Information hiding in Swift by nesting class declaration inside a function

gistfile1.swift

protocol Counter {
    var value: Int { get }
    func moveTowardZero()
}

func makeCounterStartingAt(value: Int) -> Counter {
    // Since this class declaration is nested in the generator function, the returned value is only accessible via the Counter protocol
    class CountImplementation: Counter {
        var value: Int
        
        init(value: Int) {
            self.value = value
        }
        
        func countDown() {
            value -= 1
        }
        
        func countUp() {
            value += 1
        }
        
        func moveTowardZero() {
            switch value {
            case 0:
                return
            case let x where x < 0:
                countUp()
            default:
                countDown()
            }
        }
    }
    
    return CountImplementation(value: value)
}

func demonstrateCounter(counter: Counter) {
    println(counter.value)
    while counter.value != 0 {
        counter.moveTowardZero()
        println(counter.value)
    }
}

let counter1 = makeCounterStartingAt(3)
demonstrateCounter(counter1)
/* prints:
3
2
1
0
*/
let counter2 = makeCounterStartingAt(-4)
demonstrateCounter(counter2)
/* prints:
-4
-3
-2
-1
0
*/