- From: Pat Hayes <phayes@ihmc.us>
- Date: Tue, 29 May 2012 20:38:37 -0500
- To: Sandro Hawke <sandro@w3.org>
- Cc: Andy Seaborne <andy.seaborne@epimorphics.com>, RDF-WG <public-rdf-wg@w3.org>
On May 29, 2012, at 7:41 PM, Sandro Hawke wrote: > On Wed, 2012-05-16 at 17:33 +0100, Andy Seaborne wrote: >> 1/ Scenario: >> >> A site publishes a page, in RDFa, with triples in it. >> >> The site replaces that page, with one saying, "changes in progress, >> please come back later" with no triples. >> >> Recording this seems like a reason to have empty named graphs. > > I agree it's useful, I just don't know how to make it work with > partial-graph semantics. See "subset semantics" in recent message, > http://lists.w3.org/Archives/Public/public-rdf-wg/2012May/0650.html > > If > <g1> { } > and > <g1> { :a :b :c } > entails > <g1> { :a :b :c } > > then information was the first graph conveying? I think it was > conveying that <g1> had at least no triples, which isn't conveying much > at all. Right. Semantically, an empty graph is an empty conjunction, which is trivially true. So it amounts to a kind of RDF tautology. And a tautology, indeed, conveys no information, when asserted. Still, it isnt incohwerent, just kind of dim. Pat > > I think this use case could be addressed with a vocabulary instead of > syntax, as you suggest I think for dealing with quadstores. That is, > instead of allowing in a dataset: > > <g> { } > > we could put in the default graph something like: > > <g> a rdf:EmptyResource. > > It's certainly not as elegant, though. (and not compatible with > existing SPARQL systems.) > > Oh. I guess with partial-graph semantics (aka subset semantics) one > could also make it complete with a count of the triples. So > > <g> { :a :b 1,2,3 }. > <g> rdf:tripleCount 3. > > would tell us <g> has those three triples and ONLY those three triples. > With that, the complete-graph-semantics dataset: > > <g> { } > > could be conveyed by the partial-graph-semantics dataset: > > <g> rdf:tripleCount 0. > > Still not elegant, but it might be about as simple as possible. > > -- Sandro > > >> 2/ The default graph can be empty. >> >> 3/ A point made has been that N-quads can't represent an empty named graph. >> >> A syntax fix could be: >> >> _ <http://space> . >> >> or other style indicate a space name. >> >> >> To my reading, the spaces document works if "quads" are removed, not >> that I'm suggesting that. >> >> How about defining as a convenience of useful vocabulary rather than >> core definition? >> >> Andy >> >> > > > > ------------------------------------------------------------ IHMC (850)434 8903 or (650)494 3973 40 South Alcaniz St. (850)202 4416 office Pensacola (850)202 4440 fax FL 32502 (850)291 0667 mobile phayesAT-SIGNihmc.us http://www.ihmc.us/users/phayes
Received on Wednesday, 30 May 2012 01:39:11 UTC