Big state machines

Vance Shipley <>
Tue Apr 19 19:41:49 CEST 2005


On Tue, Apr 19, 2005 at 01:39:43PM +0200, Ulf Wiger (AL/EAB) wrote:
}  
}  OK, not exactly what you requested, but here's a quick hack of
}  gen_fsm that supports selective receive in the form of a receive
}  vector (gen_fsm.erl and test.erl attached).

Nice.

I was also working on hacking gen_fsm to handle a receive vector.  I
started by building a test module the way I would want to.  The module
below is what I had in mind.  I extended the return from init/1 and the
state handler callbacks to include two extra values; Receive & Save.

e.g.	{next_state, State, StateData, Timeout, Receive, Save}
	Receive = any | tuple()
	Save = none | any | tuple()

So far I was only supporting atom() events and used the is_element()
method of your previous contribution.  I changed the main loop in 
gen_fsm to handle the extra arguments and made the default values
any and none.  One difference is that you can now have gen_fsm consume
messages you are not interested in instead of doing it explicitly in
the state handler callback.


-module(sdl_fsm).
-export([init/1, s/2]).
-behaviour(gen_fsm).

init(_) ->
   {ok, s, [], 0, {a,b}, none};

%%         ___
%%        (_s_)
%%          |
%%       +--+--+
%%     __|_  __|_
%%     >_a_| >_b_|
%%      _|_   _|_
%%     (_x_) (_y_)
%%
s(a, S) ->
   {next_state, x, [], 0, {a,b}, any};
s(b, S) ->
   {next_state, y, [], 0, {a,b}, {c,d}}.

%%            ___
%%           (_x_)
%%             |
%%       +-----+------+
%%     __|_  __|_   __|_
%%     >_a_| >_b_| /_*_/
%%      _|_   _|_
%%     (_x_) (_y_)
%%
x(a, S) ->
   {next_state, x, [], 0, {a,b}, any};
x(b, S) ->
   {next_state, y, [], 0, {a,b}, {c,d}}.

%%            ___
%%           (_y_)
%%             |
%%       +-----+------+-----+
%%     __|_  __|_   __|_  __|_
%%     >_a_| >_b_| /_c_/ /_d_/
%%      _|_   _|_
%%     (_x_) (_y_)
%%
y(a, S) ->
   {next_state, x, [], 0, {a,b}, any};
y(b, S) ->
   {next_state, y, [], 0, {a,b}, {c,d}}.


I hadn't yet contemplated how to handle real life events.  I had been
thinking that ultimately what would be required some sort of match
spec format for the vectors and whatever (parse transforms?) else in
gen_fsm to handle it.  Your approach to mandate the events take some
certain form may well be a perfectly reasonable way to go.

	-Vance




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