Swift Functional Programming

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Swift Functional Programming @parisba

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Agenda • What is Swift • Why bother? • Installing Swift • Playgrounds/REPL • Functional Programming Fundamentals • Swift Functional Programming

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This is mostly a “how Swift does it” talk

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A quick overview. Not all-inclusive.

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Swift?! • Similar to Rust, Perl 6, and a bunch of other languages… • Another trendy, hip, cool, modern programming language… • Actually pretty good…

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Why does Swift matter?

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Swift is open source. swift.org

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Swift is easy to teach.

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iOS.

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http://blog.appannie.com/app-annie-2015-retrospective/ Annual App Downloads, 2013 - 2015

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http://blog.appannie.com/app-annie-2015-retrospective/ Annual App Revenue, 2013 - 2015

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let = ["","",""] for in { print() }

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Swift is not really a functional language

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Nothing like a pure functional language.

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Ideals of functional programming are important

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Installing Swift

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Available on Linux and OS X (OS X installation: just download Xcode)

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Binaries available for Ubuntu

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$ sudo apt-get install clang libicu-dev Download a binary from https://swift.org/download/#linux; extract it $ tar xzf swift--.tar.gz $ export PATH=/path/to/usr/bin:"${PATH}" This creates a usr/ directory; add it to your path: Swift is now installed! Download the pre-requisites:

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Useful Swifty Things

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Playgrounds

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$ swift Welcome to Apple Swift version 2.1.1 (swiftlang-700.1.101.15 clang-700.1.81). Type :help for assistance. 1> let text = "Don t worry if you are not computer" text: String = "Don t worry if you are not computer" 2> text.uppercaseString $R0: String = "DON T WORRY IF YOU ARE NOT COMPUTER" 3>

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What does functional programming mean?

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Functions

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First-class functions

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Higher-order functions

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Pure functions

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Pure functions have no side effects.

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No I/O, no changing outside state.

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Why does functional programming matter?

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–John Carmack “No matter what language you work in, programming in a functional style provides beneﬁts. You should do it whenever it is convenient, and you should think hard about the decision when it isn't convenient.”

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yeah ok but WHY

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“Pure functions have a lot of nice properties.”

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• Thread safety • Reusability • Testability • Understandability • Maintainability

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Important Language Support

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A really nice type system.

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First-class functions. See also: lambda calculus, Haskell, OCaml, Scala, and so on.

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func add(num1: Int, _ num2 : Int) -> Int { return num1 + num2 }

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func add(num1: Int, _ num2 : Int) -> Int { return num1 + num2 } var addNumbers : (Int,Int) -> Int

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func add(num1: Int, _ num2 : Int) -> Int { return num1 + num2 } var addNumbers : (Int,Int) -> Int addNumbers = add

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func add(num1: Int, _ num2 : Int) -> Int { return num1 + num2 } var addNumbers : (Int,Int) -> Int addNumbers = add addNumbers(1,2)

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Closures

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Inline ﬁrst-class functions

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var add = { (num1 : Int, num2 : Int) -> Int in return num1 + num2 }

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Swift will infer as much type info as it can

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var add = { (num1 : Int, num2 : Int) -> Int in return num1 + num2 }

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var add = { (num1 : Int, num2 : Int) -> Int in return num1 + num2 }

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var add = { (num1 : Int, num2 : Int) -> Int in return num1 + num2 }

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var add : (Int,Int) -> Int add = { $0 + $1 }

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var add : (Int,Int) -> Int add = (+)

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Value capturing

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typealias Adder = (Int) -> Int

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typealias Adder = (Int) -> Int func makeAdder(number: Int) -> Adder { return { $0 + number } }

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typealias Adder = (Int) -> Int func makeAdder(number: Int) -> Adder { return { $0 + number } } let addTwo = makeAdder(2)

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typealias Adder = (Int) -> Int func makeAdder(number: Int) -> Adder { return { $0 + number } } let addTwo = makeAdder(2) addTwo(5) // 7

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First-class functions means we get higher-order functions!

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typealias TwoIntegerFunction = (Int,Int) -> Int

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typealias TwoIntegerFunction = (Int,Int) -> Int func applyOperationToNumbers(num1 : Int, _ num2 : Int, operation: TwoIntegerFunction) -> Int { return operation(num1, num2) }

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typealias TwoIntegerFunction = (Int,Int) -> Int func applyOperationToNumbers(num1 : Int, _ num2 : Int, operation: TwoIntegerFunction) -> Int { return operation(num1, num2) } let add : TwoIntegerFunction = { $0 + $1 }

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Optionals

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In Swift, variables are never allowed to be nil, unless they’re Optionals.

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var text = "As you might know, I am a full time Internet" text.uppercaseString // "AS YOU MIGHT KNOW, I AM A FULL TIME INTERNET"

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let textFromDisk = loadTextFromDisk() textFromDisk.uppercaseString // Error!!

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Very similar to Haskell’s Maybe

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let textFromDisk = loadTextFromDisk() textFromDisk?.uppercaseString // "EVERYTHING HAPPENS SO MUCH"

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Some Contrived Examples using Swift

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func incrementArray(theArray: [Int]) -> [Int] { var result: [Int] = [] for item in theArray { result.append(item+1) } return result }

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func doubleArray(theArray: [Int]) -> [Int] { var result: [Int] = [] for item in theArray { result.append(item*2) } return result }

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typealias TransformFunction = Int -> Int func manipulateArray(theArray: [Int], transform: TransformFunction) -> [Int] { var result: [Int] = [] for item in theArray { result.append(transform(item)) } return result }

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func doubleArray(xs: [Int]) -> [Int] { return manipulateArray(xs, transform:{ $0*2 }) }

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Limitations…

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[ ] , , , Int Int Int Int [ ] , , , Int Int Int Int

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[ ] , , , Int Int Int Int [ ] , , , ? Bool Bool Bool Bool

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Writing duplicate versions sucks

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Generics!

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func manipulateArray(theArray: [Int], transform: TransformFunction) -> [Int] { var result: [Int] = [] for item in theArray { result.append(transform(item)) } return result }

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func map(xs: [Int], transform: Int->T) -> [T] { var result: [T] = [] for x in xs { result.append(transform(x)) } return result }

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func map(xs: [Int], transform: Int->T) -> [T] { var result: [T] = [] for x in xs { result.append(transform(x)) } return result }

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But we can take it further…

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func map(xs: [Element], transform: Element->T) -> [T] { var result: [T] = [] for x in xs { result.append(transform(x)) } return result }

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let doubledArray = map([1,2,3,4], transform: { $0 * 2} )

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extension Array { func map(transform: (Element) -> T) -> [T] { var result: [T] = [] for x in self { result.append(transform(x)) } return result } }

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[1,2,3,4].map({ $0 * 2 })

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Map actually comes with Swift Hopefully you were expecting me to say that… … the point is, there’s no secrets to Map!

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Composing functions

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10 11 +1 ×2

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5 10 11 +1 ×2

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5 10 11 +1 ×2

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5 10 11 +1 ×2

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5 10 11 +1 ×2

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×2 +1

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11 ×2 +1

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5 11 ×2 +1

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5 11 ×2 +1

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// Compose a function from two functions func compose(funcA: T->T, _ funcB: T->T) -> T->T { // Return a new function that calls them both return { funcB(funcA($0)) } }

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// Compose a function from two functions func compose(funcA: T->T, _ funcB: T->T) -> T->T { // Return a new function that calls them both return { funcB(funcA($0)) } } // Two dumb lil functions let double = { $0 * 2 } let addOne = { $0 + 1 }

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Limitations!

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func compose(funcA: T->T, _ funcB: T->T) -> T->T These have to be the same type, for no good reason!

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Is Even? Flip Boolean Bool Int Bool Bool

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No Is Even? Flip Boolean

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4 No Is Even? Flip Boolean

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4 No Is Even? Flip Boolean

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func compose(funcA: T->T, _ funcB: T->T) -> T->T { return { funcB(funcA($0)) } }

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func compose(funcA: T->U, _ funcB: U->V) -> T->V { return { funcB(funcA($0)) } }

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func compose(funcA: T->U, _ funcB: U->V) -> T->V { return { funcB(funcA($0)) } } let isEven = { $0 % 2 == 0 } // Int -> Bool let flipBool = { !$0 } // Bool -> Bool let isNotEven = compose(isEven, flipBool) // Int -> Bool isNotEven(4) // false

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Third-party Stuff

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Swiftz https://github.com/typelift/swiftz

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Better types and tools for FP

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import struct Swiftz.List // Cycles a finite list of numbers into an infinite list. let finite : List = [1, 2, 3, 4, 5] let infiniteCycle = finite.cycle() // Lists also support the standard map, filter, and reduce operators. let l : List = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] let twoToEleven = l.map(+1) // [2, 3, 4, 5, 6, 7, 8, 9, 10, 11] let even = l.filter((==0) • (%2)) // [2, 4, 6, 8, 10] let sum = l.reduce(curry(+), initial: 0) // 55 // Plus a few more. let partialSums = l.scanl(curry(+), initial: 0) // [0, 1, 3, 6, 10, 15, 21, 28, 36, 45, 55] let firstHalf = l.take(5) // [1, 2, 3, 4, 5] let lastHalf = l.drop(5) // [6, 7, 8, 9, 10]

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Promissum https://github.com/tomlokhorst/Promissum

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Dollar.swift dollarswift.org

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Result https://github.com/antitypical/Result

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Learning more • Learn You a Haskell for Great Good! • Pearls of Functional Algorithm Design • Programming in Haskell (Hutton) • Apple’s Swift Book(s) • IBM Swift Resources: https://developer.ibm.com/swift/

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Thank you @parisba / http://www.secretlab.com.au