Map over KeyPaths in Swift 4.0.swift

extension Sequence {
    func map<T>(_ keyPath: KeyPath<Element, T>) -> [T] {
        return self.map {
            $0[keyPath: keyPath]
        }
    }
    
    func flatMap<T>(_ keyPath: KeyPath<Element, T?>) -> [T] {
        return self.flatMap {
            $0[keyPath: keyPath]
        }
    }
}

struct Person {
    let firstname: String
    let lastname: String?
}

let persons = [
    Person(firstname: "Tobias", lastname: "Due Munk"),
    Person(firstname: "John", lastname: "Doe"),
    Person(firstname: "Jane", lastname: nil)
]

let firstnames = persons.map(\Person.firstname) // ["Tobias", "John", "Jane"]
let lastnames = persons.flatMap(\Person.lastname) // ["Due Munk", "Doe"]

Shorter version

let firstnames = persons.map(\.firstname) // ["Tobias", "John", "Jane"]
let lastnames = persons.flatMap(\.lastname) // ["Due Munk", "Doe"]