Implementing a state machine in scala DSL Subscribe Pub

Consolidating my various blogs into one. This was originally posted on 2010/2/26

I had a bit of fun in the past few days writing a telnet server implementation in #scala. Part of that was implementing a state machine, which I then cleaned up so the definitions are simple.

The entire code, fairly independent and reusable, is at SM.scala

Here's a sample state machine:

/** sample state machine */
class SampleSM extends razie.SM.StateMachine {
  import razie.SM._
  implicit val sm = this

  val states@(s1, s2, s3) = ("s1", "s2", "s3")
  override def start = state("s1") // TODO there's an issue if i use s1 instead of "s1"

  override val transitions : Seq[Transition] =
     (s1, 2)               -> s2 ::
     (s1, 0)               -> s1 :: // NOP
     (s2, 1)               -> s1 + echo ("back to s1") :: // CR
     (s2, 3)               -> s3 ::
     (s3, Seq(4, 5, 6, 7)) -> s2 ::
     (s3, AnyEvent)        -> s1 ::
     (s2, {_:Event=>true}) -> s2 + eatChar + echo ("s2 eats all others") :: // eating stuff
     (""".*""".r, -1)      -> s1 :: // some reset event applies to all states
     Nil

  def eatChar(sm: StateMachine, t: Transition, e: Event) =
    println("ate char: " + e.asInstanceOf[IEvent].i.toChar)
}

object SampleSMain extends App {
  implicit def e(i: Int) = SM.IEvent(i)
  val sm = new SampleSM()
  sm move 2 move 1 move 3 move -1 move 2 move 3 move 6 move 59
}

Try this online at bit.ly/1jv1AED.

The basic structure is:

 (state, event-matcher) -> newstate + callback ++ unitcalback... + callback :: Nil

The event-matcher can be:

• an event
• a sequence of events
• a function taking an event and returning...Boolean, of course

The callback is a function with either of these signatures:

def push (sm:StateMachine, t:Transition, e:Event) - use + for these

def reset : Unit - use ++ for these (hey, I'm still getting my head around advanced scala stuff)

Enjoy, and if you likee, contribute, eh? If you want to see more state machine DSL, see Implementing the Telnet state machine.

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