- From: Ivan Herman <ivan@w3.org>
- Date: Sat, 13 Mar 2010 10:19:29 +0100
- To: Chimezie Ogbuji <ogbujic@ccf.org>
- CC: Birte Glimm <birte.glimm@comlab.ox.ac.uk>, SPARQL Working Group WG <public-rdf-dawg@w3.org>, Axel Polleres <axel.polleres@deri.org>, Sandro Hawke <sandro@w3.org>
- Message-ID: <4B9B58A1.4090904@w3.org>
Chime, I do not understand... On 2010-3-12 21:10 , Chimezie Ogbuji wrote: > So as I was taking another pass through the Simple RIF Core section, I > noticed a problem that I will need to addressed. In short, without some > amount of 'embedding', RIF-Simple entailment will behave in a > counter-intiuitive way if there is no correspondence between statements and > frames, rdf:type and #. > > Consider the example below > > Scoping Graph > ------------- > _:a rdf:type _:b > <> rif:imports <emptyRules> > My understanding of the proposed semantics (by Axel) for rif:imports is that this combination is transformed as follows: 1. Starting point G: _:a rdf:type _:b . <> rif:imports <R> . R: empty 2. Apply the semantics G': _:a rdf:type _:b R': Import(G, <http://www.w3.org/2007/rif-import-profile#Simple>) (whether the <> rif:imports <R> is removed from G is still an open question but does not seem to influence this issue) 3. From the RIF point of view, that is equivalent to: R'' : _a # _b . (using RIF's unique id-s which look very much like skolemization to me). Which means that... > Where emptyRules.rif is an empty document > > A SPARQL query such as SELECT ?THING ?CLASS { ?THING a ?CLASS } will return > nothing! In fact (as I understand it - I'm hoping someone else can confirm > this) *all* queries will return nothing with the current language: > > "A solution mapping μ is a?possible solution [...] such that sk(P(BGP)) are > well-formed RDF triples that are RIF-Simple entailed by the?RIF-RDF > combination formed from sk(SG) and the?strongly safe?RIF core documents > referenced from SG via the rif:imports predicate" > > As a result of condition 7 for common-RIF-RDF-interpretations <I',I> (where > I' is a RIF-BLD semantic structure and I is a simple interpretation), the > scoping graph is not satisfiable by such an I in a > common-RDF-RDF-interpretation. In particular IEXT(IS(rdf:type)) is *not* > equal to the set of all pairs (a,?b) in?Dind?×?Dind (since the latter is an > empty set). This is not an empty set, see above... Do I severely miss something here? Actually, if what you say was true, then I think there is a problem in the RIF-RDF document. That has to be signalled to the RIF group Ivan > This condition, along with others requires this correspondence > in order for an common RIF-RDF-interpretation to be valid. > > As a result, the combination will not entail anything and since the answers > are defined with respect to RIF-Simple entailment, there will be no answers. > > The mechanisms in section 9.1.3 Embedding Simple Entailment in the > compatibility document provide some tools to address the problem and I'm > inclined to re-use them rather than simply refer to that informative section > since the goal of embedding (to demonstrate how a RIF-compliant translator > without native support for RDF can process RIF-RDF combinations) seems very > different from what we are doing here as I understand it. > > My suggested solution is to form the combination differently: > > SG' = sk(SG) > <merge(R,?R_{simple-core},tr(SG')), SG'> > > Where merge is the same function that is defined towards the end of 9 of the > appendix and tr is the same injective function described in the appendix as > well. The difference between how tr is used in 9.1.3 and how it is used > here is that nothing will need to be done with BNodes (since they will be > replaced with new IRIs by the time the function is evaluated over SG'). > > R is the same strongly, safe RIF Core document referenced from SG. > R_{simple-core} is essentially the same ruleset as R_{Simple} (from 9.1.3) > except for the exclusion of the rule involving ## (which is not allowed in > the RIF Core syntax anyways). So it is: > > Forall??x??y (?x[rdf:type ->??y]?:-??x #??y) > Forall??x??y (?x #??y?:-??x[rdf:type ->??y]) > > This effectively ensures that condition 7 is met. So, for our example, > > merge(R,?R_{simple-core},tr(sk(SG))) becomes > > Forall??x??y (?x[rdf:type ->??y]?:-??x #??y) > Forall??x??y (?x #??y?:-??x[rdf:type ->??y]) > <skolem_iri_1>[rdf:type -> <skolem_iri_2> > <skolem_iri_1> # <skolem_iri_2> > > And without the issue of the conditions regarding the correspondence between > frames and statements, the subgraph characteristics of simple entailment > hold, sk(P({?THING rdf:type ?CLASS})) *will* be RIF-simple entailed, and > answers would be provided. > > Any thoughts before I try to incorporate this? > > -- Chime > > > =================================== > > P Please consider the environment before printing this e-mail > > Cleveland Clinic is ranked one of the top hospitals > in America by U.S.News & World Report (2009). > Visit us online at http://www.clevelandclinic.org for > a complete listing of our services, staff and > locations. > > > Confidentiality Note: This message is intended for use > only by the individual or entity to which it is addressed > and may contain information that is privileged, > confidential, and exempt from disclosure under applicable > law. 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Received on Saturday, 13 March 2010 09:21:38 UTC