rust 47 lines · 9 steps

A turnstile state machine in Rust

A finite state machine models a coin-operated turnstile by matching on the current state paired with an incoming event.

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Intermediate state machine enums pattern matching exhaustiveness
1#[derive(Debug, Clone, Copy, PartialEq, Eq)]
2enum State {
3 Locked,
4 Unlocked,
5}
6 
7enum Event {
8 Coin,
9 Push,
10}
11 
12struct Turnstile {
13 state: State,
14 coins_collected: u32,
15 passages: u32,
16}
17 
18impl Turnstile {
19 fn new() -> Self {
20 Turnstile {
21 state: State::Locked,
22 coins_collected: 0,
23 passages: 0,
24 }
25 }
26 
27 fn dispatch(&mut self, event: Event) {
28 self.state = match (self.state, event) {
29 (State::Locked, Event::Coin) => {
30 self.coins_collected += 1;
31 State::Unlocked
32 }
33 (State::Locked, Event::Push) => {
34 eprintln!("still locked, insert a coin first");
35 State::Locked
36 }
37 (State::Unlocked, Event::Push) => {
38 self.passages += 1;
39 State::Locked
40 }
41 (State::Unlocked, Event::Coin) => {
42 eprintln!("already unlocked, coin refunded");
43 State::Unlocked
44 }
45 };
46 }
47}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Modeling states and events as enums makes illegal states unrepresentable and every transition explicit.
  2. 2Matching on a tuple of state and event lets the compiler enforce that you handle every combination.
  3. 3Each match arm can both compute the next state and perform side effects like counting.

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