A simpler swifty lens pattern

KeypathUpdatable.swift

public protocol KeypathUpdatable {
    func update<T>(_ keyPath: WritableKeyPath<Self, T>, to value: T) -> Self
}

public extension KeypathUpdatable {
    public func update<T>(_ keyPath: WritableKeyPath<Self, T>, to value: T) -> Self {
        var copy = self
        copy[keyPath: keyPath] = value
        return copy
    }
}

Usage.swift

// User.swift
struct User {
    private(set) var name: String
    private(set) var pet: Pet
}
extension User: KeypathUpdatable { }

// Pet.swift
struct Pet {
    private(set) var name: String
}
extension Pet: KeypathUpdatable { }

// elsewhere...
let user = User(name: "John", pet: Pet(name: "Fido"))
user.name // John
user.pet.name // Fido

let newName = user.update(\.name, to: "Jane")
newName.name // Jane
newName.pet.name // Fido

let newPetName = newName.update(\.pet.name, to: "Tabby")
newPetName.name // Jane
newPetName.pet.name // Tabby