit class Promise<T> { func then<U>(body: T -> U) -> Promise<U> func then<U>(body: T -> Promise<U>) -> Promise<U> } // RxSwift extension ObservableType { func map<R>(transform: E -> R) -> Observable<R> func flatMap<O: ObservableConvertibleType>(selector: E -> O) -> Observable<O.E> } // Interstellar extension Observable { func map<U>(transform: T -> U) -> Observable<U> func flatMap<U>(transform: T -> Observable<U>) -> Observable<U> } #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
86. What are they useful for? #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
87. ⛓ Chaining #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
88. But you've said the same thing about Higher Order Functions... #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
89. Because most types implementing them are Monads! #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
90. And that's the beauty of FP #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
91. (Actually, it's thanks to Category Theory) #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
92. #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
93. Quick summary... #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
94. In Swift, you fundamentally work in a procedural/OOP paradigm, and the whole language is built around that. There are some tools available to let you jump over to a functional style (without the full power of functional programming) when it’s useful. — Rob Napier #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
97. ❔ Qestions ❔ #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017
98. · http://robnapier.net/swi!-is-not-functional · http://www.mokacoding.com/blog/functor- applicative-monads-in-pictures/ · https://github.com/alskipp/Swi!-Adventures-In- Monad-Land · http://khanlou.com/2017/03/that-one-optional- property/ · https://github.com/ijoshsmith/function-composition- in-swi! · http://learnyouahaskell.com #mobiconf2017 MOBICONF.ORG | 5-6 OCTOBER 2017