How do you implement a state machine?

Question

Accepted Answer

**Enum + match** — implement state machines.

**Definition:**
```rust
enum State {
    Idle,
    Running,
    Paused,
    Stopped,
}

impl State {
    fn start(self) -> State {
        match self {
            State::Idle => State::Running,
            _ => self,
        }
    }

fn pause(self) -> State {
        match self {
            State::Running => State::Paused,
            _ => self,
        }
    }

fn stop(self) -> State {
        match self {
            State::Running | State::Paused => State::Stopped,
            _ => self,
        }
    }
}
```

**Usage:**
```rust
let state = State::Idle;
let state = state.start();  // Running
let state = state.pause();  // Paused
let state = state.stop();   // Stopped
```

**Key:** Enum for states, match for transitions.

Rust: How do you implement a state machine?