Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!apple!portal!cup.portal.com!pgl
From: pgl@cup.portal.com (Peter G Ludemann)
Newsgroups: comp.lang.prolog
Subject: Re: Standards question: behavior of arg/3
Message-ID: <35019@cup.portal.com>
Date: 19 Oct 90 03:31:20 GMT
References: <9888@bunny.GTE.COM> <3992@goanna.cs.rmit.oz.au> <1920@tuvie>
Organization: The Portal System (TM)
Lines: 67

| In article <3992@goanna.cs.rmit.oz.au> ok@goanna.cs.rmit.oz.au 
| (Richard A. O'Keefe) writes:
| [...discussion of design principles for BIPs...]
| >	?- 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?

I implemented such an arg/3 at university -- it never even
occured to me to check how Edinburgh does it because it seemed
the natural way to do things.  And IBM-Prolog also provides
such a facility (not implemented nor designed by me, by the
way) but it also provides the conventional Edinburgh flavour
for those who demand compatibility.  [Richard O'Keefe notes
that his proposal is not compatible with Edinburgh Prolog,
by the way -- "a foolish consistency is the hobgoblin of small
minds" (Emerson, I think).]

Richard might not like the fact that IBM's arg/3 allows
arg(0,f(a),f) -- that's easily fixed by using a delayed inequality:
    wait:(N ==/ 0) & arg(N,F,X)

IBM also a delayed version of arg/3 which will wait until
the functor is instantiated.  It allows things like:
    :- wait:arg(N, F, X) , F = f(a) , N = 1.
to succeed with N=1, F=f(a), X=a.

Here's an annotated console trace:

  % --- switch to Edinburgh syntax and predicates:

  <- switch(synt2).
   goal : <- switch(synt2) .
   48ms success
  <- switch(synt2) .

  % --- demonstrate compatibility with Edinburgh predicate arg/3:

  :- arg(0, [a,b,c], N) .
   goal : <- arg(0,[a,b,c],V1) .
   0ms fail

  <- arg(N, [a,b,c], X) .  % no error message!
   goal : <- arg(V1,[a,b,c],V2) .
   0ms fail

  :- arg(1, [a,b,c], X) .  % succeeds with X = a
   goal : :- arg(1,[a,b,c],V1) .
   0ms success
  :- arg(1,[a,b,c],a) .

  % --- now show what happens with the non-Edinburgh flavour
  % --- of arg/3.  We must use the "built" prefix to get it
  % --- because the Edinburgh flavour (built2:arg/3) is default
  % --- when we're in Edinburgh syntax.  Alternatively, we could
  % --- have declared "built" to be the default prefix for arg/3.

  :- built:arg(N, [a,b,c], X) , built:write(N:X) , fail .
   goal : :- built : arg(V1,[a,b,c],V2) , built : write(V1 : V2) , fail .
  0 : '.' .
  1 : a .
  2 : [b,c] .
   0ms fail

- Peter Ludemann    ludemann@mlpvm1.iinus1.ibm.com