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Key path expressions as functions

Available from Swift 5.2

Paul Hudson      @twostraws

SE-0249 introduced a marvelous shortcut that allows us to use keypaths in a handful of specific circumstances.

The Evolution proposal describes this as being able to use “\Root.value wherever functions of (Root) -> Value are allowed”, but what it means is that if previously you sent a Car into a method and got back its license plate, you can now use Car.licensePlate instead.

This is best understood as an example, so here’s a User type that defines four properties:

struct User {
    let name: String
    let age: Int
    let bestFriend: String?

    var canVote: Bool {
        age >= 18
    }
}

We could create some instance of that struct and put them into an array, like this:

let eric = User(name: "Eric Effiong", age: 18, bestFriend: "Otis Milburn")
let maeve = User(name: "Maeve Wiley", age: 19, bestFriend: nil)
let otis = User(name: "Otis Milburn", age: 17, bestFriend: "Eric Effiong")
let users = [eric, maeve, otis]

Now for the important part: if you want to get an array of all the users names, you can do so by using a key path like this:

let userNames = users.map(\.name)
print(userNames)

Previously you would have had to write a closure to retrieve the name by hand, like this:

let oldUserNames = users.map { $0.name }

This same approach works elsewhere – anywhere where previously you would have received a value and passed back one of its properties, you can now use a key path instead. For example, this will return all users who can vote:

let voters = users.filter(\.canVote)

And this will return the best friends for all users who have one:

let bestFriends = users.compactMap(\.bestFriend)
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