Re: [eclipse-clp-users] Propagators with a state

Wit Jakuczun wrote:
> 2009/2/16 Kish Shen <kisshen_at_cisco.com>:
>> Wit Jakuczun wrote:
>>
>>> But I would love to have other features:
>>> - initial call of a propagator (could result in suspending). This would be
>>>  especially useful for propagators based on demons.
>> As propagators are normal ECLiPSe goals, you can simply write your
>> propagator such that it is executed once before suspension, something like:
>>
>> propagator(...) :-
>>        do_propagation(....),
>>        suspend(do_propagation(....)...).
>>
> 
> The code looks like that:
> 
> propagator(Vars, state(State0)) :-
>     update_state(Vars, State0, State1),
>     ( ground(Vars) -> check_solution(Vars)
>     ; do_propagate(Vars, State1),
>       suspend(propagator(Vars, state(State1), Susp), 3, Vars->inst, Susp)).
> 
> propagator(Vars, State,  Susp) :-
>     State = state(State0),
>     update_state(Vars, State0, State1),
>     ( ground(Vars) -> kill_suspension(Susp), check_solution(Vars)
>     ; do_propagate(Vars, State1), setarg(1, State, State1)).
> 
> update_state/3 must be called as there can be more that
> one variable instantiated comparing to previous call of a propagator.
> This is due to priorities!
> Now, propagator/2 is called and if there is a need for suspension
> a propagator/3 (demon) is suspended. If there were guarantee
> that propagators are called for each variable change separately
> then I could do as you proposed.
> I hope I clarified the issue.
> 
> Best regards
Hi Wit,

You can avoid writing two versions of propagator by checking if there
is a suspension in the propagator/3 code, e.g.:

propagator(Vars, State) :-
	propagator(Vars, State, _Susp).

propagator(Vars, State, Susp) :-
	State = state(State0),
	update_state(Vars, State0, State1),
	( ground(Vars) ->
		( is_suspension(Susp) -> kill_suspension(Susp) ; true ),
		check_solution(Vars)
	;
		do_propagate(Vars, State1),
		(is_suspension(Susp) ->
			true
		;
			suspend(propagator(Vars, ....)
		)
	).

On a more general point, if I understood you correctly, you are doing 
the update_state/3 in propagator/2 in your original code because the
state can change between calling propagator/2 and executing the goals
in propagator/2, because other propagators can wake up and update the 
State. If this is your concern, then I am not sure if your code would 
avoid the problem, because the update can happen after you do the
update_state/3.

To avoid this problem, you should use call_priority/2 to call
propagator/2 with a higher priority than what you are suspending your
propagators at -- so 2 in your case. In this case, you can also
miss out the initial update_state, and simplify the code a little.

Cheers,

Kish
	
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