[SOLVED] how do you write a KeyPath to reference the value of a dictionary

how do you write a KeyPath to reference the value of a dictionary

Say I have a struct like this:

struct Person {
  var kids: [KidId: KidName] = [:]
}

Is it possible to construct a keypath to reference the value of something in the User.kids property? Something like \User.kids[KidId]?

Specifically, I'd like to construct a WritableKeyPath<User,KidName>. I thought you could keypath into both dictionaries and arrays, but I can't for the life of me find the syntax anywhere on the internet. Maybe it's not possible?

Solution

Your question is unclear because you refer to User in your text, but have Person in your code. That suggests that you might have the syntax right but a hierarchy wrong.

You also may just be forgetting the question mark.

To get the type you're asking about, you also need a KidID (the D is capitalized in Swift) value, not the KidID type.

typealias KidID = String
enum KidName { case goatBoy }
\Person.kids["🐐"] as WritableKeyPath<Person, KidName?>
Person(kids: ["🐐": .goatBoy])[keyPath: \.kids["🐐"]] // .goatBoy

You don't have to supply a value in the key path, but that is a different type.

\Person.kids as WritableKeyPath<Person, [KidID: KidName]>
Person(kids: ["🐐": .goatBoy])[keyPath: \.kids]["🐐"] // .goatBoy