Question

When should I use process-wide dictionary and when should my process state be in the parameters of the loop function?

This:

loop() ->
  receive
     {From, subscribe} ->
       put(listeners, [From|get(listeners)]),
       ?MODULE:loop()
  end.

Or this:

loop(Listeners) ->
  receive
    {From, subscribe} ->
       ?MODULE:loop([From|Listeners])
  end.

?

Parameters to the loop function has the benefit of being explicit in that nothing outside it can change the parameters (unless you’re doing strange magic bouncing on another function like a trampoline), but state tends to add up, and the parameters with them.

How about a dict as a parameter? Best of both worlds or worst of both worlds?

Answer 1

I would suggest you to use a record to store your loop data. That way if you ever need to add a new “variable” to your loop data all you have to do is add it to the record, without touching any of your clauses. Dict would only be a better choice if your “keys” recently change, which is unlikely in the case of loop data.

-record(state, {
           listeners = [],
           new_var   = undefined,
           new_var2  = ""
           ...
        }).

init() ->
  loop(#state{}).

loop(State) ->
  receive
    {From, subscribe} ->
       loop(State#state{listeners = [From|State#state.listeners]});
    somethingelse ->
       do_nothing(),
       loop(State)
  end.