How to Use Enums for State Machines in Rust

Use an enum to define valid states and a match expression to enforce transitions, ensuring the compiler rejects invalid state changes.

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

impl TrafficLight {
    fn next(&self) -> TrafficLight {
        match self {
            TrafficLight::Red => TrafficLight::Green,
            TrafficLight::Green => TrafficLight::Yellow,
            TrafficLight::Yellow => TrafficLight::Red,
        }
    }
}

fn main() {
    let mut light = TrafficLight::Red;
    light = light.next();
    println!("{light:?}");
}

An enum acts like a strict list of allowed statuses for your program, such as 'Red', 'Yellow', or 'Green'. You use a match statement to define exactly how the program moves from one status to the next, preventing it from ever getting into a broken or impossible state. Think of it like a traffic light that can only change to the next color in the sequence, never skipping steps.