state.rs

enum State {
    Init, Started, Stopped
}

enum InitEvent {
    Start, Stop
}

enum StartedEvent {
    Stop
}

enum StoppedEvent {
    Start
}

fn init_event() -> InitEvent {
    return InitEvent::Start;
}

fn started_event() -> StartedEvent {
    return StartedEvent::Stop;
}

fn stopped_event() -> StoppedEvent {
    return StoppedEvent::Start;
}

fn apply(s: State) -> State {
    match s {
        State::Init => match init_event() {
            InitEvent::Start => State::Started,
            InitEvent::Stop => State::Stopped,
        },
        State::Started => match started_event() {
            StartedEvent::Stop => State::Stopped,
        },
        State::Stopped => match stopped_event() {
            StoppedEvent::Start => State::Started,
        }
    }
}

fn state_loop(init: State) {
    let mut s = init;
    loop {
        s = apply(s);
    }
}

fn main() {
    state_loop(State::Init)
}