Path: utzoo!utgpu!attcan!uunet!husc6!bbn!rochester!udel!burdvax!zeta!lang
From: lang@zeta.PRC.Unisys.COM (Francois-Michel Lang)
Newsgroups: comp.lang.prolog
Subject: Determining order of argument unification
Message-ID: <8192@burdvax.PRC.Unisys.COM>
Date: 3 Nov 88 21:56:57 GMT
Sender: news@PRC.Unisys.COM
Lines: 26

There are essentially two (reasonable) ways for a Prolog system to attempt
to unify two compound terms which have the same functor and arity, e.g.,

    p(T11, ..., T1n)  and  p(T21, ..., T2n)

(1) Unifying the arguments in left-to-right order, and
(2) Unifying the arguments in right-to-left order.

Is it possible to determine (by writing a Prolog program)
which of these two orders is used for given a Prolog system,
and, if so, how?

Affirmative answers determined by looking in the manual,
poking through source code, calling the implementors, etc.
are *not* considered legitimate!

I know one valid (I think) affirmative answer,
but I'm sure there are lots of others...

Credit is due to Kim Marriott and Harald Sondergaard
(Department of Computer Science, University of Melbourne)
for inspiring the question.
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Francois-Michel Lang
Paoli Research Center, Unisys Corporation lang@prc.unisys.com (215) 648-7256
Dept of Comp & Info Science, U of PA      lang@cis.upenn.edu  (215) 898-9511