Re: Solicitation of feedback

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
> 
> 
>