This document provides 7 habits for writing more functional Swift code. It discusses avoiding mutability and for-loops in favor of map, filter and reduce functions. It also covers being lazy, currying functions, writing domain-specific languages, and thinking about code in terms of data and functions rather than objects.

1. 7 Habits For a More Functional Swift

2. Jason Larsen
@jarsen

3. 7 Habits
1. Avoid mutability
2. Avoid for-loops
3. Combine map/filter/reduce
4. Be lazy
5. Curry functions
6. Write DSLs
7. Stop objectifying code

4. What is a Function?
f(x) = x * x

5. Functions Are Mappings
4 Do not mutate input
4 Do not change external state
4 Determined only by explicit inputs

6. Consequences of Pure Functions
4 Return the same values every time for input
4 No Side Effects
4 Purity allows laziness (since the value will be the
same whenever its computed, we can compute it only
when we need it)
4 Concurrency is easy, b/c no shared state

7. Consequences of Pure Functions
4 No I/O (user input, printing, random values, etc)
4 No state
4 No variables (writing to variables is a side effect)
4 No Side Effects

8. Is Swift Functional?

9. #1
Let it be

10. Bad
// find the bug, and don't tell me you've never done this
func square(x: Int) -> Int {
return x * x
}
var a = [1,2,3,4,5]
var b = [Int]()
for x in a {
a.append(square(x))
}

11. Good
func square(x: Int) -> Int {
return x * x
}
let a = [1,2,3,4,5]
let b = a.map({x in square(x)})

12. Beautiful
func square(x: Int) -> Int {
return x * x
}
let a = [1,2,3,4,5]
let b = a.map(square)

13. Immutable structs
struct Person {
let name: String
let age: Int
}
let alice = Person(name: "Alice", age: 22)
let alice2 = Person(name: alice.name, age: 23) // transform data

14. Transforming Immutable Objects
extension Dictionary {
func dictionaryByUpdatingKey(key: Key, value: Value) -> Dictionary {
var mutable = self
mutable.updateValue(value, forKey: key)
return mutable
}
}
let animalNoiseMap = ["cow" : "moo", "cat" : "meow"]
let animalNoiseMapImproved = animalNoiseMap.dictionaryByUpdatingKey("dog", value: "woof")

15. Transforming Immutable Objects
struct Person {
let name: String
let age: Int
func age(age: Int) -> Person {
return Person(name: self.name, age: age)
}
}
let bob = Person(name: "Bob", age: 25)
let birthdayBob = bob.age(bob.age + 1)

16. Transforming Immutable Objects
struct Person {
let name: String
let age: Int
static func age(person: Person, age: Int) -> Person {
return Person(name: person.name, age: age)
}
}
let bob = Person(name: "Bob", age: 25)
let birthdayBob = Person.age(bob, age: bob.age + 1)

17. Transforming Immutable Objects
class Person {
let name: String
let age: Int
init(name: String, age: Int) {
self.name = name
self.age = age
}
init(person: Person, name: String? = nil, age: Int? = nil) {
self.name = name ?? person.name
self.age = age ?? person.age
}
}
let bob = Person(name: "Bob", age: 25)
let birthdayBob = Person(person: bob, age: bob.age + 1)

18. #2
for the love of loops!

19. Map
Map each item in an existing collection to something
else.

20. Filter
Find objects in a collection that match your criteria by
filtering out everything that doesn't match.

21. Ugly
var bestStudents = [Student]()
for student in students {
if (student.grade > 90) {
bestStudents.append(student)
}
}

22. Beautiful
let bestStudents = students.filter { $0.grade > 90 }

23. Also Beautiful
func isBestStudent(student: Student) -> Bool {
return student.grade > 90
}
let bestStudents = students.filter(isBestStudent)

24. Reduce
Reduces all sequence elements into one value. Takes an
initial value, passes that value through as an
accumulator, which may be updated in each iteration.

25. Ugly
let a = [1,2,3,4,5]
var sum = 0
for x in a {
sum += x
}

26. Calculating Sums With Reduce
let a = [1,2,3,4]
let sum = a.reduce(0, combine: { (accumulator, value) in
return accumulator + value
})

27. Calculating Sums With Reduce
let a = [1,2,3,4]
let sum = a.reduce(0, combine: { (accumulator, value) in accumulator + value })

28. Calculating Sums With Reduce
let a = [1,2,3,4]
let sum = a.reduce(0, combine: { $0 + $1 })

29. Calculating Sums With Reduce
let a = [1,2,3,4]
let sum = a.reduce(0, +)

30. Finding the Max Value With Reduce
let numbers = [1,4,15,23,9]
if let initial = numbers.first {
let numMax = numbers.reduce(initial) { (m, x) in
return x > m ? x : m
}
}

31. Finding the Max Value With Reduce
let numbers = [1,4,15,23,9]
if let initial = numbers.first {
let numberMax = numbers.reduce(initial) { (m, x) in
return max(m, x)
}
}

32. Finding the Max Value With Reduce
let numbers = [1,4,15,23,9]
if let initial = numbers.first {
let numberMax = numbers.reduce(initial, max)
}

33. Counting Frequencies with Reduce
let numbers = [1,4,15,23,1,1,9,9,23,9]
let histogram = numbers.reduce([Int: Int]()) { (acc, x) in
if let count = acc[x] {
return acc.dictionaryByUpdatingKey(x, value: count + 1)
}
else {
return acc.dictionaryByUpdatingKey(x, value: 1)
}
}

34. Composing Filters
typealias Filter = CIImage -> CIImage
let filters: [Filter] = [colorOverlay, blur, drawTitle]
let filteredImage = filters.reduce(image, combine: { $1($0) } )

35. #3
By our powers combined

36. struct Person {
let name: String
let age: UInt
}
let people = [Person(name: "Alice", age: 22),
Person(name: "Bob", age: 23),
Person(name: "Mallory", age: 25)]
let ageSum = people.map({$0.age}).reduce(0, combine: +)

37. let people = [Person(name: "Alice", age: 22),
Person(name: "Bob", age: 23),
Person(name: "Mallory", age: 25)]
let namesBeforeJason = people.map({$0.name}).filter { name in
name.compare("Jason") == NSComparisonResult.OrderedAscending
}

38. Zip it up
let a = Array(1...5)
let b = Array(6...10)
let result = map(Zip2(a,b), +) // [7, 9, 11, 13, 15]

39. #4
Be Lazy

40. class EvenNaturalNumbers: SequenceType {
typealias GeneratorType = EvenNaturalNumbersGenerator
func generate() -> EvenNaturalNumbersGenerator {
return EvenNaturalNumbersGenerator()
}
}
class EvenNaturalNumbersGenerator : GeneratorType {
var current = 2
typealias Element = Int
func next() -> Int? {
let ret = current
current += 2
return ret
}
}

41. class Fibonacci : SequenceType {
typealias GeneratorType = FibonacciGenerator
func generate() -> FibonacciGenerator {
return FibonacciGenerator()
}
}
class FibonacciGenerator : GeneratorType {
var current = 0, nextValue = 1
typealias Element = Int
func next() -> Int? {
let ret = current
current = nextValue
nextValue = nextValue + ret
return ret
}
}

42. func take<T, S : SequenceType where S.Generator.Element == T>(n: Int, sequence: S) -> [T] {
var gen = sequence.generate()
var values = [T]()
for _ in (1...n) {
if let value = gen.next() {
values.append(value)
}
}
return values
}
take(5, [1,2,5,12,31,4,2])
take(10, EvenNaturalNumbers())
take(10, Fibonacci())

43. func filter<S : SequenceType>
(source: S, includeElement: (S.Generator.Element) -> Bool) -> [S.Generator.Element]
func map<S : SequenceType, T>
(source: S, transform: (S.Generator.Element) -> T) -> [T]

44. #5
Curried Functions. Yum.

45. func addNormal(x:Int, y : Int) -> Int {
return x + y
}
let sum = addNormal(1, 2)

46. func addCurried(x:Int) -> Int -> Int {
return {y in return x + y}
}
let sum = addCurried(1)(2)

47. let numbers = Array(0...5)
let numbersIncrementedBy1 = numbers.map(addCurried(1))
let numbersIncrementedBy2 = numbers.map(addCurried(2))

48. // taken from the excellent WIP "Functional Programming in Swift"
// http://www.objc.io/books/
typealias Filter = CIImage -> CIImage
func blur(radius: Double) -> Filter {
return { image in
let parameters : Parameters = [kCIInputRadiusKey: radius, kCIInputImageKey: image]
let filter = CIFilter(name:"CIGaussianBlur", parameters:parameters)
return filter.outputImage
}
}
let blurredImage = blur(2.0)(image)

49. Instance Methods are Curried
class BankAccount {
var balance: Double = 0.0
func deposit(amount: Double) {
balance += amount
}
}
let account = BankAccount()
account.deposit(100) // balance is now 100
let depositor = BankAccount.deposit
depositor(account)(100) // balance is now 200

50. #6
Domain-Specific Langauges

51. Custom Flow Control
func unless(condition: Bool, then: () -> ()) {
if (!condition) {
then()
}
}
unless(1 != 1) {
println("Phew. Identity holds.")
}

52. Cucumber-Style BDD Framework
given("I have entered (.*) into the calculator") { n in
let calculator = Calculator()
calculator.push(n)
}

53. Sinatra-Style Web Framework
GET("/greetings/:name") { request, params in
let name = params["name"] ?? "Anonymous"
let greeting = "<h1>Hello, (name)!</h1>"
return Response(body: greeting, code: 200)
}

54. Custom Operators
infix operator |> { associativity left }
func |> (filter1: Filter, filter2: Filter) -> Filter {
return {img in filter1(filter2(img))}
}
let myFilter = blur(blurRadius) |> colorOverlay(overlayColor)
let result = myFilter(image)

55. #7
Stop Objectifying Code

56. Objectification
4 Class - Noun
4 Properties - Nouns related to noun above
4 Instance Methods - Actions instance of Class can
perform
4 Class Methods - Actions related to Class in general

57. Functionalization
4 Data
4 Functions transform data

58. Thinking About Data

59. Arrays
great for variable length data of the same type
4 list of students in a class
4 lines in a document
4 search results in a JSON response

60. Tuples / Named Tuples
fixed length list. can hold mixed types, but probably best
to prefer same types
4 Points/Vectors
4 functions with multiple return values
typealias Vector2D = (x: Double, y: Double)
let foo = Vector2D(2, 4)

61. Dictionaries
Dictionaries are maps.
4 anything that needs to be mapped to something else
4 a JSON response

62. Structs
Encapsulate properties of multiple types. No
inheritance. Free constructor for all the immutable
properties.
4 Anything you might use a tuple for
4 Data related to a student - grade, first name, last
name

63. Enums
Anytime something has a set of options
4 HTTP Methods
4 Errors
4 Optionals

64. Classes
Objects. Object Oriented Programming. Not a bad thing,
but not terribly functional.

65. Typealias All The Things
typealias Filter = Request->Request
typealias Handler = (Request,Parameters)->Response
typealias Model = [String : [String]]
typealias Renderer = Model -> String
typealias Parameters = [String: String]

66. Resources
4 http://www.drewag.me/posts/practical-use-for-curried-
functions-in-swift
4 https://www.skillsmatter.com/skillscasts/5678-an-introduction-
to-haskell
4 http://matt.might.net/articles/implementing-laziness
4 http://www.scottlogic.com/blog/2014/06/26/swift-sequences.
html