Question

I would like to translate some actions specified in TLA in Erlang. Can you think of any natural way of doing this directly in Erlang or of any framework available for such? In a nutshell (a very small one), TLA actions are conditions on variables, some of which are primed, meaning that they represent the values of the variables in the next state. For example:

Action(x,y,z) ->
    and PredicateA(x),
    and or PredicateB(y)
        or PredicateC(z)
    and x' = x+1

This action means that, whenever the state of the system is such that PredicateA is true for variable x and either PredicateB is true for y or PredicateC is true for z, then the system may change it’s state so that everything remains the same except that x changes to the current value plus 1.

Expressing that in Erlang requires a lot of plumbing, at least in the way I’ve found. For example, by having a loop that evaluates conditions before triggering them, like:

what_to_do(State,NewInfo) ->
    PA = IsPredicateA(State,NewInfo),
    PB = IsPredicateB(State,NewInfo),
    PC = IsPredicateC(State,NewInfo),
    [{can_do_Action1, PA and (PB or PC}, %this is the action specified above.
     {can_do_Action2, PA and PC},        %this is some other action
     {can_do_Action3, true}]             %this is some action that may be executed at any time.

 loop(State) ->
     NewInfo = get_new_info(),
     CanDo = what_to_do(State,NewInfo),
     RandomAction = rand_action(CanDo),

     case RandDomAction of
          can_do_Action1 -> NewState = Action(x,y,z);
          can_do_Action2 -> NewState = Action2(State);
          can_do_Action3 -> NewState = Action3(State)
     end,
     NewestState = clean_up_old_info(NewState,NewInfo),
     loop(NewestState).

I am thinking writing a framework to hide this plumbing, incorporating message passing within the get_new_info() function and, hopefully, still making it OTP compliant. If you know of any framework that already does that or if you can think of a simple way of implementing this, I would appreciate to hear about it.

Answer 1

I believe gen_fsm(3) behaviour could probably make your life slightly easier.

FSM from Finite State Machine, not Flying Spaghetti Monster, though the latter could help, too.