- From: Graham Klyne <Graham.Klyne@Baltimore.com>
- Date: Thu, 26 Jul 2001 16:57:21 +0100
- To: RDF core WG <w3c-rdfcore-wg@w3.org>
At 07:06 PM 7/20/01 +0100, Graham Klyne wrote: >Per today's teleconference, these are the questions that I feel need to be >answered to resolve issue #rdfms-identity-anon-resources [1]. > > >1. Does the current model and syntax spec sanction the use of an anonymous >resource to convey anything other than an assertion of the existence of a >resource with the properties given? c.f. Frank Manola's comment in >[2]. See also Brian McBride's survey of relevant parts of M&S [3]. I think not. >2. Does the use of an anonymous resource as a query variable represent an >extension to the idea an assertion of the existence of a resource with the >properties given? Opinions vary: e.g. [4-9]. I think this is an extension to the idea of expressing existence. >3 If the answer to Q1 is "no" and to Q2 is "yes", should we undertake to >extend M&S to allow this usage? If so, why and how? I think such extension would have far deeper ramifications than we can currently see; e.g. maybe requiring introduction of universally quantified variables. I am also thinking there is a possible relationship with use-without-assertion of RDF statements, which is currently a murky area. My (tentative) answer, then, would be no, don't try and do this now (but note the issue for further exploration in RDF V2.0. >4. Can we agree whether or not unique generated IDs (in the style of >Skolem constants) are equivalent to existentially quantified variables for >the purpose of asserting the existence of a resource with properties >given? (See Frank's message [10] for a discussion -- I discount the >option of dropping anonymous resources.) At the level of making simple assertions using the existential-conjunctive subset of FOL, I think they are equivalent. But I'm not sure there won't be future developments of RDF that might expose subtle differences. >If we agree they are equivalent: > >4.1 What are the advantages and difficulties with each approach? > >4.2 Which approach do we prefer? > >4.3 How does the preferred approach affect M&S? I lean toward having a special "variable name" form along the lines of _:genid, for two reasons: (a) it's easier, more compact and ultimately more dependable then trying to generate unique identifiers. (Unique identifier algorithms are statistical rather than definite in their performance. And the statistics aren't always as favourable as one might expect: see, e.g., appendix A of RFC 2938.) (b) it seems possible that differences will emerge if future developments extend the logical capabilities of RDF. ... I think that the variable name form would (for now) behave like a URI in most respects, except one. Assuming an operation that takes two independent N-triple expressions (= set/bag of statements) and combines them into a single N-triple expression containing all of the statements from each, names of the special form are changed/reassigned so that they are unique in the new expression; this description needs tightening up to maybe something like: (1) two variables are DISTINCT if: (a) they have different names, OR (b) they appear in different N-triples expressions (2) when combining two N-triples expressions into a single expression, all variables are assigned arbitrary names (which may be the same as their original names) such that DISTINCT variables always have different names in the resulting expression. By this approach, the variables are maintained with distinguishable forms, but may be treated equivalently for the purposes of the model theory. Processing of such variables is a syntactic operation on N-triples expressions. >5. Are there any other uses of anonymous resources that have been overlooked? I see none that are consistent with my understanding of the expressive capability of RDF 1.0. #g -- >[1] http://www.w3.org/2000/03/rdf-tracking/#rdfms-identity-anon-resources > >[2] http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jul/0124.html > >[3] http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jul/0244.html > > >Opinions on representation of query variables: > >[4] Dan >C: http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jun/0435.html > >[5] Pat >H: http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jun/0543.html > >[6] Pat >H: http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jun/0518.html > >[7] Dan >C: http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jun/0537.html > >[8] >Frank: http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jun/0538.html > >[9] >Frank: http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jun/0559.html > > >[10] http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jul/0218.html > > > >------------ >Graham Klyne >(GK@ACM.ORG) ------------------------------------------------------------ Graham Klyne Baltimore Technologies Strategic Research Content Security Group <Graham.Klyne@Baltimore.com> <http://www.mimesweeper.com> <http://www.baltimore.com> ------------------------------------------------------------
Received on Thursday, 26 July 2001 11:58:53 UTC