Continuation-Passing Style and Design By Contract(English ver.) / 20190319(E) #tryswift_pre

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Continuation-Passing Style and Design By Contract by. 2019/3/19 try! Swift Pre Talks 2019 1

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takasek @takasek Works OSS: ActionClosurable౳ App: PasteTheType Articles ʮίϯύΠϧΤϥʔ΍ϥϯλΠϜΤϥʔΛ௚͍ͯ͘͠ ͚ͩͰiOSΞϓϦͷ࡞Γํ͕Θ͔ΔϓϩδΣΫτʯ ʮ͓લΒ͕ModelͱݺͿΞϨΛͳΜͱݺͿ΂͖͔ʯ ʮiOSΞϓϦઃܭύλʔϯೖ໳ʯʢڞஶʣ 2

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ʮiOSΞϓϦઃܭύλʔϯೖ໳ʯ ON SALE!! https://peaks.cc/takasek/iOS_architecture takasek୲౰ষ Chap 1ʮTo designʯ Chap 2ʮBefore applying patterns to your designʯ Chap 3ʮDesigning swiftyʯ Chap 4ʮOverview through history of architecturesʯ 3

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Continuation- Passing Style (CPS) 4

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// Without CPS func doSomething(i: Int) -> String { sleep(100) return "\(i)" } // With CPS func doSomething(i: Int, completion: @escaping (String) -> Void) { DispatchQueue.global().async { sleep(100) completion("\(i)") } } 5

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! 6

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func doSomething( i: Int, completion: @escaping (String) -> Void ) This interface isn't the best 7

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ʮI know what you meanʯ 8

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ʮAbout Callback Hell, huh?ʯ 9

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Nop. 10

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Design By Contract 11

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Design By Contract The idea to construct reliable system introduced by Bertrand Meyer 12

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Design By Contract Asserts; • Preconditionɿ a caller should ensure it. described by Argument Type or so on. • Postconditionɿ a callee should ensure it. described by Return Type or so on. If precondition is violated, throw Exception. 13

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For example, func half(of i: Int) throws -> Int { assert(i.isMultiple(of: 2)) if !i.isMultiple(of: 2) { throw Error.isOdd } return i / 2 } • Precondition: • accepts Int. • accepts Even. (not describable as type) • Postcondition: returns Int. • Exception: thrown if the argument is odd. 14

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Swifty type system helps Design by Contract. 15

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By the way, 16

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func half(of i: Int) throws -> Int { if !i.isMultiple(of: 2) { throw Error.isOdd } return i / 2 } translate it to CPS. 17

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func half(of i: Int, completion: (Int) -> Void) { if !i.isMultiple(of: 2) { return } completion(i / 2) } 18

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OK, in the case of Odd value...? 20

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Not called...? 22

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completion hadn't be called 24

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func half(of i: Int, completion: (Int) -> Void) { if !i.isMultiple(of: 2) { return // ! } completion(i / 2) } 25

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enum Result { case success(T) case error(Error) } func half(of i: Int, completion: (Result) -> Void) { if !i.isMultiple(of: 2) { return // ! } completion(.success(i / 2)) } 26

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enum Result { case success(T) case error(Error) } func half(of i: Int, completion: (Result) -> Void) { if !i.isMultiple(of: 2) { completion(.error(.isOdd)) // } completion(.success(i / 2)) } 27

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Okay, check again! 28

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Called twice...? 30

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func half(of i: Int, completion: (Result) -> Void) { if !i.isMultiple(of: 2) { completion(.error(.isOdd)) // ! } completion(.success(i / 2)) } 31

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func half(of i: Int, completion: (Result) -> Void) { if !i.isMultiple(of: 2) { completion(.error(.isOdd)) return // ! } completion(.success(i / 2)) } 32

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CPS avoids benefits of Static Typing 33

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ͳͥͳͷ͔ ໭Γ஋ͷܕʹΑͬͯ ࣄޙ৚݅ΛදݱͰ͖ͳ͍͔Β (T, (U) -> Void) -> Void Void͸࠷ऑͷࣄޙ৚݅ 34

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Ͳ͏͢Ε͹͍͍͔ ໭Γ஋ͷܕʹΑͬͯ ࣄޙ৚݅Λදݱ͢Ε͹͍͍ (T) -> XXX ʮ͍ͣΕ݁Ռ͕ಘΒΕΔʯ͜ͱΛܕ෇͚ͰࣔͤΕ͹ɺ ඇಉظॲཧͰ΋໭Γ஋Λػೳͤ͞ΒΕΔ 35

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func half(of i: Int) -> RxSwift.Single { if !i.isMultiple(of: 2) { return .error(Error.isOdd) } return .just(i / 2) } ͜ΕͳΒ੩తܕ෇͚ͷԸܙΛड͚ͭͭɺ ʮඇಉظͰ͋Δ͜ͱʯΛදݱͰ͖Δ 36

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• Promise(Future) ύλʔϯ • PromiseK • Hydra • RxSwift • etc • async/await • comming soon in Swift 5.x!? • ͜ͷݴޠ࢓༷Λϕʔεʹ Future ܕΛߏஙՄೳ CPSΛશ໘తʹஔ͖͔͑Մೳ͔ͱ͍͏ͱ೉͍͕͠ɺ બ୒ࢶΛ஌ͬͨ͏͑ͰύλʔϯΛબͼ͍ͨ 37

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݁࿦ ɾਓؒ͸ϛεΛ͠·͢ ɾϛεΛ͢Δલʹɺ ද໌ΛίϯύΠϥ͕෼͔ΔܗͰॻ͚ͳ͍͔ ҙࣝ͠ͳ͕Βઃܭ͠·͠ΐ͏ 38

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͓·͚ ͱͯ΋5෼Ͱ͸ޠΓ͖Εͳ͍࿩ 39

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ʮද໌ʯʹ͍ͭͯͷิ଍ • Ҿ਺ɾ໭Γ஋ͷܕ͚͕ͩද໌Ͱ͸ͳ͍ͱݴ͚ͬͨΕͲ… • assert ͚ͩͰͳ͘ɺSwiftͰݴ͑͹ generics ΍ conditional conformance΋ද໌ͷํ๏ • ΂ͭʹɺίϯύΠϥղऍෆೳͳද໌͸ྑ͘ͳ͍…Θ͚Ͱ͸ͳ͍ • දݱྗΛ্͛Δͱֶशίετɾෳࡶੑ͕ϖΠ͠ͳ͍͜ͱ΋ଟ͍ͷͰɺͦ͜͸όϥϯε • ͨͱ͑͹ɺڽͬͨprotocolͷwhere۟ɺৗਓʹཧղग़དྷΔ͔ͬͯݴΘΕΔͱݫ͍͠ΑͶ… ɹ 40

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ิ଍ͷิ଍ ٯʹݴ͑͹ɺ ܖ໿ʹΑΔઃܭ͸ ݴޠΛ໰Θͣద༻Մೳͳ֓೦Ͱ͢ t_wada͞ΜʹΑΔ ͋͑ͯPHPΛϕʔεʹͨ͠εϥΠυ PHPer͡Όͳͯ͘΋ΦεεϝͰ͢ https://speakerdeck.com/twada/php- conference-2016 41

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ʮܧଓ౉͠ʯʹ͍ͭͯͷิ଍ • ࣮͸ async/await͸ʮ಺෦తͳCPSม׵ʯͦͷ΋ͷͱ͍͑·͢ Continuation Passing Style Revisited – Fabulous Adventures In Coding https://blogs.msdn.microsoft.com/ericlippert/2010/10/21/ continuation-passing-style-revisited-part-one/ ͦͷ࿨༁ matarillo.com: ܧଓ౉͠ελΠϧ(CPS)ͱඇಉظߏจ(async/await) https://matarillo.com/general/cps 42

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Swiftͷ async/await ͱ ܧଓͱ Future ͷؔ܎ Chris Lattnerͷproposal2ʹΑΔͱɺ • ଞͷݴޠͰ͸ Future ͕·ͣ͋ͬͯɺͦͷ্ʹasync/await͕৐͔͍ͬͬͯΔ • Swiftͷasync/awaitͷઃܭͰ͸ͦΕΛલఏͱ͸ͤͣɺܧଓΛͦͷ··ѻ͏खஈΛ࿐ग़͍ͤͯ͞ Δ • ͱ͸͍͑ Future ͱ͍͏ܕ෇͚͞Εͨঢ়ଶͰѻ͏ΠϯλϑΣʔε΋ߏஙՄೳͰ͢Αɺͱ͍͏͜ͱ 2 https://gist.github.com/lattner/429b9070918248274f25b714dcfc7619 yimajo͞Μͷʮasync/awaitݚڀಡຊʯ΋͋ΘͤͯͲ͏ͧ 43

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ؔ࿈ɿʮܖ໿ʹΑΔઃ ܭʯΛΩʔϫʔυͰ֦ ு͢Δ࿩ ʮܖ໿ʹΑΔઃܭʯʹ͍ͭͯৄղ͞Ε ͍ͯΔʮΦϒδΣΫτࢦ޲ೖ໳ʯʹ ͸ɺ ඇಉظʢฒߦʣॲཧΛ΋ͱʹ֦ுͨ͠ ৔߹ͷܖ໿ͷϙϦγʔʹ͍ͭͯͷٞ࿦ ΋͋Γ·͢ ໘ന͍Αʂ 44