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From: vu0310@bingvaxu.cc.binghamton.edu (R. Kym Horsell)
Newsgroups: comp.lang.prolog
Subject: Re: Arrays in Prolog
Message-ID: <3920@bingvaxu.cc.binghamton.edu>
Date: 30 Aug 90 20:34:36 GMT
References: <90239.175243SCHMIED@DB0TUI11.BITNET> <3899@bingvaxu.cc.binghamton.edu> <1990Aug28.065353.13951@sics.se> <3904@bingvaxu.cc.binghamton.edu> <1162@cluster.cs.su.oz> <3906@bingvaxu.cc.binghamton.edu> <3321@skye.ed.ac.uk>
Reply-To: vu0310@bingvaxu.cc.binghamton.edu.cc.binghamton.edu (R. Kym Horsell)
Organization: SUNY Binghamton, NY
Lines: 36

In article <3321@skye.ed.ac.uk> richard@aiai.UUCP (Richard Tobin) writes:
>I have never used setarg, but it seems you are confusing two things:
>the ability to change the value of an instantiated variable, and the
>ability to preserve the value of a variable when backtracking.

In my original post my point was that setarg/3 as implemented in
the compiler I'm working on works as follows (a bit hard to
write this in Prolog, of course):

	setarg(Index,Term,Arg) :-
		arg(Index,Term,OldArg),
		/* save "OldArg" somewhere */,
		/* destructively change argument number
		``Index'' of ``Term'' to ``Arg'' via
		some dirty internal mechanism */
	;	/* destructuvely change argument number
		``Index'' of ``Term'' to ``OldArg'' via
		some dirty internal mechanism */.

Thus I am perfectly correct to say a cut after *my* setarg/3
will lose a backtrack point (note the ;/2) and the dirty
internal stuff that changes the relevant arg *back* to its
original value is lost.

If we therefore use *my* setarg/3 information *can* be
transferred from one disjunctive branch to another by
means other than assert/retract.

I have no intention of *defending* the implementation
of such a dirty mechanism, which I realize destroys both
the *forward* and even *backward* declarative reading of
Prolog programs in which it is included. 

I hope this post saves some bandwidth in the long run.

-Kym Horsell