type_state_machine.rs

mod states {
    pub struct Red;
    pub struct Yellow;
    pub struct Green;
}

fn red(_traffic_light: &TrafficLight<states::Red>) {}

fn yellow(_traffic_light: &TrafficLight<states::Yellow>) {}

fn green(_traffic_light: &TrafficLight<states::Green>) {}

struct TrafficLight<COLOR> {
    _color: COLOR,
}

impl TrafficLight<states::Red> {
    fn new() -> TrafficLight<states::Red> {
        TrafficLight {
            _color: states::Red,
        }
    }
}

impl TrafficLight<states::Red> {
    fn next(&self) -> TrafficLight<states::Yellow> {
        TrafficLight {
            _color: states::Yellow,
        }
    }
}

impl TrafficLight<states::Yellow> {
    fn next(&self) -> TrafficLight<states::Green> {
        TrafficLight {
            _color: states::Green,
        }
    }
}

impl TrafficLight<states::Green> {
    fn next(&self) -> TrafficLight<states::Red> {
        TrafficLight {
            _color: states::Red,
        }
    }
}

fn main() {
    let traffic_light = TrafficLight::new();
    red(&traffic_light);
    // compile error
    // yellow(&traffic_light);

    let traffic_light = traffic_light.next();
    yellow(&traffic_light);
    // compile error
    // green(&traffic_light);

    let traffic_light = traffic_light.next();
    green(&traffic_light);
    // compile error
    // red(&traffic_light);

    let traffic_light = traffic_light.next();
    red(&traffic_light);
    // compile error
    // yellow(&traffic_light);
}