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From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.prolog
Subject: Re: Standards question: behavior of arg/3
Keywords: ANSI, standard
Message-ID: <4018@goanna.cs.rmit.oz.au>
Date: 19 Oct 90 02:46:32 GMT
References: <9888@bunny.GTE.COM> <3992@goanna.cs.rmit.oz.au> <1920@tuvie>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 65

In article <1920@tuvie>, brock@tuvie (Inst.f.Prakt.Info 1802) writes:
> In article <3992@goanna.cs.rmit.oz.au> ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) writes:
> [...discussion of design principles for evaluable predicates...]
> >	?- arg(N, [a,b,c], X).
> >makes perfect sense and has two solutions
> >	N = 1, X = a ;
> >	N = 2, X = [b,c] .

> Which Prolog systems have arg/3 defined in this way?

Re-read that carefully.  I said that the query makes sense and it has
two solutions.  I didn't say anything about implementations.  I _have_
seen a Prolog system which did this, but can no longer recall which.
Quintus Prolog has an operation that does this, it just calls it
genarg/3 rather than arg/3.

The thing which would make extending arg/3 tricky on the WAM is that
WAM registers don't survive choice point creation, so that open-coding
non-determinate evaluable predicates doesn't help a lot.  If a compiler
can do data-flow analysis, it can pick the determinate case a lot of
the time, and I'm not even talking about inter-clause analysis.
Consider

	first_N_args_the_same(N, T1, T2) :-
		integer(N),
		first_N_args_the_same(0, T1, T2, N).

	first_N_args_the_same(I, T1, T2, N) :-
		(   I < N ->
		    J is I+1,
		    arg(J, T1, X),	% number(J) is *known* here
		    arg(J, T2, X),	% and here, so open-coding pays off
		    first_N_args_the_same(J, T1, T2, N)
		;   I =:= N
		).

As for the benefit of making arg/3 logical, consider

	path_arg([], Term, Term).
	path_arg([Selector|Selectors], Term, SubTerm) :-
		arg(Selector, Term, Arg),
		path_arg(Selectors, Arg, SubTerm).

If arg/3 can solve for its first argument, then so can path_arg/3.

The wording in my 1984 document (BSI number PS/6) was very carefully
chosen to insist that when the logic says there is a unique answer,
an implementation must find it, but when there are many answers, an
implementation may give up, provided it admits to the fault.  For
example, a query like

	times(X, Y, 1000000000000000000000000000)

has a well defined finite solution set, but an implementation may not
be prepared to go to the trouble of finding it.  (The next revision
of that document defined times/3 to work on rationals, not integers,
so the solution set is infinite.  This is just an example.)

(For the record, when you ask
	?- arg(N, [a,b,c], X).
SICStus Prolog 0.7 fails quietly, and NU Prolog reports that the
query ``floundered''.)

-- 
Fear most of all to be in error.	-- Kierkegaard, quoting Socrates.