- From: Jos de Bruijn <debruijn@inf.unibz.it>
- Date: Fri, 13 Feb 2009 10:08:08 +0100
- To: Axel Polleres <axel.polleres@deri.org>
- CC: RIF WG <public-rif-wg@w3.org>
- Message-ID: <49953878.9010602@inf.unibz.it>
Then about SPARQL: you were citing the wrong piece of text; it does not specify how the default data set is obtained from the FROM clause. The following text in section 8.2.1 does seem to support your argument: "Each FROM clause contains an IRI that indicates a graph to be used to form the default graph. " one wonders, though, what it means for an IRI to "indicate" a graph. Note that we are not talking about named graphs, but about RDF graphs (which do not have a name). I did not find any formal definitions about evaluating SPARQL queries including FROM clauses, so I could really verify what they mean precisely with "indicate". Best, Jos Jos de Bruijn wrote: > As a clarification: > the OWL2 document you are referring to is merely a "normal" working > draft; it is not in last call, and I know for a fact that there have > already been substantial (and substantive) changes in the document since > the working draft of December. So, this is not something we should base > our decisions on. > > The section from the OWL semantics you are referring to is concerned > with the abstract syntax, not with RDF documents on the web. In fact, > when going to the RDF world in section 5.4 [1] I read: > "[A collection of OWL DL ontologies] O is said to be imports closed iff > for any URI, u, in an imports directive in any ontology in O the RDF > parsing of the document accessible on the Web at u results in T(K), > where K is the ontology in O with name u." > > This is very much in line with what we wrote in the RDF and OWL > compatibility document. > > all that said, I don't care too much about this issue. But we need to > get our facts straight when referring to other specifications. > > > Best, Jos > > [1] http://www.w3.org/TR/owl-semantics/rdfs.html#5.4 > > Axel Polleres wrote: >> I am a bit surprised to see that we refer to a "RDF graphs accessible >> from the locations u1,...,un" in RIF imports, which suggests that we >> talk about URLs here, rather than URIs. >> >> Neither OWL, nor SPARQL, nor OWL2 do this: >> >> OWL [2, Section 3,4]: >> "The imported ontology is the one, if any, that has as name the argument >> of the imports construct. (This treatment of imports is divorced from >> Web issues. The intended use of names for OWL ontologies is to make the >> name be the location of the ontology on the Web, but this is outside of >> this formal treatment.)" >> >> SPARQL [3, section 8]: >> "A SPARQL query is executed against an RDF Dataset which represents a >> collection of graphs. An RDF Dataset comprises one graph, the default >> graph, which does not have a name, and zero or more named graphs, where >> each named graph is identified by an IRI." >> >> OWL2 [4, Section 9]: >> "Definition 3.1 (Import Closure): Let K be a collection of RDF graphs. K >> is imports closed iff for every triple in any element of K of the form x >> owl:imports u then K contains a graph that is referred to by u. The >> imports closure of a collection of RDF graphs is the smallest imports >> closed collection of RDF graphs containing the graphs." >> >> Neither of these specs require the URI/IRI of an ontology (or for a >> named graph in the case of SPARQL) to be dereferenceable on the Web, but >> this is - IMO intentionally - left open in the specs, just mentioning >> that the URI/IRI at identifies a graph/ontology. How this identification >> is specified is not part of the specs. While accessing the URI as a URL >> from the Web might be the default behavior, there are use cases where >> this may not be desirable (e.g. in a Triple store which has several >> named graphs stored, these graphs may not be (web) accessible, but only >> be called by these "names" within the triple store.) >> >> Likewise, I would be reluctant if we made any stronger assumptions here, >> which might be restrictive. I rather suggest to adopt something similar >> to the formulation in OWL above. >> >> Long written, briefly summarized: >> I suggest to replace >> "accessible from the locations u1,...,un" >> by >> "referred to by u1,...,un" >> >> >> This is though not directly related to your question, I see. But we >> could state e.g. something like "If there is no RDF graph (or, resp. >> ontology) referred to by uri u_i in an imports statement, the respective >> graph SHOULD be treated as empty." (in case this is the behavior we >> want to advocate) >> >> Axel >> >> 2. http://www.w3.org/TR/owl-semantics/direct.html >> 3. http://www.w3.org/TR/rdf-sparql-query/ >> 4. http://www.w3.org/TR/2008/WD-owl2-rdf-based-semantics-20081202/ >> Jos de Bruijn wrote: >>> In section 5.2 of the RDF-OWL document [1] we specify how RIF documents >>> with two-ary import statements must be interpreted. however, we do not >>> say anything about the case that an import statement refers to a >>> location does not have an RDF graph. >>> >>> So, if >>> >>> Import(<u1> <p1>) >>> ... >>> Import(<un> <pn>) >>> >>> are the 2-ary import statements and one of u1,...,un does not point to >>> an RDF graph, what should happen? Do we say that the document could be >>> rejected, or do we leave this unspecified? >>> >>> related: what if pi denotes the OWL DL profile, but ui does not point to >>> an OWL DL ontology? Should the document be rejected? I think so. >>> >>> Best, Jos >>> >>> >>> [1] http://www.w3.org/2005/rules/wiki/SWC#Interpretation_of_Profiles >> > -- Jos de Bruijn debruijn@inf.unibz.it +390471016224 http://www.debruijn.net/ ---------------------------------------------- No one who cannot rejoice in the discovery of his own mistakes deserves to be called a scholar. - Donald Foster
Received on Friday, 13 February 2009 09:08:11 UTC