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Re: [TF-ENT] Agenda 24th Feb teleconf

From: Chimezie Ogbuji <ogbujic@ccf.org>
Date: Wed, 24 Feb 2010 10:12:03 -0500
To: "Axel Polleres" <axel.polleres@deri.org>, "Sandro Hawke" <sandro@w3.org>
cc: "Birte Glimm" <birte.glimm@comlab.ox.ac.uk>, "SPARQL Working Group" <public-rdf-dawg@w3.org>
Message-ID: <C7AAABF3.10002%ogbujic@ccf.org>
Some responses below

On 2/24/10 4:18 AM, "Axel Polleres" <axel.polleres@deri.org> wrote:
> On 24 Feb 2010, at 06:29, Sandro Hawke wrote:
>> Anyway, the mapping is perhaps orthogonal to the Ent issue, since graphs
>> which merely describe RIF documents don't have any special semantics.
>> For the semantics, yeah, we want rif:imports, as Axel has mentioned.

> One reason why I am still slightly afraid of the RDF/RDF embedding and which
> is a pro for
> simply rif:imports is:
> 
> if you encode rif into triples of the graph at hand, these triples will also
> contribute to 
> the rule based answers/inferences, this would bring us into the same issues as
> OWL has, i.e.
> there'd be suddenly two ways to deal with this:
> (a) simply treating all as RDF (RDF-based semantics, ir "RIF/RDF Full")
> (b) first extracting the rules, and not considering them as part of the graph,
> which includes checking whether the RIF rules indeed form a well-formed
> ruleset, etc. and then use the RDF/OWL combination-semantics as defined in [1]
> (RIF "direct semantics"?)?
> 
> I am not sure whether I want to get into the issues of (a) when we still have
> a lot to sort out for (b) alone, such as
> a reasonable restriction to finite answers for example.

I'm not quite sure what you have in mind for rif:imports here, but if I
understand things correctly the main problem is that we don't have a
straight forward way to refer to a RIF document from  SPARQL query that
wishes to use a RIF entailment regime and this is mostly due to the fact
that RIF defines importing *from* a RIF document *to* an RDF graph, which
would not work for the SPARQL scenario (since we really want something more
like the reverse).  So we would need a different mechanism and there are a
number of ways to do this, but this mostly a problem of reference and not
semantics.

>>>   o Will each rule set be an entailment regime, e.g., the SD says something
>>>     like: myEndpoint sd:EntailmentRegime <http://example.org/myRules.rif>?

I would think that the entailment regime would be comprised of the RIF-RDF
combination consisting of the RIF document (referred to in some way that
also specifies the entailment profile) and the active/scoping graph as well
as the entailment relationship determined by the profile specified.

>>>   o Not all RIF dialects are based on a model-theory (e.g., RIF PRD), so
>>>     they do not come with an entailment relation, but have a procedural
>>>     semantics. Can we still use the procedural semantics to define
>>>     something like an entailment regime?

I don't think (just my $0.02) that we want to support (at least not
initially) any other RIF dialect beyond RIF Core.  It is a quick win, and
anything else becomes very complicated very fast (monotonic, etc..)
 
>>>   o Which RIF profiles should be included? Only RIF Core? Does RIF Core
>>>     coincide with OWL RDF-Based or Direct Semantics? How many profiles are
>>>     there?

I would think we only want to include (safe) RIF-Core.

> The advantage of (strongly safe) RIF Core is that it lends itself
> straightforwardly to the
> idea of defining the entailment regime via the closure (i.e. minimal Herbrand
> model), since 
> it is always finite. For other entailment regimes, restricting to a finite
> answer set is trickier.

Yes.

> I wouldn't be too worried to leave that open... as we did leave open the
> definition of 
> more dialects in RIF. I'd be happy to go with RIF strongly safe Core, RIF Core
> and 
> RIF BLD for a start. (If we find a volunteer to tackle RIF PRD, also fine).

What is the delta between RIF Core and RIF BLD?

>>>   o What effects do the non-monotonic features of some RIF dialects have?
>>>     E.g., RIF PRD and (anticipated) RIF dialects with default negation.
>>>     How does that interact with SPARQL's non-monotonic features?
>>>     This probably affects issue-43: Should entailment-regimes be declared
>>>     over the whole dataset or individual graphs?
>> 
>> Eeeeek, scary stuff.

>> One little concern I have is Condition 4.  I don't see how one can
>> possibly define a finite answer set for RIF.   For example, given the
>> document:
>> 
>>     if p(?n) then p(?n+1).
>>     p(1).
>> 
>> and the query
>> 
>>     p(?x).

The RIF Core safeness criteria are meant to guarantee finiteness.

>> well, I don't think there's any sensible way to limit this to a finite
>> set of answers.  Is there?

Yes, but requiring that the RIF core documents used to form RIF-RDF
combinations are safe.  I think this is the way to address C4.

-- Chime


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Received on Wednesday, 24 February 2010 15:12:53 GMT

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