Learning CQS from mutating keyword by. 2018/3/9 try! Swift After Talks 1

takasek iOS Developer @takasek OSS ActionClosurable౳ Xcode Extension PasteTheType 2

CQS Command Query Separation ίϚϯυΫΤϦ෼཭ݪଇ 3

Commands 1 • Change the state of a system • but do not return a value Queries • Return a result • do not change the observable state of the system • (are free of side effects) 1 https://martinfowler.com/bliki/CommandQuerySeparation.html 4

Side effects ෭࡞༻ 5

෭࡞༻͸ɺιϑτ΢ΣΞͷਖ਼͠͞ͷ ࠜڌʢʹ਺ֶతଐੑʣΛࣦΘͤΔ Side effects ruin a ground for mathematical validity of the software. 6

Referential transparency ࢀরಁաੑ Queries shouldn't change answers. ʮ໰͍Λൃ͢Δ͜ͱʹΑͬͯ ౴͕͑ม͑ΒΕΔ΂͖Ͱ͸ͳ͍ʯ 7

Martin Fowler says... 1 It would be nice if the language itself would support this notion. I could imagine a language that would detect state changing methods, or at least allow the programmer to mark them. ʮCQSΛαϙʔτͯ͘͠ΕΔݴޠ͕͋Ε͹͍͍ͷʹʯ 1 https://martinfowler.com/bliki/CommandQuerySeparation.html 8

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mutating keyword “if you need to modify the properties of your structure or enumeration within a particular method, you can opt in to mutating behavior for that method” 3 func ͕஋ܕΦϒδΣΫτʹมߋΛՃ͑Δ͜ͱΛ໌ࣔ 3 "The Swift Programming Language (Swift 4.1)" 10

4 4 https://swift.org/documentation/api-design-guidelines/ 11

Commands 1 • Change the state of a system • but do not return a value Queries • Return a result • do not change the observable state of the system • (are free of side effects) 1 https://martinfowler.com/bliki/CommandQuerySeparation.html 12

Commands 1 • Change the state of a system • but do not return a value ! Queries • Return a result • do not change the observable state of the system • (are free of side effects) 1 https://martinfowler.com/bliki/CommandQuerySeparation.html 13

find ./stdlib/public/core | grep .swift | xargs cat | grep "public mutating func" public mutating func append(_ newElement: Element) public mutating func insert(_ newElement: Element, at i: Int) public mutating func removeFirst() public mutating func sort() public mutating func subtract(_ other: Self) public mutating func multiply(by other: Self) public mutating func divide(by other: Self) public mutating func negate() public mutating func round(_ rule: FloatingPointRoundingRule) public mutating func write(_ other: String) public mutating func encode(_ value: T, forKey key: Key) throws 14

Using Swift value types Swiftͷ஋ܕΛ࢖͏ = Geting along with CQS CQSʹ׳Ε਌͠Ή 15

How about reference types (classes)? 16

Do queries have side effects? ΫΤϦ͕෭࡞༻Λ͍࣋ͬͯͳ͍͔ʁ Do commands have return value? ίϚϯυ͕໭Γ஋Λ͍࣋ͬͯͳ͍͔ʁ 17

Learn from Swift. Swiftʹֶ΅͏ Stand on the shoulders of giants. աڈͷڊਓʹֶ΅͏ 18

׬ 19

! 20

find ./stdlib/public/core | grep .swift | xargs cat | grep "public mutating func" public mutating func popFirst() -> Element? public mutating func popLast() -> Element? public mutating func update(with newMember: Element) -> Element? public mutating func remove(at index: Int) -> Element public mutating func remove(_ member: Element) -> Element? public mutating func removeFirst() -> Element public mutating func removeValue(forKey key: Key) -> Value? public mutating func next() -> Element? public mutating func decode(_ input: inout I) -> UnicodeDecodingResult public mutating func superEncoder() -> Encoder public mutating func nestedContainer(keyedBy keyType: NestedKey.Type) -> KeyedEncodingContainer 21

Exceptions. ྫ֎΋͋Δɻ 22

Popping a stack is a good example of a query that modifies state. Meyer correctly says that you can avoid having this method, but it is a useful idiom. So I prefer to follow this principle when I can, but I'm prepared to break it to get my pop.1 ʮ෭࡞༻ආ͚ΒΕΔͬͯMeyer͸ݴͬͯΔ͚Ͳɺ popʹ໭Γ஋͕͋ͬͨΒਖ਼௚ศརͳͷͰ ʰCQSݪଇ͸कΕΔͳΒकΔʱʯ 1 https://martinfowler.com/bliki/CommandQuerySeparation.html 23

कഁ཭ 24

͓͖ͯ͞ɺجຊతʹ͸༗ޮͳͷ͕ ʮݪଇʯ Principles work fine principally. 25

ϝϦοτҙࣝͯ͠ ࢖͍͜ͳ͠·͠ΐ͏ Get along with them! 26