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Demystifying Tomás Ruiz-López @tomasruizlopez Functional Programming Swifters December 2020

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HARD… Functional Programming is

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EASY! Functional Programming is

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Functional Programming? What is

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Functions Programming with

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Provide output for every input Total

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Total func divide(_ x: Double, _ y: Double) -> Double

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Total func divide(_ x: Double, _ y: Double) -> Double { guard y != 0 else { fatalError("Denominator cannot be zero") } return x / y }

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Total func divide(_ x: Double, _ y: Double) throws -> Double { guard y != 0 else { throw DivideError.divisionByZero } return x / y }

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Total func divide(_ x: Double, _ y: Double) -> Result { guard y != 0 else { return .failure(.divisionByZero) } return .success(x / y) }

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Deterministic Provide the same output for a given input

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Deterministic func makeFilename(prefix: String, extension ext: String) -> String { let now = Date() return "\(prefix)-\(now).\(ext)" }

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Deterministic func makeFilename(prefix: String, extension ext: String, now: Date) -> String { return "\(prefix)-\(now).\(ext)" }

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Pure Only compute output, and nothing else

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Pure “How can I do something useful in my app without side e f f ects?”

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Pure func greet(name: String) -> Void { print("Hello \(name)!") } greet(name: “Swifters") > Hello Swifters!

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Pure func greet(name: String) -> AnyPublisher { Future { promise in print("Hello \(name)!") promise(.success(())) }.eraseToAnyPublisher() }

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Pure func greet(name: String) -> AnyPublisher { Future { promise in print("Hello \(name)!") promise(.success(())) }.eraseToAnyPublisher() } let publisher = greet(name: “Swifters") > Hello Swifters!

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Pure func greet(name: String) -> AnyPublisher { Deferred { Future { promise in print("Hello \(name)!") promise(.success(())) } }.eraseToAnyPublisher() }

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Pure func greet(name: String) -> AnyPublisher { Deferred { Future { promise in print("Hello \(name)!") promise(.success(())) } }.eraseToAnyPublisher() } let publisher = greet(name: “Swifters") >

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Pure func greet(name: String) -> AnyPublisher { Deferred { Future { promise in print("Hello \(name)!") promise(.success(())) } }.eraseToAnyPublisher() } let cancellable = greet(name: “Swifters").sink { _ in } > Hello Swifters!

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Pure Why does it matter?

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reasoning We want to have a sound about our code

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Quiz How many times does it print “Hello Swifters!”? func greet(name: String) -> AnyPublisher { Future { promise in print("Hello \(name)!") promise(.success(())) }.eraseToAnyPublisher() }

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Quiz How many times does it print “Hello Swifters!”? func greet(name: String) -> AnyPublisher { Future { promise in print("Hello \(name)!") promise(.success(())) }.eraseToAnyPublisher() } let cancellable = greet(name: "Swifters").flatMap { _ in greet(name: “Swifters") }.sink { _ in }

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Quiz How many times does it print “Hello Swifters!”? func greet(name: String) -> AnyPublisher { Future { promise in print("Hello \(name)!") promise(.success(())) }.eraseToAnyPublisher() } let cancellable = greet(name: "Swifters").flatMap { _ in greet(name: “Swifters") }.sink { _ in } Solution: 2

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Quiz How many times does it print “Hello Swifters!”? func greet(name: String) -> AnyPublisher { Future { promise in print("Hello \(name)!") promise(.success(())) }.eraseToAnyPublisher() } let publisher = greet(name: “Swifters") let cancellable = publisher.flatMap { _ in publisher }.sink { _ in }

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Quiz How many times does it print “Hello Swifters!”? func greet(name: String) -> AnyPublisher { Future { promise in print("Hello \(name)!") promise(.success(())) }.eraseToAnyPublisher() } let publisher = greet(name: “Swifters") let cancellable = publisher.flatMap { _ in publisher }.sink { _ in } Solution: 1

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Referential Transparency Eager evaluation breaks

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Quiz How many times does it print “Hello Swifters!”? func greet(name: String) -> AnyPublisher { Deferred { Future { promise in print("Hello \(name)!") promise(.success(())) } }.eraseToAnyPublisher() }

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Quiz How many times does it print “Hello Swifters!”? let cancellable = greet(name: "Swifters").flatMap { _ in greet(name: “Swifters") }.sink { _ in } func greet(name: String) -> AnyPublisher { Deferred { Future { promise in print("Hello \(name)!") promise(.success(())) } }.eraseToAnyPublisher() }

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Quiz How many times does it print “Hello Swifters!”? let cancellable = greet(name: "Swifters").flatMap { _ in greet(name: “Swifters") }.sink { _ in } Solution: 2 func greet(name: String) -> AnyPublisher { Deferred { Future { promise in print("Hello \(name)!") promise(.success(())) } }.eraseToAnyPublisher() }

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Quiz How many times does it print “Hello Swifters!”? func greet(name: String) -> AnyPublisher { Deferred { Future { promise in print("Hello \(name)!") promise(.success(())) } }.eraseToAnyPublisher() } let publisher = greet(name: “Swifters") let cancellable = publisher.flatMap { _ in publisher }.sink { _ in }

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Quiz How many times does it print “Hello Swifters!”? Solution: 2 func greet(name: String) -> AnyPublisher { Deferred { Future { promise in print("Hello \(name)!") promise(.success(())) } }.eraseToAnyPublisher() } let publisher = greet(name: “Swifters") let cancellable = publisher.flatMap { _ in publisher }.sink { _ in }

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Pure func greet(name: String) -> IO { IO.invoke { print("Hello \(name)!") } } greet(name: “Swifters").unsafeRunSync() > Hello Swifters! bow-swift.io

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side effects Model as values

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Functional Programming This is all you need to know about

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Composition Everything else is

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Composition func compose( _ g: @escaping (B) -> C, _ f: @escaping (A) -> B ) -> (A) -> C { { a in g(f(a)) } }

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Composition func compose( _ g: @escaping (B) -> C, _ f: @escaping (A) -> B ) -> (A) -> C { { a in g(f(a)) } } B C A

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Composition func compose( _ g: @escaping (B) -> C, _ f: @escaping (A) -> B ) -> (A) -> C { { a in g(f(a)) } } B C A g

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Composition func compose( _ g: @escaping (B) -> C, _ f: @escaping (A) -> B ) -> (A) -> C { { a in g(f(a)) } } B C A f g

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Composition func compose( _ g: @escaping (B) -> C, _ f: @escaping (A) -> B ) -> (A) -> C { { a in g(f(a)) } } B C A f g g · f

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Composition func andThen( _ f: @escaping (A) -> B, _ g: @escaping (B) -> C ) -> (A) -> C { { a in g(f(a)) } } B C A f g g · f

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Higher-order Functions Possible thanks to

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Composition is not always as easy

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Composition func compose( _ f: @escaping (A) -> B?, _ g: @escaping (B) -> C ) -> (A) -> ??? { // What goes here? }

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Composition B? A f func compose( _ f: @escaping (A) -> B?, _ g: @escaping (B) -> C ) -> (A) -> ??? { // What goes here? }

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Composition B? A f C g B func compose( _ f: @escaping (A) -> B?, _ g: @escaping (B) -> C ) -> (A) -> ??? { // What goes here? }

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Composition B? A f C g B func compose( _ f: @escaping (A) -> B?, _ g: @escaping (B) -> C ) -> (A) -> ??? { // What goes here? }

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Composition B? A f C g B C? map func compose( _ f: @escaping (A) -> B?, _ g: @escaping (B) -> C ) -> (A) -> ??? { // What goes here? }

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Composition func compose( _ f: @escaping (A) -> B?, _ g: @escaping (B) -> C ) -> (A) -> C? { { a in f(a).map(g) } } B? A f C g B C? map

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Composition func compose( _ f: @escaping (A) -> [B], _ g: @escaping (B) -> C ) -> (A) -> [C] { { a in f(a).map(g) } } func compose( _ f: @escaping (A) -> Result, _ g: @escaping (B) -> C ) -> (A) -> Result { { a in f(a).map(g) } }

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Functor Compose functions using map func compose( _ f: @escaping (A) -> F, _ g: @escaping (B) -> C ) -> (A) -> F { { a in f(a).map(g) } }

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Higher-Kinded Types Not possible in Swift, we need

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Functor Compose functions using map func compose( _ f: @escaping (A) -> Kind, _ g: @escaping (B) -> C ) -> (A) -> Kind { { a in f(a).map(g) } } bow-swift.io

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Composition func compose( _ f: @escaping (A) -> B?, _ g: @escaping (B) -> C? ) -> ??? { // What goes here? }

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Composition B? A f func compose( _ f: @escaping (A) -> B?, _ g: @escaping (B) -> C? ) -> ??? { // What goes here? }

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Composition B? A f C? g B func compose( _ f: @escaping (A) -> B?, _ g: @escaping (B) -> C? ) -> ??? { // What goes here? }

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Composition B? A f C? g B func compose( _ f: @escaping (A) -> B?, _ g: @escaping (B) -> C? ) -> ??? { // What goes here? }

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Composition B? A f C? g B C? f latMap func compose( _ f: @escaping (A) -> B?, _ g: @escaping (B) -> C? ) -> ??? { // What goes here? }

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Composition B? A f C? g B C? f latMap func compose( _ f: @escaping (A) -> B?, _ g: @escaping (B) -> C? ) -> (A) -> C? { { a in f(a).flatMap(g) } }

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Composition func compose( _ f: @escaping (A) -> [B], _ g: @escaping (B) -> [C] ) -> (A) -> [C] { { a in f(a).flatMap(g) } } func compose( _ f: @escaping (A) -> Result, _ g: @escaping (B) -> Result ) -> (A) -> Result { { a in f(a).flatMap(g) } }

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Monad Compose functions using f latMap func compose( _ f: @escaping (A) -> F, _ g: @escaping (B) -> F ) -> (A) -> F { { a in f(a).flatMap(g) } }

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Monad Compose functions using f latMap func compose( _ f: @escaping (A) -> Kind, _ g: @escaping (B) -> Kind ) -> (A) -> Kind { { a in f(a).flatMap(g) } } bow-swift.io

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Composition func zip( _ a: A?, _ b: B?, with f: @escaping (A, B) -> C ) -> ??? { // What goes here? }

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Composition func zip( _ a: A?, _ b: B?, with f: @escaping (A, B) -> C ) -> ??? { // What goes here? } A? B?

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Composition func zip( _ a: A?, _ b: B?, with f: @escaping (A, B) -> C ) -> ??? { // What goes here? } A? B? A B C f

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Composition func zip( _ a: A?, _ b: B?, with f: @escaping (A, B) -> C ) -> ??? { // What goes here? } A? B? A B C f

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Composition func zip( _ a: A?, _ b: B?, with f: @escaping (A, B) -> C ) -> ??? { // What goes here? } A? B? A B C f C? zip

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Composition func zip( _ a: A?, _ b: B?, with f: @escaping (A, B) -> C ) -> C? { a.flatMap { x in b.map { y in f(x, y) } } } A? B? A B C f C? zip

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Composition func zip( _ a: [A], _ b: [B], with f: @escaping (A, B) -> C ) -> [C] { a.flatMap { x in b.map { y in f(x, y) } } } func zip( _ a: Result, _ b: Result, with f: @escaping (A, B) -> C ) -> Result { a.flatMap { x in b.map { y in f(x, y) } } }

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cumbersome Functional Programming terminology is

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unfamiliar Functional Programming terminology is

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Composition func loadWebResource(_ path: String) -> Resource? func decodeImage(_ r1: Resource, _ r2: Resource) -> Image? func dewarpAndCleanup(_ image: Image) -> Image? func processImageData() -> Image? { loadWebResource("dataprofile.txt").flatMap { dataRes in loadWebResource("imagedata.dat").flatMap { imageRes in decodeImage(dataRes, imageRes).flatMap { imageTmp in dewarpAndCleanup(imageTmp) } } } }

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Composition func loadWebResource(_ path: String) -> Resource? func decodeImage(_ r1: Resource, _ r2: Resource) -> Image? func dewarpAndCleanup(_ image: Image) -> Image? func processImageData() -> Image? { loadWebResource("dataprofile.txt").flatMap { dataRes in loadWebResource("imagedata.dat").flatMap { imageRes in decodeImage(dataRes, imageRes).flatMap { imageTmp in dewarpAndCleanup(imageTmp) } } } }

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Composition func loadWebResource(_ path: String) -> Resource? func decodeImage(_ r1: Resource, _ r2: Resource) -> Image? func dewarpAndCleanup(_ image: Image) -> Image? func processImageData() -> Image? { if let dataRes = loadWebResource("dataprofile.txt"), let imageRes = loadWebResource("imagedata.dat"), let imageTmp = decodeImage(dataRes, imageRes) { return dewarpAndCleanup(imageTmp) } else { return nil } }

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Composition func loadWebResource(_ path: String) -> Result func decodeImage(_ r1: Resource, _ r2: Resource) -> Result func dewarpAndCleanup(_ image: Image) -> Result func processImageData() -> Result { loadWebResource("dataprofile.txt").flatMap { dataRes in loadWebResource("imagedata.dat").flatMap { imageRes in decodeImage(dataRes, imageRes).flatMap { imageTmp in dewarpAndCleanup(imageTmp) } } } }

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Composition func loadWebResource(_ path: String) -> AnyPublisher func decodeImage(_ r1: Resource, _ r2: Resource) -> AnyPublisher func dewarpAndCleanup(_ image: Image) -> AnyPublisher func processImageData() -> AnyPublisher { loadWebResource("dataprofile.txt").flatMap { dataRes in loadWebResource("imagedata.dat").flatMap { imageRes in decodeImage(dataRes, imageRes).flatMap { imageTmp in dewarpAndCleanup(imageTmp) } } } }

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Monad Comprehensions Composition meets imperative syntax

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Composition func loadWebResource(_ path: String) async throws -> Resource func decodeImage(_ r1: Resource, _ r2: Resource) async throws -> Image func dewarpAndCleanup(_ image: Image) async throws -> Image func processImageData() async throws -> Image { let dataRes = await try loadWebResource("dataprofile.txt") let imageRes = await try loadWebResource("imagedata.dat") let imageTmp = await try decodeImage(dataRes, imageRes) return await try dewarpAndCleanup(imageTmp) }

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Composition func processImageData() -> IO { let dataRes = IO.var() let imageRes = IO.var() let imageTmp = IO.var() let image = IO.var() return binding { dataRes <- loadWebResource("dataProfile.txt") imageRes <- loadWebResource("imagedata.dat") imageTmp <- decode(dataRes.get, imageRes.get) image <- dewarpAndCleanup(imageTmp.get) } yield: { image.get }^ } bow-swift.io

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awesome Swift is for Functional Programming

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Why choose? Take what’s best from both

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Demystifying Tomás Ruiz-López @tomasruizlopez Functional Programming Swifters December 2020