Type-Safe FSM with Phantom Types 👻

phantom-fsm.swift

// inspired by https://blog.jle.im/entry/introduction-to-singletons-1.html 

protocol DoorState {}
enum Opened: DoorState {}
enum Closed: DoorState {}
enum Locked: DoorState {}

struct Door<State: DoorState> {
    let id: Int
}

extension Door where State == Closed {
    func open() -> Door<Opened> { .init(id: id) }
    func lock() -> Door<Locked> { .init(id: id) }

    func forceLock() -> Door<Locked> { lock() }
}

extension Door where State == Opened {
    func close() -> Door<Closed> { .init(id: id) }

    func forceLock() -> Door<Locked> { close().lock() }
}

extension Door where State == Locked {
    func unlock() -> Door<Closed> { .init(id: id) }

    func forceLock() -> Door<Locked> { self }
}

Door<Closed>(id: 0)
    .open()   // Door<Opened>
    .close()  // Door<Closed>
    .lock()   // Door<Locked>
    .unlock() // Door<Closed>

Door<Opened>(id: 1).forceLock() // Door<Locked>