🚀 go-pugleaf

In article <1593@aipna.ed.ac.uk> cam@aipna.ed.ac.uk (Chris Malcolm) writes:

   Path: opus!lanl!cmcl2!nrl-cmf!think!ames!claris!apple!usc!cs.utexas.edu!uunet!mcsun!ukc!edcastle!aipna!cam
   From: cam@aipna.ed.ac.uk (Chris Malcolm)
   Newsgroups: comp.lang.prolog
   Date: 8 Nov 89 17:03:02 GMT
   References: <EIVERSON.89Oct30145311@hades.nmsu.edu>
   Reply-To: cam@aipna.ed.ac.uk (Chris Malcolm)
   Distribution: comp
   Organization: Dept of AI, Edinburgh University, UK.
   Lines: 51

   [Apologies if you've seen this before -- I suffer from whimsical network
   connections.]

   In article <EIVERSON.89Oct30145311@hades.nmsu.edu> eiverson@nmsu.edu (Eric Iverson) writes:
   >This may be a baby problem, but here goes:  Is there any possible way
   >to skip levels while backtracking?  By this I mean that if I run into
   >a problem at level 4 and realize that level 2 is causing the problem,
   >is there anyway that I can go directly to level 2 without having level
   >3 exhaustively fail first?

   A very interesting question. There are lots of different ways of doing
   it. When I was faced with this problem (in a robot assembly planner) a
   crucial requirement of the implementation was that it be FAST, which
   unfortunately ruled out - so I concluded - any elegant solutions. 
	...


not so firmly, if you have some special help.

prolog dialects such as sicstus and nu allow a goal to be frozen
pending the instantiation of a variable.  this facility can be used to
implement a sort of instantaneous failure on a higher level at the
cost of handing the variable to be instantiated down to the
descendants.  for example,


parent :- set_failure_point(X), first_child(X), write(didnt_backtrack),nl.
parent :- write(back_tracked), nl.

first_child(X) :- second_child(X).

second_child(X) :- write(deep), nl, cause_failure(X), write(not_here), nl.


set_failure_point(X) :- freeze(X,X=1).
cause_failure(2).