Lenses - Re-imagined with Swift 4

lenses.swift

// Motivation:
// http://chris.eidhof.nl/post/lenses-in-swift/
// https://www.youtube.com/watch?v=ofjehH9f-CU&feature=youtu.be

protocol Changeable {
    func changed<A>(changes: [KeyPath<Self, A>: A]) -> Self
}

extension Changeable {
    func changed<A>(changes: [KeyPath<Self, A>: A]) -> Self {
        var modified = self
        changes.forEach { readOnlyKeyPath, newValue in
            // Will this continue to work or is this a bug in the compiler?
            // https://bugs.swift.org/browse/SR-6502
            guard let writeableKeyPath = readOnlyKeyPath as? WritableKeyPath<Self, A> else { return }
            modified[keyPath: writeableKeyPath] = newValue
        }
        
        return modified
    }
}


struct User: Changeable {
    let firstName: String
    let lastName: String
    let age: Int
    let someInt: Int
    let address: Address
}

struct Address: Changeable {
    let street: String
    let zipCode: String
}

let oneMarketStreet = Address(street: "1 Market Street", zipCode: "94105")
let jane = User(firstName: "Jane", lastName: "Doe", age: 35, someInt: 123, address: oneMarketStreet)
let john = jane.changed(changes: [\.firstName: "John", \.lastName: "Poe"])
               .changed(changes: [\.age: 40])
               .changed(changes: [\.address.zipCode: "94103"])
print(jane) // User(firstName: "Jane", lastName: "Doe", age: 35, someInt: 123, address: __lldb_expr_244.Address(street: "1 Market Street", zipCode: "94105"))
print(john) // User(firstName: "John", lastName: "Poe", age: 40, someInt: 123, address: __lldb_expr_244.Address(street: "1 Market Street", zipCode: "94103"))