Collection+KeyPathSubscripting.swift

import Foundation

extension Collection {
  subscript<T>(_ keyPath: KeyPath<Element, T>) -> [T] {
    return map { $0[keyPath: keyPath] }
  }
}

struct Person {
  let firstName: String
}

let expectedFirstNames = ["Alice", "Bob", "Charlie"]
let people = expectedFirstNames.map(Person.init)
assert(expectedFirstNames == people[\.firstName])

let counts = people[\.firstName.count]
assert(expectedFirstNames.map(\.count) == counts)