Swift Optional chaining == flatMap

MonkeyBananaPeel.swift

// An (admittedly silly) demonstration of how optional chains in Swift
// are equivalent to flatMapping over each optional.
// 
// You can, in your mind, replace every `?.x` by a `flatMap { $0.x }` call.

class Peel {
    var color: String
    init(color: String) { self.color = color }
}
class Banana {
    var peel: Peel?
}
class Monkey {
    var banana: Banana?
}

let monkey = Monkey()
monkey.banana = Banana()
monkey.banana?.peel = Peel(color: "brown")

var s1 = monkey.banana?.peel?.color // "brown" - equivalent to next line
var s2 = monkey.banana.flatMap({ $0.peel }).flatMap({ $0.color }) // "brown"

s1.debugDescription // Optional("brown")
s2.debugDescription // Optional("brown")