KeyPathUpdatableTwoLevelExample.swift

protocol KeyPathUpdatable {}

extension KeyPathUpdatable {
    func updating<LeafType>(_ keyPath: WritableKeyPath<Self, LeafType>, to value: LeafType) -> Self {
        var copy = self
        copy[keyPath: keyPath] = value
        return copy
    }
}

struct Author {
    var name: String
}

struct Book: KeyPathUpdatable {
    var title: String
    var author: Author
    var isbn: Int
    var publishedYear: Int
}

let book = Book(title: "Something", author: Author(name: "Noah"), isbn: 1, publishedYear: 2020)
let updated = book.updating(\.author.name, to: "Other")

print(book)
print(updated)