Practical Functional Programming (dotSwift 2015)

A practical introduction to functional programming in Swift and how it can be applied to the real world. Going over what being functional means and how it allows you to write clearer, declarative and testable code.

Usually, functional programming is taught by abstract functional techniques. Instead this talk goes for a different approach showing examples of unfunctional, imperative code people may have written in the past and how that can be improved with a functional style.

Transcript

1. Practical Functional
   Programming
   @kylefuller
   

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2. Imperative
   

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3. Declarative
   

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4. Describe what to do
   Not how to do it.
   

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5. What makes up functional
   programming?
   

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6. What makes up functional programming?
   4 first class functions
   4 immutable data
   4 reducing
   4 pipelining
   4 recursing
   4 currying
   4 monads
   

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7. What makes up functional programming?
   4 first class functions
   4 immutable data
   4 reducing
   4 pipelining
   4 recursing
   4 currying
   4 monads
   

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8. Side-effects
   

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9. What is not functional
   function?
   

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10. Un-functional Function
    var value = 0
    func increment() {
    value += 1
    }
    

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11. Un-functional Function
    func increment() {
    let center = NSNotificationCenter.defaultCenter()
    let value = center.valueForKey("key") as? Int ?? 0
    center.setValue(value + 1, forKey: "key")
    }
    

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12. Functional Function
    func increment(value:Int) -> Int {
    return value + 1
    }
    

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13. High Order Functions
    

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14. High Order Functions
    4 Functions that take other functions as arguments
    4 Functions that returns a functions
    

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15. Everything is a closure
    

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16. State of callables in
    Objective-C
    

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17. @implementation Object
    - (void)message:(NSString *)value {
    }
    @end
    

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18. @implementation Object
    - (void)message:(NSString *)value {
    }
    @end
    [object message:@“Hello”];
    

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19. void message(NSString *value) {
    }
    

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20. void message(NSString *value) {
    }
    message(@“Hello”);
    

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21. void (^message)(NSString *value) = ^void(NSString *value) {
    };
    

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23. void (^message)(NSString *value) = ^void(NSString *value) {
    };
    message(@“Hello”);
    

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24. Callables in Swift
    

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25. (String) -> ()
    

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26. let message:(String -> ())
    

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27. let message:(String -> ()) = { value in
    }
    

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28. func message(value:String) {
    }
    

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29. message("dotSwift")
    

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30. Interchangeability
    

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31. func message1(value:String) {
    println(“Hello \(value)”)
    }
    

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32. func message1(value:String) {
    println(“Hello \(value)”)
    }
    var message2:(String -> ()) = { value in
    println(“Hi \(value)”)
    }
    

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33. func message1(value:String) {
    println(“Hello \(value)”)
    }
    var message2:(String -> Void) = { value in
    println(“Hi \(value)”)
    }
    message2 = message1
    message2(“dotSwift”)
    // Hello dotSwift
    

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34. Functional programming by Example
    

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35. let groups = [
    ["Kyle", "Maxine"],
    ["André", "Katie", "Ash"]
    ]
    

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36. How many people are in
    each group?
    

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37. How many people are in each group?
    let groups = [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
    var count = [Int]()
    for group in groups {
    let people = countElements(group)
    count.append(people)
    }
    count
    // [2, 3]
    

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38. map
    

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39. map
    (source, transform) -> ([T])
    

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40. transform
    (item) -> (T)
    

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41. How many people are in each group?
    let groups = [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
    map(groups) {
    countElements($0)
    }
    // [2, 3]
    

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42. countElements
    (sequence) -> (Int)
    

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43. transform
    (item) -> (T)
    

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44. (item) -> (T)
    (sequence) -> (Int)
    

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45. How many people are in each group?
    let groups = [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
    map(groups, countElements)
    // [2, 3]
    

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46. Order the numbers of
    people in each group
    

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47. [2, 3] -> [3, 2]
    

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48. sorted
    

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49. sorted
    (sequence, isOrderedBefore) -> (sequence)
    

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50. isOrderedBefore
    (lhs, rhs) -> (Bool)
    

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51. Order the numbers of people in each group
    let count = [2, 3]
    

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52. Order the numbers of people in each group
    let count = [2, 3]
    sorted(count) { (lhs, rhs) in
    lhs > rhs
    }
    // largest group is [3, 2].first
    

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53. Operators are closures
    

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54. >
    (lhs, rhs) -> (Bool)
    

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55. let count = [2, 3]
    sorted(count, >)
    // largest group is [3, 2].first
    

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56. Building an array of all people
    

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57. Building an array of all people
    let groups = [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
    var people = [String]()
    for group in groups {
    people += group
    }
    // ["Kyle", "Maxine", "André", "Katie", "Ash"]
    

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58. reduce
    

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59. reduce
    (sequence, initial(U), combine) -> (U)
    

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60. combine
    (U, value) -> (U)
    

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61. +
    (U, U) -> (U)
    

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62. reduce(groups, [], +)
    

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63. Building an array of all people
    let groups = [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
    reduce(groups, [], +)
    // ["Kyle", "Maxine", "André", "Katie", "Ash"]
    

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64. let input = "Kyle,Maxine\nAndré,Katie,Ash"
    

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65. let input = "Kyle,Maxine\nAndré,Katie,Ash"
    var groups = [[String]]()
    for line in input.componentsSeparatedByString("\n") {
    let group = line.componentsSeparatedByString(",")
    groups.append(group)
    }
    groups
    // [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
    

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66. let input = "Kyle,Maxine\nAndré,Katie,Ash"
    let groups = map(input.componentsSeparatedByString("\n")) { line in
    line.componentsSeparatedByString(",")
    }
    groups
    // [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
    

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67. let input = "Kyle,Maxine\nAndré,Katie,Ash"
    func commaSeparator(input:String) -> [String] {
    return input.componentsSeparatedByString(",")
    }
    map(input.componentsSeparatedByString("\n"), commaSeparator)
    // [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
    

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68. High Order Functions
    4 Functions that returns a functions
    

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69. func separateBy(separator:String) -> ((String) -> [String]) {
    func inner(source:String) -> [String] {
    return source.componentsSeparatedByString(separator)
    }
    return inner
    }
    

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70. func separateBy(separator:String) -> ((String) -> [String]) {
    func inner(source:String) -> [String] {
    return source.componentsSeparatedByString(separator)
    }
    return inner
    }
    let lineSeparator = separateBy("\n")
    let commaSeparator = separateBy(",")
    

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71. func separateBy(separator:String)(source:String) -> [String] {
    return source.componentsSeparatedByString(separator)
    }
    let lineSeparator = separateBy("\n")
    let commaSeparator = separateBy(",")
    

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72. let input = "Kyle,Maxine\nAndré,Katie,Ash"
    map(lineSeparator(input), commaSeparator)
    // [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
    

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73. Why are map and reduce
    better?
    

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74. Declarative
    

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75. Immutability
    

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76. Don’t iterate over arrays
    

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77. Don’t iterate over arrays
    4 Use map
    

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78. Don’t iterate over arrays
    4 Use map
    4 Use reduce
    

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79. Write declaratively
    Not imperatively
    

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