Implementing a state machine in scala DSL

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.

Here's a sample state machine:





The code is at http://code.google.com/p/razie/source/browse/trunk/razie/src/razie/SM.scala

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?

Here's the definition (in-work) of the high-level telnet state machine, a little more complicated but not much.

The picture (thanks to http://tomi.vanek.sk/index.php?page=telnet):



The state machine (when it's ready I'll post the link to the entire code):

// based on diagram at http://tomi.vanek.sk/index.php?page=telnet

implicit val sm = this

val sstates @ (data, cmd, app, param, neg, subneg) = ("data", "cmd", "app", "param", "neg", "subneg")
override def start = state("data")

override val transitions : Seq[Transition] =
     (data, IAC)             -> cmd ::
     (data, 0)               -> data :: // NOP - don't know why i get these after CR
     (data, 10)              -> data :: // NOP - LF ignored?
     (data, 13)              -> data + eatLine + echo ("") :: // CR
     (data, {_:Event=>true}) -> data + eatChar + echo ("") :: // remaining chars
     (cmd, IAC)              -> data ::
     (cmd, Seq(WILL, WONT, DO, DONT)) -> neg + push ::
     // TODO 3-2 should negociate stuff, i.e. reply with will/won't
     (neg, {_:Event=>last==SM(DO)})   -> data + mode(true) + pop ::
     (neg, {_:Event=>last==SM(DONT)}) -> data + mode(false) + pop ::
     (neg, AnyEvent) -> data + echo("interesting sequence...") + pop ::  // What is this?
     (cmd, SB) -> subneg ::
     (""".*""".r, CR) -> data :: // it's important to reset the thing on ENTRE
      Nil

def eatChar (sm:StateMachine, t:Transition, e:Event)
def eatLine (sm:StateMachine, t:Transition, e:Event)

Enjoy and, more importantly, have fun!

If you'd like to read more of my rants about scala, see http://wiki.homecloud.ca/thinking-in-scala