KeyPath Lenses

KeyPathLenses.swift

import Foundation

precedencegroup LeftApplyPrecedence {
    associativity: left
    higherThan: AssignmentPrecedence
    lowerThan: TernaryPrecedence
}

precedencegroup FunctionCompositionPrecedence {
    associativity: right
    higherThan: LeftApplyPrecedence
}

precedencegroup LensSetPrecedence {
    associativity: left
    higherThan: FunctionCompositionPrecedence
}

infix operator |> : LeftApplyPrecedence
infix operator .~ : LensSetPrecedence

public func |><A,B>(a:A, f:(A) -> B) -> B {
    return f(a)
}

public func .~ <Whole, Part> (lens: WritableKeyPath<Whole,Part>, part: Part) -> (Whole) -> Whole {
    return { whole in
        var copy = whole
        copy[keyPath:lens] = part
        return copy
    }
}

struct User {
    private(set) var name:String
    private(set) var id:Int

    init(id:Int, name:String) {
        self.name = name
        self.id = id
    }
}

let user = User(id: 1, name: "Terry")
let newUser =
    user
        |> (\User.name) .~ "Bob"
        |> (\User.id) .~ 2

dump(user)  // Name:"Terry", id:1
dump(newUser)  // Name:"Bob", id:2

This is awesome, thanks! Is there any way to set up the precedence groups so we don't need the parentheses around the keypaths?