A media-player transport state machine

media_player.cell

// A media-player transport state machine.
// stopped -> playing <-> paused, with stop reachable from play/pause.
// A typed (sum-type) machine so an illegal transport command is caught
// statically by the variant name. This is a reactive cyclic system: the
// proof obligation is deadlock-freedom.

cell type Transport {
    variants { Stopped  Playing  Paused }
}

cell MediaPlayer {
    face {
        signal play_track(id: String) -> String
        signal pause_track(id: String) -> String
        signal resume_track(id: String) -> String
        signal stop_track(id: String) -> String
        signal transport_of(id: String) -> String
        signal next(id: String) -> List<String>
    }

    state transport: Transport {
        initial: Stopped
        Stopped -> Playing
        Playing -> Paused
        Paused  -> Playing
        Playing -> Stopped
        Paused  -> Stopped
    }

    on play_track(id: String)   { transition(id, Playing) return get_status(id) }
    on pause_track(id: String)  { transition(id, Paused)  return get_status(id) }
    on resume_track(id: String) { transition(id, Playing) return get_status(id) }
    on stop_track(id: String)   { transition(id, Stopped) return get_status(id) }
    on transport_of(id: String) { return get_status(id) }
    on next(id: String)         { return valid_transitions(id) }
}

cell test MediaPlayerTests {
    rules {
        // play, pause, resume, stop
        assert play_track("m1") == "Playing"
        assert pause_track("m1") == "Paused"
        assert resume_track("m1") == "Playing"
        assert stop_track("m1") == "Stopped"

        // pause then stop directly
        assert play_track("m2") == "Playing"
        assert pause_track("m2") == "Paused"
        assert stop_track("m2") == "Stopped"

        // illegal: cannot pause something that is stopped
        assert_fails pause_track("m3")
        // illegal: a stopped transport cannot jump straight to Paused
        assert_fails transition("m4", Paused)
    }
}