At one point I built a type-based state machine core for Java. The idea was to u...

At one point I built a type-based state machine core for Java. The idea was to use interfaces and classes as the basis for defining what transitions are allowed, and as a way to store the associated fields. It relied on Java's type-checking to guide you into using it correctly.

    public interface TurnstileState {}
    
    public interface LockedState extends TurnstileState {}
    public class UnlockedState implements TurnstileState {
        public final int credits;
        public UnlockedState(int credits) {
            this.credits = credits;
        }
    }

    public static final LockedState LOCKED = new LockedState() {};
    
    StateMachine<TurnstileState> turnstile = StateMachine.<TurnstileState>.newBuilder()
            .addValidTransition(LockedState.class, UnlockedState.class)
            .addValidTransition(UnlockedState.class, LockedState.class)
            .addValidTransition(UnlockedState.class, UnlockedState.class)
            .buildWithInitialState(LOCKED);

    turnstile.transition(new UnlockedState(1));

I never got around to building out examples in Kotlin. I feel like the type checking there would streamline a ton of this even further.

https://github.com/Hounshell/st8 if anyone wants to see the gory bits.