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Re: Solicitation of feedback

From: Axel Polleres <axel.polleres@deri.org>
Date: Tue, 20 Jul 2010 22:43:04 +0100
Cc: Dave Reynolds <dave.e.reynolds@googlemail.com>, public-rif-wg <public-rif-wg@w3.org>, Ivan Herman <ivan@w3.org>, Birte Glimm <birte.glimm@comlab.ox.ac.uk>, Lee Feigenbaum <lee@thefigtrees.net>
Message-Id: <EA63612B-4B17-477E-903D-B4C2838C4F05@deri.org>
To: Sandro Hawke <sandro@w3.org>, Chimezie Ogbuji <ogbujic@ccf.org>
At the same time as emphasizing Chime's request for feedback from RIF members (ideally disjoint from those also in SPARQL, who may be biased)
let me make some additions from what I remember from discussions in RIF here... i.e., wearing RIF member hat for the rest of the mail ...

On 20 Jul 2010, at 21:45, Sandro Hawke wrote:

> On Tue, 2010-07-20 at 17:10 +0100, Dave Reynolds wrote:
> > [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 [...]

uniqueness, I would think yes, finiteness obviously no.
Or, what exactly do you mean by uniqueness here, Chime, could you give an example?

> 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).
> 
> That sounds right, yes.

Yup, overall, a majority of the group found the finiteness restriction 
too strong for RIF Core, which is why it ended up as being informative only.

> > 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.

I would rather say "interoperation is undefined by the SPARQL entailment spec" 
than "not guaranteed" (though that probably boils down to the same)

> > 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?

... sound both like feasible options.

Axel

> Yes, exactly. Strong safety is one way to get certain guarantees, but
> it's very limiting. I have the impression that neither rule system
> vendors nor rule systems users have any interest in sticking to
> strongly-safe rules, and a lot of them are not interested in sticking to
> RIF Core (or even BLD or PRD).   For the SPARQL WG to come in and
> restrict them to a subset of what they want to do seems, well, let's
> just say "sub-optimal". 
> 
> I'm pretty sure the right thing for SPARQL is to leave it to RIF, which
> in this case means that systems doing RIF must implement RIF core, and
> may implement additional features (eg PRD, or xml-data, or their own
> built-ins.).  Rule authors have to decide what their audience is, and
> pick the appropriate dialect; if they want the maximal audience, they
> should stuck to RIF Core.
> 
>     - Sandro
> 
> 
> 
Received on Tuesday, 20 July 2010 21:43:40 GMT

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