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Lens
Shinjuku.LT#17
@to4iki
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Me
• Takezawa
•
•
!
•
☕
•
♨
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what Lens?
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Def. / Feature
• getter/setterͷநԽɾম͖͠(൚༻Խ)
• ༷ʑͳσʔλΛڞ௨ͷI/FͰૢ࡞Ͱ͖Δ
• ಛఆͷݴޠʹґଘ͠ͳ͍֓೦/ܕ
• functional
• ෆมߏ / ߹Մೳ
• Lens Laws
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ekmett/lens
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• Monocle.Lens - elm-monocle
• julien-truffaut/Monocle
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https://github.com/julien-truffaut/Monocle/blob/master/image/class-
diagram.png
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getter
setter
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getter
• ΦϒδΣΫτS͔ΒAΛऔΓग़͢
• get: (S) -> A
setter
• ΦϒδΣΫτSͷҙͷΛAʹมߋ͢Δ
• set: (S, A) -> S == set: (S) -> A -> S
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͜ΕΒͷ͍͢͝
getter/setter1Λ
ҙͷΦϒδΣΫτͷ
ҙͷϝϯόʹద༻͢Δͷ͕
Lens
1 https://qiita.com/to4iki/items/f0cc28e1102cf53be85d
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࣮ͯ͠ΈΔ
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struct Lens {
private let _get: (S) -> T
private let _set: (S, T) -> S
init(get: @escaping (S) -> T, set: @escaping (S, T) -> S)
func get(_ source: S) -> T
func set(_ source: S, value: T) -> S
}
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͜Μ͚ͩ
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struct User {
let name: String
}
let user = User(name: "hoge")
let nameLens = Lens(
get: { $0.name },
set: { User(name: $1) }
)
nameLens.get(user) // "hoge"
nameLens.set("fuga") // User(name:"fuga)"
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Usecase?
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ωετͨ͠σʔλͷ͋ΔҰ෦
͚ͩΛมߋ͍ͨ͠
struct Street { let name: String }
struct Address { let street: Street }
struct Company { let address: Address }
struct Employee { let company: Company }
let employee = Employee(company: Company(address: Address(street: Street(name: "street"))))
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!
let updatedEmployee =
Employee(company:
Company(address:
Address(street:
Street(name: employee.company.address.street.name.capitalizedString)
)
)
)
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Lens߹Λ͍ɺ
ෆมߏΛอͪͳ͕Β
ΞΫηεग़དྷΔ
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struct Lens {
...
func compose(_ other: Lens) -> Lens {
return Lens(
get: { (source: S) -> U in
other.get(self.get((source)))
},
set: { (source: S, value: U) -> S in
self.set(source, value: other.set(self.get(source), value: value))
}
)
}
}
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(operatorఆٛ͠ͱ͘)
/// Left-to-Right Composition
infix operator >>> : MultiplicationPrecedence
func >>> (lhs: Lens, rhs: Lens) -> Lens {
return lhs.compose(rhs)
}
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Demo
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!
let nameLens = Lens(get: { $0.name }, set: { Street(name: $1) } )
let streetLens = Lens ...
let addressLens = Lens ...
let companyLens = Lens ...
let lens: Lens = companyLens >>> addressLens >>> streetLens >>> nameLens
let employee = Employee(company: Company(address: Address(street: Street(name: "street"))))
let updatedEmployee = lens.modify(employee) { $0.capitalized }
// Employee(Company(Address(Street(name: "Street")
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https://github.com/
to4iki/LensKit
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Conclusion
• Lens ͍͢͝getter/setter
• ෆมߏΛอͪͳ͕Β
• ਂ͍ߏͰ
• ʹΞΫηεग़དྷΔ
• Ϣʔεέʔεਖ਼͍͠
• ςετ࣌ͷαϙʔτͱͯ͠͏?
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See also
• ࠓ͔Β࢝ΊΔ Lens/Prism
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