🚀 go-pugleaf

ludemann@quintus.com (Peter Ludemann) writes:

>In article <525@tower1.uucp>, arjen@disnet.uucp (Arjen Duursma) writes:
>> I am not sure because I do not know the appication at hand, but
>> I think that a proper implementation of a cleanup would function like
>> the next example.
>> 
>> goal :- (sideeffects,somegoal);(undosideeffects,othergoal).
>> 
>> If you find out that the sideeffects (like opening a file) where not
>> needed, fail, and undo them.
>> 
>> Hope that this is what you have been looking for.
>> 

>If only it were so simple!

>Consider:  ?- goal, !, fail.

>The cut will have removed the choice point created by the ";"
>in goal/0, so undosideeffects/0 will not be executed.

So define:

oncegoal :-
	(sideeffects,somegoal,undosideeffects);(undosideeffects,othergoal),!.

and use: ?- oncegoal.

>What you need is something on the "choice point" stack which is executed
>upon backtracking.  Various Prologs have this for internal use, but I'm
>not aware of any which provide it has a defined user predicate.  (If your
>Prolog has an interface to an RDBMS in which backtracking gets new rows,
>chances are it has such a hidden predicate.)

What you need is a good description of the use of the predicate you want
to use. This description may tell you that have to use two predicates,
one to make the mess (produce the answers) and one to clean things up.

Just like: see/1 and seen/0, tell/1 and told/0.

Any answers to the question that started this discussion is quite
useless until there is a good description of what the predicate should
do. The answer depends on the question.

Anyway, I am still far from convinced that any special predicates to
implement the requested funcionality. Just some experience in working
with prolog.

>-- 
>	Peter Ludemann  +1-415-813-3800  ludemann@quintus.com

Arjen