A simple finite state machine in Rust (from https://youtu.be/xuu3FrTKb50)

fst.rs

#[derive(Debug)]
enum TrafficSignal {
    Green,
    Yellow,
    Red,
}

impl TrafficSignal {
    fn step(&mut self) {
        use TrafficSignal::*;

        *self = match self {
            Green => Yellow,
            Yellow => Red,
            Red => Green,
        }
    }
}

fn main() {
    let mut light = TrafficSignal::Green;
    light.step();
    light.step();

    println!("{light:?}");
}