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flatMap ͷ࿩ USAMI Kosuke Fenrir Inc.

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͸͡Ίʹ • Swi% ͷ flatMap ʹ͍ͭͯ࿩͠·͢ • flatMap ͦͷ΋ͷ͸͍Ζ͍Ζͳݴޠʹ͋Γ·͢

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flatMap • Α͘Θ͔Βͳ͍Πϝʔδ͕͋Δʁ • ίʔυ͕ಡΈͮΒ͍ʁ • map ͯ͠ fla'en ͢Δ΍ͭʁ • ؔ਺ܕʁ Ϟφυʁ

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flatMap • ݸਓతʹ͸ɺؾָʹ࢖͑͹͍͍ͱࢥ͏

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Op#onal ͷ flatMap

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Op#onal ͷ flatMap • ม׵લ : Optional • ม׵ॲཧ : T -> Optional • flatMap : Optional -> Optional • ม׵લ͕ non-nil ͳΒม׵ॲཧΛద༻͢Δ • ม׵લ͕ nil ͳΒม׵ॲཧΛద༻͠ͳ͍

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ྫ • ม׵લ : Optional • ม׵ॲཧ : String -> Optional : { Int($0) } • string.flatMap { Int($0) } • "123" -> Int("123") -> 123 • "abc" -> Int("abc") -> nil • nil -> nil

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nil ͷೖྗΛҙࣝ͠ͳ͍͍ͯ͘ // string ͸ Optional ͚ͩͲɾɾɾ let string: String? = "123" let integer: Int? = string.flatMap { Int($0) // $0 ͸ unwrap ͞Ε͍ͯΔ }

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if let Ͱॻ͘ͱ৑௕ʹͳΔ let string: String? = "123" let integer: Int? if let string = string { integer = Int(string) } else { integer = nil }

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Op#onal chaining ෩ʹ࢖͏ // Optional chaining string?.isEmpty // ಉ༷ʹ͜Μͳ΋ͷ͕ཉ͘͠ͳΔʁ string?.toInt // ୅ΘΓʹ͜͏͢Δ string.flatMap { Int($0) }

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Array ͷ flatMap

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Array ͷ flatMap • 2 छྨͷ flatMap ͕ଘࡏ͢Δ

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Array ͷ flatMap (1) • ม׵લ : Array • ม׵ॲཧ : T -> Array • flatMap : Array -> Array

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ྫ • ม׵લ : Array • ม׵ॲཧ : Int -> Array : { [$0, $0 * 10] } • [1, 2, 3].flatMap { [$0, $0 * 10] } • 1 -> [1, 10], 2 -> [2, 20], 3 -> [3, 30] • [1, 10, 2, 20, 3, 30]

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Op#onal ͱ Array ͷ flatMap ൺֱ • ม׵લ : Optional • ม׵ॲཧ : T -> Optional • flatMap : Optional -> Optional • ม׵લ : Array • ม׵ॲཧ : T -> Array • flatMap : Array -> Array

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flatMap ͷجຊܗ • ܕ T ΍ U ͕ೖͬͨԿΒ͔ͷശʢBoxʣ͕͋Δͱ͖ʹ • ม׵લ : Box • ม׵ॲཧ : T -> Box • flatMap : Box -> Box

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Array ͷ flatMap (2) • ม׵લ : Array • ม׵ॲཧ : T -> Optional • flatMap : Array -> Array • ม׵ॲཧͷ݁Ռ͕ non-nil ͷ΋ͷ͕ Array ͷཁૉʹͳΔ • ม׵ॲཧͷ݁Ռ͕ nil ͷ΋ͷ͸ࣺͯΒΕΔ

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ྫ • ม׵લ : Array • ม׵ॲཧ : String -> Optional : { Int($0) } • ["123", "abc"].flatMap { Int($0) } • "123" -> 123, "abc" -> nil • [123]

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Array ͷ 2 ͭͷ flatMap ͷؔ܎ • flatMap (2) ͸ flatMap (1) Ͱ࣮ݱͰ͖Δ • non-nil Λฦ͢୅ΘΓʹ1ཁૉͷ Array Λฦ͢ • nil Λฦ͢୅ΘΓʹۭͷ Array Λฦ͢ • flatMap (2) ͷํ͕ίʔυ͕γϯϓϧʹͳΔ • flatMap (1) ͸࣮͸͋Μ·Γ࢖Θͳ͍

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׆༻ྫ • ["A", "B", nil, "C"] : [String?] • ͔͜͜Β nil ΛऔΓআ͍ͨ [String] ͕ཉ͍͠ // flatMap ͳ͠ͳΒɺ͜Μͳײ͡Ͱॻ͘ʁ ["A", "B", nil, "C"] .filter({ $0 != nil }) as! [String]

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׆༻ྫ // ୯ʹ͜͏ॻ͚͹ྑ͍ ["A", "B", nil, "C"].flatMap { $0 } • ม׵લ : Array> • ม׵ॲཧ : Optional -> Optional • flatMap : Array> -> Array

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flatMap Λ׆༻͢Δ৔໘ • Op$onal ͱͷ૬ੑ͕ྑ͍ • Op$onal Λѻ͏ॲཧ͕͋Δͱ͖ʹ࢖͑ͳ͍͔ߟ͑Δͱྑ͍

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ࢀߟɿଞʹ flatMap ͕࢖͑Δ΋ͷ • Result ܕʢan+typical/Resultʣ • Op+onal ͱ΄΅ಉ͡ʢnil ͷ୅ΘΓʹ۩ମతͳ Error Λฦ͢ʣ • Observable ܕʢRxSwi:ʣ • ඇಉظετϦʔϜɺ్தͰΤϥʔʹͳͬͨ৔߹ͷରԠ͕༰қ

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flatMap Λ্खʹ׆༻͠Α͏