🚀 go-pugleaf

In article <10539@burdvax.PRC.Unisys.COM> armagan@PRC.Unisys.COM (Armagan Ozdinc) writes:
>In article <PX.89Jun9194119@uniq.unl.fctunl.rccn.pt>, px@unl.fctunl.rccn.pt (Joaquim Baptista (pxQuim)) writes:
>> 
>> Variations on this problem also arise with asserta and assertz, as in:
>> 
>> /* this will loop forever, exhausting your heap space,
>>    writing 1211111111... */
>> 	b(1). b(2).
>> 	exec2:- b(X), write(X), assertz(b(X)), fail.
>
>In Quintus Prolog 2.4, exec2 doesn't behave as you describe above. It
>prints 12 and fails.


An article that is very relevant to this entire discussion
of retractall and backtracking is the following:

@INPROCEEDINGS{DefensibleSemantics,
        AUTHOR="Timothy G. Lindholm and Richard A. O'Keefe",
        TITLE="Efficient Implementation of a Defensible Semantics
         for Dynamic Prolog Code",
        BOOKTITLE=ilpc4,
        EDITOR="Jean-Louis Lassez",
        PAGES="21-39",
        PUBLISHER="The MIT Press",
        ADDRESS="Cambridge, MA",
        YEAR=1987
        }

Lindholm and O'Keefe describe several different approaches to
dynamic code---i.e., a predicate whose definition can be modified at runtime,
in particular by asserting or retracting clauses for that predicate.

Any program that depends specifically on whether the Prolog system
that is being used implements the "immediate update" or "logical"
approach to handling dynamic code will *not* be very portable!
----------------------------------------------------------------------------
Francois-Michel Lang
Paoli Research Center, Unisys Corporation lang@prc.unisys.com (215) 648-7256
Dept of Comp & Info Science, U of PA      lang@cis.upenn.edu  (215) 898-9511