Lazy Evaluation in Swift

Transcript

1. Lazy Evaluation in Swift
   

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2. • ΪΞ
   • @nghialv (twitter: nghialv2607) ΞΧ΢ϯτ
   • 2014೥11݄ʹCyberAgentʹ৽ଔೖࣾ
   • AmebaΞϓϦνʔϜ
   

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3. A programming language uses an evaluation strategy to determine
   when to evaluate the argument(s) of a function call
   (for function, also read: operation, method, or relation)
   and what kind of value to pass to the function.
   “Wikipedia”
   Evaluation Strategy:
   

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4. A programming language uses an evaluation strategy to determine
   when to evaluate the argument(s) of a function call
   (for function, also read: operation, method, or relation)
   and what kind of value to pass to the function.
   “Wikipedia”
   Evaluation Strategy:
   call-by-value
   call-by-reference
   

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5. When?
   

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6. Eager evaluation
   Call-by-name
   Call-by-need
   

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7. Eager Evaluation
   func dosomething() -> Int {
   println("dosomething")
   return 1
   }
   func foo(x: Int, status: String) {
   println("foo")
   }
   foo(dosomething(), "200")
   output
   dosomething
   foo
   

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8. Eager Evaluation
   func dosomething() -> Int {
   println("dosomething")
   return 1
   }
   func foo(x: Int, status: String) {
   println("foo")
   }
   foo(dosomething(), "200")
   output
   dosomething
   foo
   fooؔ਺ͷ࣮ߦલʹdosomething͕ධՁ͞ΕΔ
   

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9. Eager Evaluation
   func expensiveComputation() -> Int {
   println("expensiveComputation")
   return 1
   }
   func foo(x: Int, status: String) {
   println("foo")
   let result = status == "200" ? x : 0
   println("result: \(result)")
   }
   foo(expensiveComputation(), "404")
   output
   expensiveComputation
   foo
   result: 0
   

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10. Eager Evaluation
    func expensiveComputation() -> Int {
    println("expensiveComputation")
    return 1
    }
    func foo(x: Int, status: String) {
    println("foo")
    let result = status == "200" ? x : 0
    println("result: \(result)")
    }
    foo(expensiveComputation(), "404")
    output
    expensiveComputation
    foo
    result: 0
    x ͕࢖ΘΕ͍ͯͳ͍ͷʹධՁ͞Εͨʂ
    

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11. Eager Evaluation
    func expensiveComputation() -> Int {
    println("expensiveComputation")
    return 1
    }
    func foo(x: Int, status: String) {
    println("foo")
    let result = status == "200" ? x : 0
    println("result: \(result)")
    }
    foo(expensiveComputation(), "404")
    output
    expensiveComputation
    foo
    result: 0
    x ͕࢖ΘΕ͍ͯͳ͍ͷʹධՁ͞Εͨʂ
    ϞολΠφΠʂ
    

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12. Function Currying
    func dosomething() -> Int {
    println("dosomething")
    return 1
    }
    func foo(a: Int)(b: Int) {
    println("add")
    }
    let cFoo = foo(dosomething())
    //cFoo(b: 10)
    output
    dosomething
    

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13. Function Currying
    func dosomething() -> Int {
    println("dosomething")
    return 1
    }
    func foo(a: Int)(b: Int) {
    println("add")
    }
    let cFoo = foo(dosomething())
    //cFoo(b: 10)
    output
    dosomething
    ͜ͷ࣌఺ʹ a ͕ධՁ͞ΕΔ
    

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14. ඞཁ࣌ͷΈධՁ͍ͨ͠ʂ
    ධՁλΠϛϯάมߋ͍ͨ͠ʂ
    

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15. Call-by-name
    ඞཁ࣌ͷΈධՁ͍ͨ͠ʂ
    ධՁλΠϛϯάมߋ͍ͨ͠ʂ
    

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16. Call-by-name
    func expensiveComputation() -> Int {
    println("expensiveComputation")
    return 1
    }
    func foo(x: () -> Int, status: String) {
    println("foo")
    let result = status == "200" ? x() : 0
    println("result: \(result)")
    }
    foo({ expensiveComputation() }, "404")
    output
    foo
    result: 0
    

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17. Call-by-name
    func expensiveComputation() -> Int {
    println("expensiveComputation")
    return 1
    }
    func foo(x: () -> Int, status: String) {
    println("foo")
    let result = status == "200" ? x() : 0
    println("result: \(result)")
    }
    foo({ expensiveComputation() }, "404")
    output
    foo
    result: 0
    ClosureΛ࢖͏ʂ
    

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18. Call-by-name
    func expensiveComputation() -> Int {
    println("expensiveComputation")
    return 1
    }
    func foo(x: () -> Int, status: String) {
    println("foo")
    let result = status == "200" ? x() : 0
    println("result: \(result)")
    }
    foo({ expensiveComputation() }, "404")
    output
    foo
    result: 0
    ClosureΛ࢖͏ʂ
    ؾ࣋ͪѱ͍
    

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19. @autoclosure
    

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20. Call-by-name
    func expensiveComputation() -> Int {
    println("expensiveComputation")
    return 1
    }
    func foo(x: @autoclosure () -> Int, status: String) {
    println("foo")
    let result = status == "200" ? x() : 0
    println("result: \(result)")
    }
    foo(expensiveComputation(), "404")
    output
    foo
    result: 0
    

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21. Call-by-name
    func expensiveComputation() -> Int {
    println("expensiveComputation")
    return 1
    }
    func foo(x: @autoclosure () -> Int, status: String) {
    println("foo")
    let result = status == "200" ? x() : 0
    println("result: \(result)")
    }
    foo(expensiveComputation(), "404")
    output
    foo
    result: 0
    ؾ࣋ͪྑ͘ͳͬͨʂ
    

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22. Call-by-name
    func expensiveComputation() -> Int {
    println("expensiveComputation")
    return 1
    }
    func foo(x: @autoclosure () -> Int, status: String) {
    println("foo")
    let result = status == "200" ? x() : 0
    let t1 = x()
    let t2 = x()
    println("result: \(result)")
    }
    foo(expensiveComputation(), "404")
    output
    foo
    expensiveComputation
    expensiveComputation
    result: 0
    

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23. Call-by-name
    func expensiveComputation() -> Int {
    println("expensiveComputation")
    return 1
    }
    func foo(x: @autoclosure () -> Int, status: String) {
    println("foo")
    let result = status == "200" ? x() : 0
    let t1 = x()
    let t2 = x()
    println("result: \(result)")
    }
    foo(expensiveComputation(), "404")
    output
    foo
    expensiveComputation
    expensiveComputation
    result: 0
    x ͕࠶ධՁ͞ΕΔʂ
    

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24. ࠶ධՁΛආ͚͍ͨʂ
    

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25. Call-by-need
    ࠶ධՁΛආ͚͍ͨʂ
    

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26. Call-by-need
    lazy property
    

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27. Call-by-need
    class Foo {
    lazy var tmp: Int = {
    println("tmp init")
    return 1
    }()
    }
    println("before init")
    let foo = Foo()
    println("after init")
    foo.tmp
    foo.tmp
    output
    before init
    after init
    tmp init
    

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28. Call-by-need
    class Foo {
    lazy var tmp: Int = {
    println("tmp init")
    return 1
    }()
    }
    println("before init")
    let foo = Foo()
    println("after init")
    foo.tmp
    foo.tmp
    output
    before init
    after init
    tmp init
    ̎ճݺ͹Ε͍ͯΔ͚ͲɼॳظԽ͸̍ճ͚ͩ
    ඞཁ࣌ͷΈධՁ͢Δ
    

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29. Call-by-need
    lazy property
    global variable
    static property
    struct Foo {
    static var tmp: Int = {
    println("tmp init")
    return 1
    }()
    static func log() {
    println("foo")
    }
    }
    let foo = Foo()
    Foo.log()
    println("access tmp")
    let x = Foo.tmp
    output
    foo
    access tmp
    tmp init
    

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30. Memoization
    func memoize( body: (T)->U ) -> (T)->U {
    var memo = Dictionary()
    return { x in
    if let q = memo[x] { return q }
    let r = body(x)
    memo[x] = r
    return r
    }
    }
    Fibonacci
    link: WWDC2014 Advanced Swift (from page 123)
    Ωϟογϡ͞ΕͨͷͰ࠶ܭࢉ͕ෆཁ
    

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31. When
    • Eager evaluation
    • evaluated completely before the function is applied
    • Call-by-name
    • evaluate only when needed
    • evaluate each time value is accessed
    • Call-by-need
    • evaluate only when needed
    • cache result for repeated access
    

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32. ͝੩ௌ͋Γ͕ͱ͏͍͟͝·ͨ͠
    

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