Passing function to function arguments

This is the English version of "関数を引数として渡す書き方のポイント" which was presented at iOSDC 2017 (2017/09/17)

- https://speakerdeck.com/yoching/guan-shu-woyin-shu-tositedu-sushu-kifang-falsepointo

Transcript

1. Passing functions to function
   arguments
   Yoshikuni Kato
   1
   

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2. Notice
   This is the English version of "樛හΨ୚හ;ͭͼჁͯ䨗ͣො
   ΄ϪαЀϕ" which was presented at iOSDC 2017
   (2017/09/17)
   — https://speakerdeck.com/yoching/guan-shu-woyin-
   shu-tositedu-sushu-kifang-falsepointo
   2
   

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3. Who am I?
   — Yoshikuni Katoҁےᡕኧ懺҂
   @yoshikuni_kato
   — iOS Engineer (3 years)
   — Yahoo! Japan -> Ohako, inc.
   — "Radi-Hey" →
   — Interests: Class Design / FRP /
   Coordinator Pattern / UI
   implementation
   3
   

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4. Wave of writing functionally
   — More opportunity to write functionally
   — FRP (RxSwift / ReactiveSwift)
   — map / filter / reduce
   — reducing if and for
   — more declarative way
   4
   

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5. What I talk?
   — how to write functions as arguments of other
   functions
   -> a little change, much declarative way
   — example: map of array
   5
   

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6. How to write #1: Write closures directly
   let array: [Int] = [1, 2, 3]
   array.map { number -> Int in
   return number * 2
   }
   6
   

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7. Definition of map of array
   func map(_ transform: (Element) throws -> T) rethrows -> [T]
   — arguments of map: closure that takes Element and
   return T
   — functions are like "named closures"
   — possible to pass function directly
   7
   

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8. How to write #2: Pass function
   // declare function first
   func twoTimes(of number: Int) -> Int {
   return number * 2
   }
   let array: [Int] = [1, 2, 3]
   array.map(twoTimes) // pass the function
   8
   

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9. In case of multiple parameters
   func someFunc(a: Int, b: Int) -> String {
   return "a = \(a), b = \(b)"
   }
   let array: [Int] = [1, 2, 3]
   array
   .map { number -> (a: Int, b: Int) in
   return (a: number, b: number) // make a tuple
   }
   .map(someFunc)
   9
   

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10. In case of initializer
    struct Sample {
    let number: Int
    init(number: Int) {
    self.number = number
    }
    }
    10
    

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11. How to write #1: Write closure directly
    let array: [Int] = [1, 2, 3]
    array.map { number -> Sample in
    return Sample(number: number)
    }
    11
    

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12. How to write #2: Pass function
    let array: [Int] = [1, 2, 3]
    array.map(Sample.init)
    — initializer(.init) = function which returns the object
    12
    

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13. Comparison 1
    array.map { number -> Sample in
    return Sample(number: number)
    }
    array.map(Sample.init)
    13
    

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14. Comparison 2
    array
    .map { number -> Int in
    return number * 2
    }
    .map { number -> Sample in
    return Sample(number: number)
    }
    .map { sample -> Foo in
    return Foo(sample: sample)
    }
    array
    .map(twoTimes)
    .map(Sample.init)
    .map(Foo.init)
    14
    

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15. Wrap up
    — a little change, much declarative way
    — example: convert Model to ViewModel
    ̴ model.map(ViewModel.init)
    — extracting logics as methods, as a result
    — feeling of passing function (different from imperative
    programming style)
    15
    

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16. References
    — Connecting View Controllers, Swift Talk1
    — From Runtime Programming to Functions, Swift Talk2
    2 https://talk.objc.io/episodes/S01E19-from-runtime-programming-to-functions
    1 https://talk.objc.io/episodes/S01E05-connecting-view-controllers
    16
    

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17. One more thing
    func someFunc(a: Int, b: Int) -> String {
    return "a: \(a), b: \(b)"
    }
    // cannot pass tuples to function itself
    let parameters = (a: 0, b: 0)
    someFunc(parameters) //
    !
    (swift3~)
    // can pass tuples when using map
    let array: [(Int, Int)] = [(0, 0)]
    array.map(someFunc) //
    "
    (even in swift3)
    — Please tell me if you know how this is possible
    17
    

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