struct Person {
    private(set) var name: String
    init(name: String) {
        self.name = name
    }
}
let keyPath = \Person.name
var person = Person(name: "oldName")
person[keyPath: keyPath] = "newName"

error: My Page.xcplaygroundpage:22:7: error: cannot assign through subscript: 'keyPath' is a read-only key path
person[keyPath: keyPath] = "newName" // Cannot assign through subscript: 'keyPath' is a read-only key path
      ^         ~~~~~~~


My Page.xcplaygroundpage:20:1: note: change 'let' to 'var' to make it mutable
let keyPath = \Person.name
^~~
var