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4XJGUͷNeverͱ
 ϘτϜܕ 2019/04/14 ୈ7ճΧϯϑΝϨϯεಈըؑ৆ձ Mobile Act OSAKA #9 (ิܽ) try! Swift Tokyo 2019 (Backup)

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About Me w ͻΖΜʢҰٶߒڭʣ w 5XJUUFS(JU)VC
 2JJUB4MJEF4IBSF 4QFBLFS%FDL
 ˠIJSPOZUJD w ಙౡࡏॅ

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ݱ࣌఺ͷ4XJGUͰ͸
 Never͸ϘτϜܕͰ͸͋Γ·ͤΜ

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Agenda w NeverܕͬͯԿ͚ͩͬʁ w ϘτϜܕͬͯԿΑʁ w ΋͠Never͕ϘτϜܕͩͬͨΒʁ

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Never Type

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func foo() -> Type foo͸TypeͷΠϯελϯεΛฦ͢

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func foo() -> Int foo͸IntͷΠϯελϯεΛฦ͢ -3, 0, 10, 42, …

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func foo() -> Void foo͸஋Λฦ͞ͳ͍

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func foo() -> Type foo͸TypeͷΠϯελϯεΛฦ͢

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func foo() -> Void foo͸VoidͷΠϯελϯεΛฦ͢

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Void Type w ۭͷλϓϧܕͷΤΠϦΞε w ۭͷλϓϧͷܕʜ() w ͦͷ།Ұͷ஋ʜ()

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No content

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func foo() -> Never foo͸ฦͬͯ͜ͳ͍

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func foo() -> Type foo͸TypeͷΠϯελϯεΛฦ͢

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func foo() -> Never foo͸NeverͷΠϯελϯεΛฦ͢

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Never Type w DBTFΛ࣋ͨͳ͍FOVN FOVN/FWFS\^ w ΠϯελϯεΛ࣋ͨͳ͍ w ؔ਺͸/FWFSͷΠϯελϯεΛฦͤͳ͍ w ແݶϧʔϓɺΫϥογϡɺϓϩάϥϜऴྃɺྫ֎

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Bottom Type

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Subtype class Animal { ... } class Dog: Animal { ... } Animal Dog

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Subtype Dog͸AnimalͷαϒλΠϓ DogͷΠϯελϯεΛ
 Animalͱͯ͠ѻ͑Δ Animal Dog

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Animal Dog Cat Animal Dog Cat

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Animal Dog Cat Plant Flower Star … ?

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Animal Dog Cat Plant Flower Star … Any Top Type

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Animal Dog Cat Plant Flower Star … Any Top Type ? Bottom Type

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Bottom Type w ϘτϜܕ͸͢΂ͯͷܕͷαϒλΠϓ w ϘτϜܕͷΠϯελϯεΛͲͷܕͱͯ͠΋ѻ͑Δ

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4501ࡀະຬҿञ ະ੒೥ऀͷҿञ͸๏཯Ͱې͡ΒΕ͍ͯ·͢

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Bottom Type w ϘτϜܕ͸͢΂ͯͷܕͷαϒλΠϓ w ϘτϜܕͷΠϯελϯεΛͲͷܕͱͯ͠΋ѻ͑Δ w ϘτϜܕ͸ΠϯελϯεΛ࣋ͨͳ͍ܕ

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Imagine:
 Never as a Bottom Type

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Case 1 class FooViewController: UIViewController { var label: UILabel! ... }

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Case 1 class FooViewController: UIViewController { lazy var label: UILabel = fatalError() ... }

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Case 2 let data: [String: String] = ... guard let name = data["name"] else { throw ParseError.missing("name") } speaker.name = name guard let icon = data["icon"] else { throw ParseError.missing("icon") } speaker.icon = icon

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Case 2 let data: [String: String] = ... speaker.name = data["name"] ?? try missing("name") speaker.icon = data["icon"] ?? try missing("icon") func missing(_ key: String) throws -> Never { throw ParseError.missing(key) }

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Case 2+ let data: [String: String] = ... speaker.name = data["name"] ?? throw ParseError.missing("name") speaker.icon = data["icon"] ?? throw ParseError.missing("icon") ͞Βʹthrow͕NeverΛฦࣜ͢ͳΒ

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Case 3 enum Result { case success(Value) case failure(Error) } func foo() -> Result { ... } foo͸ࣦഊ͠ͳ͍

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Case 3 • 4XJGUͰ࣮ݱ͞Ε·ͨ͠ 4& https://github.com/apple/swift-evolution/blob/master/proposals/0215-conform-never-to- hashable-and-equatable.md

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Case 3 w 4&ͷٞ࿦ʹͯ https://forums.swift.org/t/se-0215-conform-never-to-equatable-and-hashable/13586

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Protocol • ΠϯελϯεϓϩύςΟ (var) • Πϯελϯεϝιου (func) • ܕϓϩύςΟ (static var) • ܕϝιου (static func) • ΠχγϟϥΠβ (init) Πϯελϯεʹର͢Δ
 ੍໿Ͱ͸ͳ͍

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Recap

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Recap w NeverܕͬͯԿ͚ͩͬʁ w ϘτϜܕͬͯԿΑʁ w ΋͠Never͕ϘτϜܕͩͬͨΒʁ
 ˠΠϯελϯεΛ࣋ͨͳ͍ܕ ˠ͢΂ͯͷܕͷαϒλΠϓʹͳΔܕ ˠ͍Ζ͍ΖεοΩϦ͢Δ͜ͱ͕͋Δ

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Never as a Bottom Type • SE-0215 - Conform Never to Equatable and Hashable
 https://forums.swift.org/t/se-0215-conform-never-to-equatable-and-hashable/13586 • SE-0102: Remove @noreturn attribute and introduce an empty Never type
 https://github.com/apple/swift-evolution/blob/master/proposals/0102-noreturn-bottom- type.md • [Pitch] Never as a bottom type
 https://forums.swift.org/t/pitch-never-as-a-bottom-type/5920

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One more thing: Kotlin 4XJGU Void Never ,PUMJO Unit Nothing Nothing͸ϘτϜܕ

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Case 2 (Kotlin) val data: Map = ... speaker.name = data["name"] ?: throw MissingException("name") speaker.icon = data["icon"] ?: throw MissingException("icon") throw͸NothingΛฦࣜ͢

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͝ਗ਼ௌ͋Γ͕ͱ͏͍͟͝·ͨ͠