Path: utzoo!utgpu!attcan!uunet!mcvax!enea!kth!draken!ttds!jonasn
From: jonasn@ttds.UUCP (Jonas Nygren)
Newsgroups: comp.lang.prolog
Subject: The double-cut.
Message-ID: <1162@ttds.UUCP>
Date: 28 Jul 88 21:50:36 GMT
Reply-To: jonasn@ttds.UUCP (Jonas Nygren)
Organization: The Royal Inst. of Techn., Stockholm
Lines: 30


Maybe it's presumptuous of a novice Prolog programmer like myself 
to come with suggestions on how to improve the language but somehow you
have to find out if your'e dead wrong or if your ideas have any substance.

Here we go.

When trying to write procedural code for a simple curvplotting routine
I often encountered problems with backtracking over system-calls. The
remedy for this was code like:
	a :- !, b, !, c, !, d, !, e, !.
to avoid backtracking of goals a,b,c,d,e but this looks rather awkward.
If you instead could write something like:
	a :- !!, b, c, d, e.
with the same effect it wouldn't look to bad, I call it the double-cut.
Naturally the double-cut could be placed later in the goal chain, e g:
	a :- b, c, !!, d, e.
to achieve the equivalence of:
	a :- b, c, !, d, !, e, !.
	
What do you think of the double-cut:

	1	is it any good?
	2	are there any Prolog-variants with similar functionality? 
	3	would it provide means for the Prolog-implementor to do
		more optimization than without it?
	4	is there a better way to achieve the same result?
	
		
/jonas