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Re: Solicitation of feedback

From: Dave Reynolds <dave.e.reynolds@googlemail.com>
Date: Tue, 20 Jul 2010 17:10:06 +0100
To: Sandro Hawke <sandro@w3.org>, Chimezie Ogbuji <ogbujic@ccf.org>
Cc: public-rif-wg@w3.org, Ivan Herman <ivan@w3.org>, Birte Glimm <birte.glimm@comlab.ox.ac.uk>, Lee Feigenbaum <lee@thefigtrees.net>, Axel Polleres <axel.polleres@deri.org>
Message-ID: <1279642206.1546.241.camel@dave-desktop>
[For some reason I couldn't read the original forwarded email so didn't
see this question until Sandro's reply.]

On Tue, 2010-07-20 at 08:54 -0400, Sandro Hawke wrote: 
> On Mon, 2010-07-19 at 10:26 -0400, Chimezie Ogbuji wrote:

> > 7.2 (Simple) RIF Core Entailment Regime
> > 
> > "It is unclear whether safe RIF-Core rules used to form combinations for
> > this entailment regime guarantee uniqueness (up to RDF graph equivalence) on
> > answer sets [...] Without strongly safe restrictions, there may be
> > interoperability issues ... However, strong safety restrictions are only
> > defined in the informative sections of the RIF-Core specification"
> > 
> > Can you give any background into why the strong safety characteristics are
> > only an informative part of the specification that might help in informing
> > conditions for preventing trivial infinite answers as appropriate for the
> > RIF regime if the use of the strong safety criteria is not appropriate for
> > this?
> 
> I'm going to have to let someone else answer this, or take some time to
> swap this back in/figure this out, sorry.

If I recall correctly the issue was that people treat rule systems,
especially production rule systems, like programming languages. They use
the expressivity of cyclic dependencies while, in practice, ensuring
appropriate termination conditions. An artificial example being
something like:

   p(0) .
   p(?x + 1) :- p(?x), ?x >= 0, ?x < 10 .

The strong safety conditions would exclude such rule sets.

We wanted Core to be a useful subset of both PR and BLD and felt that
the restriction to strongly safe rules would eliminate too many rule
sets used in practice (that would otherwise be within Core).

I guess you could say that the SPARQL-RIF Core entailment regime is only
defined over strongly safe rule sets and that interoperation is not
guaranteed for other rule sets. 

Or could you  say that interoperability is only guaranteed over rule
sets which terminate (on the given proof engine) and that strong safety
is one way to ensure that?

Dave
Received on Tuesday, 20 July 2010 16:10:43 GMT

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