nel

Non-Empty-List

# 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

Transcript

1. "ͶΔ" ͸͍͍ͧ
   2018.12.9
   #shinjukult
   @to4iki
   1
   

   View Slide

2. Me
   • @to4iki
   • 
   • AbemaTV
   • builderscon
   2
   

   View Slide

3. ಥવͰ͕͢ɺ
   ഑ྻૢ࡞ʹؔͯ͠ɺ
   3
   

   View Slide

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

   View Slide

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

   View Slide

6. 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
   

   View Slide

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

   View Slide

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

   View Slide

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

   View Slide

10. ۭͱ͍͏ঢ়ଶ͕ى͜Γ͑ͳ͍
    ഑ྻ͕͋Ε͹Α͍ͷͰ͸
    10
    

    View Slide

11. Nel(ͶΔ)
    11
    

    View Slide

12. Non Empty List
    12
    

    View Slide

13. 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
    

    View Slide

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

    View Slide

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

    View Slide

16. ۭͷঢ়ଶΛදݱͰ͖Δͷ͸Array
    ͚ͩͰ͸ͳ͍ΑͶ
    16
    

    View Slide

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

    View Slide

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

    View Slide

19. NonEmpty
    Λඪ४ͷίϨΫγϣϯૢ࡞ʹ͚͍ۙͮͯ͘
    19
    

    View Slide

20. (ུ)
    20
    

    View Slide

21. !
    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
    

    View Slide

22. 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
    

    View Slide

23. Demo
    23
    

    View Slide

24. 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
    

    View Slide

25. Advanced
    25
    

    View Slide

26. 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
    

    View Slide

27. 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
    

    View Slide

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

    View Slide

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

    View Slide

30. 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
    

    View Slide

31. Thanks
    31
    

    View Slide