Enums with mutating methods

.swift

import Foundation

enum TrafficLight {
    case red, yellow, green // TrafficLight cases
    mutating func next() {
        switch self { // self = TrafficLight
        case .red:
            self = .green
        case .green:
            self = .yellow
        case .yellow:
            self = .red
        }
    }
}

var currentLight = TrafficLight.red
currentLight.next() // to green
print(currentLight)
currentLight.next() // to yellow
print(currentLight)
currentLight.next() // to red
print(currentLight)