Functional Programming in Swift

The source code is here: https://gist.github.com/chriseidhof/8b109d27701d745e6bcd

Transcript

1. Functional Programming in Swift Chris Eidhof - objc.io - June 28, Minsk
3. Apple decides everything, and we will like it. — Charles McCathie Nevile
4. Why? You can keep writing normal OOP code. But some things are easier when done in a functional way.
5. What is Functional Programming?
6. Functions func add(x: Int, y: Int) -> Int { return x + y }
7. No side-effects func add(x: Int, y: Int) -> Int { return x + y + Int(arc4random()) } add(1,2) 1176275535
8. Referential Transparency The same input should always give the same output.
9. Avoid State
10. Purity A pure function has no side-effects: • No use of global variables • No reading of files • No use of functions that are not pure
11. Pure functions are referentially transparent
12. In Practice Most of your code should be pure, only small bits have side-effects.
13. Example Suppose we build a library for describing diagrams: • Describing a diagram: pure • Calculating the layout of a diagram: pure • Executing the actual drawing: side-effect
14. Why? This will make your code a lot easier to change, reuse and test.
15. Functional Programming Basics
16. Our data set let cities : Dictionary<String,Int> = [ "Минск": 1834200 , "Борисов": 147100 , "Солигорск": 102300 , "Молодечно": 94200 , "Жодино": 61800 ] let names = Array(cities.keys) let populations = Array(cities.values)
17. Names [Жодино, Минск, Молодечно, Солигорск, Борисов]
18. Populations [61800, 1834200, 94200, 102300, 147100]
19. Map func addCity(s: String) -> String { return s + "is a city" } names.map(addCity) [Жодиноis a city, Минскis a city, Молодечноis a city, Солигорскis a city, Борисовis a city]
20. Filter func isMinsk(s: String) -> Bool { return s == "Минск" } names.filter(isMinsk) [Минск]
21. Filter, simplified names.filter({ (s: String) -> Bool in return s == "Минск" }) [Минск]
22. Filter, more simplified names.filter({ s in return s == "Минск" }) [Минск]
23. Filter, even more simplified names.filter({ return $0 == "Минск" }) [Минск]
24. Filter, simplest names.filter { $0 == "Минск" } [Минск] populations.filter { $0 > 100000 } [1834200, 102300, 147100]
25. Sum of an array func sum(arr: Int[]) -> Int { var result = 0 for i in arr { result += i } return result } sum(Array(1..10)) 45
26. Product of an array func product(arr: Int[]) -> Int { var result = 1 for i in arr { result *= i } return result } product(Array(1..10)) 362880
27. Reduce func reduce(initialValue: Int, combine: (Int,Int) -> Int, arr: Int[]) -> Int { var result = initialValue for i in arr { result = combine(result,i) } return result }
28. Reduce reduce(0, +, Array(1..10)) 45 reduce(1, *, Array(1..10)) 362880
29. Sum and Product let sum = { reduce(0,+,$0) } let product = { reduce(1,*,$0) }
30. Concatenate func concat(strings: String[]) -> String { var result = "" for x in strings { result += x } return result } concat(names) ЖодиноМинскМолодечноСолигорскБорисов
31. Generics func reduce<A>(initialValue: A, combine: (A,A) -> A, arr: A[]) -> A { var result = initialValue for i in arr { result = combine(result,i) } return result } reduce("", +, names) ЖодиноМинскМолодечноСолигорскБорисов
32. Adding line-breaks reduce("", { $0 + "\n" + $1 }, names) Жодино Минск Молодечно Солигорск Борисов
33. Making reduce more generic func reduce<A,R>(initialValue: R, combine: (R,A) -> R, arr: A[]) -> R { var result = initialValue for i in arr { result = combine(result,i) } return result }
34. Define map in terms of reduce func map<A,B>(array: A[], f: A -> B) -> B[] { return reduce([], { (var arr: B[], el: A) in arr += f(el) return arr }, array) } map(Array(1..10), { $0 * 2}) ( 2, 4, 6, 8, 10, 12, 14, 16, 18)
35. Define filter in terms of reduce func filter<A>(array: A[], f: A -> Bool) -> A[] { return reduce([], { (var arr, el) in if (f(el)) { arr += el } return arr }, array) } let isEven = { $0 % 2 == 0 } filter(Array(1..10), isEven) ( 2, 4, 6, 8)
36. ...
37. QuickSort func partition(v: Int[], left: Int, right: Int) -> Int { var i = left for j in (left + 1)..(right + 1) { if v[j] < v[left] { i += 1 (v[i], v[j]) = (v[j], v[i]) } } (v[i], v[left]) = (v[left], v[i]) return i } func quicksort(v: Int[], left: Int, right: Int) { if right > left { let pivotIndex = partition(v, left, right) quicksort(v, left, pivotIndex - 1) quicksort(v, pivotIndex + 1, right) } } // Source: https://gist.github.com/fjcaetano/b0c00a889dc2a17efad9
38. Functional QuickSort func qsort(var array: Int[]) -> Int[] { if array.count == 0 { return [] } let pivot = array.removeAtIndex(0) let lesser = array.filter { $0 < pivot } let greater = array.filter { $0 >= pivot } return qsort(lesser) + [pivot] + qsort(greater) } qsort(populations) [61800, 94200, 102300, 147100, 1834200]
39. let citiesAndPopulations = cities.keysAndValues > [ (Жодино, 61800) > , (Минск, 1834200) > , (Молодечно, 94200) > , (Солигорск, 102300) > , (Борисов, 147100)]
40. Sort by name func qsortWith<A>(var array: A[], compare: (A,A) -> Bool) -> A[] { if array.count == 0 { return [] } let pivot = array.removeAtIndex(0) let lesser = array.filter { !compare(pivot,$0) } let greater = array.filter { compare(pivot,$0) } return qsortWith(lesser,compare) + [pivot] + qsortWith(greater,compare) }
41. Sort by name qsortWith(citiesAndPopulations) { (l,r) in l.0 > r.0 } [(Солигорск, 102300), (Молодечно, 94200), (Минск, 1834200), (Жодино, 61800), (Борисов, 147100)]
42. Sort by population qsortWith(citiesAndPopulations) { (l,r) in l.1 > r.1 } [(Минск, 1834200), (Борисов, 147100), (Солигорск, 102300), (Молодечно, 94200), (Жодино, 61800)]
43. ...
44. Diagrams empty: Diagram func rect(#width: Double, #height: Double) -> Diagram func circle(#radius: Double) -> Diagram @infix func ||| (l: Diagram, r: Diagram) -> Diagram
45. Example diagram let blueSquare = square(1).fill(NSColor.blueColor()) let redSquare = square(2).fill(NSColor.redColor()) let greenCircle = circle(radius:1).fill(NSColor.greenColor()) let cyanCircle = circle(radius: 1).fill(NSColor.cyanColor()) example = blueSquare ||| cyanCircle ||| redSquare ||| greenCircle
46. Reduce func hcat(d: Diagram[]) -> Diagram { return reduce(empty, |||, d) } example = hcat([blueSquare, cyanCircle, redSquare, greenCircle])
47. Bar diagrams func barGraph(input: (String,Double)[]) -> Diagram { return hcat(normalize(values).map { x in return rect(width: 1, height: 3*x).alignBottom() }) }
48. Adding labels func barGraph(input: (String,Double)[]) -> Diagram { let values : Double[] = input.map { $0.1 } let bars = hcat(normalize(values).map { x in return rect(width: 1, height: 3*x).alignBottom() }) let labels = hcat(input.map { x in return text(width: 1, height: 0.3, text: x.0).alignTop() }) return bars --- labels }
49. Diagrams API Almost all functions are pure: create a diagram, combine diagrams, color diagrams. Only the rendering is impure.
50. ...
51. QuickCheck Using functional programming to automate testing
52. Check if qsort gives the same result as sort func prop_sort(ls: Int[]) -> Bool { return qsort(ls) == sort(ls) }
53. Idea Let's generate a lot of random integer arrays, and run them through our property. Makes it easier to find mistakes.
54. Running our checker check(prop_sort) true
55. Let's build QuickCheck func check<X : Arbitrary>(prop : Array<X> -> Bool) -> Bool { for _ in 0..numberOfIterations { let randomLength = Int(arc4random() % 50) let array : X[] = Array(0..randomLength).map { _ in return X.arbitrary() } if(!prop(array)) { println("Property doesn't hold: \(array)") return false } } return true }
56. Running our checker check(prop_sort) true
57. Checking our checker check { (x: Int[]) in qsort(x) == x } Property doesn't hold: [459, 9052, 1750, 3572, 3101, 3104, 1675, 148, 4781, 5031, 4769, 6738, 2946, 4396, 3190, 4217, 9975, 5143, 7339, 7180, 2041, 6552, 6780, 3839, 7279, 1060, 375, 1096, 5957, 5484, 2832, 8705, 9202, 2158, 3015, 303, 1911, 9846, 8316, 3300] false
58. Better errors protocol Smaller { func smaller() -> Self? } extension Array : Smaller { func smaller() -> Array<T>? { if self.count == 0 { return nil } var copy = self copy.removeAtIndex(0) return copy } }
59. Better errors func check1<X : Arbitrary>(prop : Array<X> -> Bool) -> Bool { for _ in 0..numberOfIterations { let randomLength = Int(arc4random() % 50) let array : X[] = Array(0..randomLength).map { _ in return X.arbitrary() } if(!prop(array)) { let smallerValue = iterateWhile(array, { !prop($0) }) { $0.smaller() } println("Property doesn't hold: \(smallerValue)") return false } } return true }
60. Let's see the results check1 { (x: Int[]) in qsort(x) == x } Property doesn't hold: [6890, 465] false
61. Conclusion
62. This is powerful stuff You can start using this in your Swift code.
63. We're just at the beginning Swift also allows for much easier OOP, and we can mix and match whatever we want.
64. There are a lot more cool things • Enums • Pattern matching • Optionals • Applicative Functors and Monads
66. @chriseidhof