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From: Mild Shock <janburse@fastmail.fm>
Newsgroups: comp.lang.prolog
Subject: Elaboration Test 1 (Re: How to project variables? [PIP-4711: Marked
 Variables])
Date: Fri, 11 Apr 2025 12:47:57 +0200
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In-Reply-To: <vtarm5$11ove$1@solani.org>

Test 1 is probably the most simple test case.
It is only the following query:

q(Y, _X) :- _X = Y.

The projection would be:

p(Y) :- q(Y, _X).

?- p(Y).
true.

Here are the testing results:

Trealla Prolog 2.68.11-5: Pass

?- _X = Y.
    true.
?- Y = _X.
    true.

SWI-Prolog 9.3.21: Fail

?- _X = Y.
true.
?- Y = _X.
Y = _X.    %%% expected true

Dogelog Player 1.3.2: Pass

?- _X = Y.
true.
?- Y = _X.
true.

Mild Shock schrieb:
> Hi,
> 
> Now I am writing a new Prolog Improvement Proposal
> (PIP), which is PIP-4711: Marked Variables. Can
> we easily specify what marked variables should
> 
> do in the top-level? Oh yes. If you have a query
> that contains unmarked and marked variables,
> just like this here:
> 
> ?- q(N1, .., Nn, _M1, .., _Mm)
> 
> Then the above query should work as:
> 
> p(N1, .., Nn) :- q(N1, .., Nn, _M1, .., _Mm).
> 
> ?- p(N1, .., Nn).
> 
> The above specification assures that no information
> is lost, because it relies on the existential
> quantifier of Clark Completion. One can read the
> 
> definition of q/n logically in first order logic
> as follows, namely with existential quantifiers:
> 
> p(N1, .., Nn) <=> EXISTS(_M1, .., _Mm):q(N1, .., Nn, _M1, .., _Mm)
> 
> Do some Prolog systems satisfy the above specification.
> Amazingly most Prolog systems cannot do it. They
> have problems archiving the above result, in very
> 
> small test cases. The main problem is some permutation
> dependency, how the query q/n+m is formulated. It
> seems most Prolog systems do not use a
> 
> permutation agnostic algorithm, whereas the
> first order logic specification doesn't have some
> permutation dependency and should be immune.
> 
>                  Test1   Test2
> Trealla Prolog  Pass    Fail
> SWI-Prolog      Fail    Pass
> Dogelog Player  Pass    Pass
> 
> Bye
> 
>