Swift array flatMap example used to cast array`s content

ArrayFlatMap.swift

class Person: CustomStringConvertible {
    let name: String
    var residence: String?

    init(name: String,residence: String) {
        self.name = name
        self.residence = residence
    }

    var description: String {
        return "\(type(of:self)) name: \(name)"
    }
}

class Programmer: Person {}

let ilya: Person = Programmer(name: "Ilya", residence: "Moscow")
let vasya: Person = Person(name: "Vasya", residence: "Ryazan")

var persons: [Person?] = [ilya, nil, vasya]

let programmers = persons.flatMap { (person) -> Programmer? in
    person as? Programmer
}

print(programmers) // [Programmer name: Ilya]