WritableKeyPath<Root, Value>

In the earlier example, our type was a WritableKeyPath. WritableKeyPaths are, as the name implies, keypaths that allow you to write information. They're formed for var properties on var instances.

struct MutableUser {

   var username: String

}

var firstUser = MutableUser(username: \"Shinji\")



firstUser[keyPath: \MutableUser.username] = \"Ikari\"

If you want to have a keypath argument to a function that allows mutating the contents, WritableKeyPath is a good choice:

func modify(user: User, keyPath: WritableKeyPath<User, String>) {

   user[keyPath: keyPath] = \"Hello World\"

}

There's another variant of the WritableKeyPath, which we will introduce next.