Lazy Evaluation in Swift

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

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:

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

When?

Eager evaluation Call-by-name Call-by-need

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

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

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

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

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

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

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

ඞཁ࣌ͷΈධՁ͍ͨ͠ʂ ධՁλΠϛϯάมߋ͍ͨ͠ʂ

Call-by-name ඞཁ࣌ͷΈධՁ͍ͨ͠ʂ ධՁλΠϛϯάมߋ͍ͨ͠ʂ

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

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

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

@autoclosure

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

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

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

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

࠶ධՁΛආ͚͍ͨʂ

Call-by-need ࠶ධՁΛආ͚͍ͨʂ

Call-by-need lazy property

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

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

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

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

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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