nel - Speaker Deck

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"ͶΔ" ͸͍͍ͧ 2018.12.9 #shinjukult @to4iki 1

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Me • @to4iki •  • AbemaTV • builderscon 2

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ಥવͰ͕͢ɺ ഑ྻૢ࡞ʹؔͯ͠ɺ 3

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e.g. 1 if xs.isEmpty { // fallback processing. } else { // anything. } 4

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e.g. 1 guard !xs.isEmpty else { fatalError("unrechable.") } // anything 5

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e.g. 2 /// GraphQL type’s fields enum UserField: String { case id, name, email } /// build up a query func query(_ fields: Set) -> String { return (["{\n"] + fields.map { " \($0.rawValue)\n" } + ["}\n"]) .joined() } 6

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e.g. 2 ! request success query([.id, .name]) // { // name // id // } ! request runtime-error query([]) // { // } 7

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! • ֓೦ϨϕϧͰ Nil or Cons(ଘࡏ͢Δ͔) Λҙࣝ͢Δඞཁ͕͋Δ • ܕ҆શͰ͸ͳ͍ ! 8

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ۭ഑ྻͱ͍͏ɺ ಉ͡഑ྻͳΜ͚ͩͲ ͦͷঢ়ଶ(ཁૉ͕༗Δ͔ແ͍͔)Λ ҙࣝ͠ͳ͍ͱ͍͚ͳ͍ 9

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ۭͱ͍͏ঢ়ଶ͕ى͜Γ͑ͳ͍ ഑ྻ͕͋Ε͹Α͍ͷͰ͸ 10

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Nel(ͶΔ) 11

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Non Empty List 12

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NonEmptyList2 /// a data type which represents a non empty list of A. /// single element (head) and optional structure (tail). struct NonEmptyArray { let head(T) let tail: [T] } 2 Swift͸ඪ४ܕͰList͕ଘࡏ͠ͳ͍ͷͰɺArrayΛࣄྫʹͯ͠ѻ͏ 13

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! // [1, 2, 3] NonEmptyArray(head: 1, tail: [2, 3]) 14

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other solution enum NonEmptyArray { case single(T) indirect case cons(T, NonEmptyArray) } // [1, 2, 3] NonEmptyArray.cons(1, .cons(2, .single(3))) 15

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ۭͷঢ়ଶΛදݱͰ͖Δͷ͸Array ͚ͩͰ͸ͳ͍ΑͶ 16

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NonEmpty struct NonEmpty { var head: C.Element var tail: C init(_ head: C.Element, _ tail: C) { self.head = head self.tail = tail } } 17

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typealias NonEmptyArray = NonEmpty> typealias NonEmptySet = NonEmpty> where T: Hashable typealias NonEmptyDictionary = NonEmpty> where K: Hashable // etc... 18

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NonEmpty Λඪ४ͷίϨΫγϣϯૢ࡞ʹ͚͍ۙͮͯ͘ 19

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(ུ) 20

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! overload non-nil property extension NonEmpty { /// overload non-nil var first: C.Element { return self.head } } extension NonEmpty where C: BidirectionalCollection { /// overload non-nil var last: C.Element { return self.tail.last ?? self.head } } 21

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Array let xs = [1, 2, 3] xs.first.map { $0 + 1 } ?? 0 xs.last! + 1 // unsafe ! NonEmptyArray let xs = NonEmptyArray(1, 2, 3) xs.first + 1 xs.last + 1 22

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Demo 23

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Other lang • Haskel: Data.List.NonEmpty • https://hackage.haskell.org/package/semigroups-0.16.0.1/docs/Data-List- NonEmpty.html • Elm: List.Nonempty • https://package.elm-lang.org/packages/mgold/elm-nonempty-list/3.1.0/List- Nonempty • scalaz / cats • functional java • etc... 24

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Advanced 25

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Validation(Nel) • Monad • Applicative Functor • https://typelevel.org/cats/datatypes/validated.html /// `Result` with success-value, `nel` as the failure type. enum Validated { case valid(Value) case invalid(NonEmptyArray) } 26

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Kotlin: arrow.Validated • https://arrow-kt.io/docs/datatypes/validated/ @higherkind sealed class Validated : ValidatedOf { data class Valid(val a: A) : Validated() data class Invalid(val e: E) : Validated() } 27

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e.g let validatedPassword: Validated ... switch validatedPassword { case .valid(let value): print(value) case .invalid(let errors): // e.g. preset `errors.first` messages } 28

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Conclusion • Ϧετ͸ ۭ or ۭͰ͸ͳ͍ͷ 2छྨͷঢ়ଶ͕ଘࡏ͢Δ • NonEmpty ʹΑΓɺܕϨϕϧͰ ۭͰ͸ͳ͍ ঢ়ଶΛ୲อ͢Δ͜ͱ͕Ͱ͖Δ • ڊਓͷݞ(ଟݴޠͰͷ࣮૷ྫ)ʹ৐͍ͬͯ͜͏ 29

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SeeAlso • https://www.pointfree.co/episodes/ep20-nonempty • http://eed3si9n.com/learning-scalaz/ja/Validation.html • http://xuwei-k.github.io/scalaz-docs/validation.html 30

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Thanks 31