🚀 go-pugleaf

In article <MATSC.92Sep2141625@vishnu.sics.se>
	matsc@sics.se (Mats Carlsson) writes:

 |   do you do anything special about this construct, or do you just
 |   consider it as an ordinary predicate with a definition like:
 |   P ;_Q :- P.
 |   _P; Q :- Q.
 |
 |SICStus Prolog does nothing special, and considers it as an ordinary
 |predicate as you describe.

Don't you have to do something special to extend the scope of cut
operators?  As I understand it, the program A below has only one
solution of X=1, while the program B should have both X=1 and X=2.

   program A:
	p(X) :- q(X), (true, !; fail).
	q(1). q(2).
	?- p.
   program B:
	p(X) :- q(X), or((true, !), fail).
	q(1). q(2).
	or(T, _F) :- T.
	or(_T, F) :- F.
	?- p.

In case of SICStus, this "something special" might have been small.
More complicated mechanisms might be needed in an implementation
without a separate choice point stack.

Takashi Chikayama
ICOT