- From: Pat Hayes <phayes@ai.uwf.edu>
- Date: Wed, 3 Apr 2002 12:15:14 -0600
- To: "Massimo Marchiori" <massimo@w3.org>
- Cc: "Dan Connolly" <connolly@w3.org>, "Pat Hayes" <phayes@ai.uwf.edu>, "w3c-rdfcore-wg" <w3c-rdfcore-wg@w3.org>, "Lynn AndreaStein" <las@olin.edu>, "Jan Grant" <Jan.Grant@bristol.ac.uk>
> > > So, summing up, since this is a fundamental architectural decision >> > (not just syntactic sugaring) concerning RDF, what is most interesting >> > is to give the reasons for this interpretation vs the easiest >> skolem one. >> > Yes, it's a classic "last call" fundamental question, because >> that spawns >> > into the data model, on which there are many things to discuss, >> but well, >> > Dan brought the matter up early ;) >> >> At the first WG F2F we had a long (and, i think, productive) argument* >> about this. Sergei produced a good set of pros and cons; my arguments >> for this are ... >> >> - supports "non-assertional" mode, ie, RDF querying by turning around >> the "X entails what?" into "what entails X?" >> >> - aesthetic reasons, and those of transparency. When I write an >> assertion with a blank node, I intend it to mean "there exists...". >> >> - DanC also claimed that skolemisation was too much of a general >> impediment to getting software written :-) I think he may have >> been dramatising for, well, dramatic effect, but I've some sympathy >> with this POV. In other words, supporting anonymous nodes requires >> some API fiddling, but is not necessarily a "simpler mechanism". > >Thanks for the initial reply, Jan. >I don't want to start a complex debate without first having seen a complete >reply, just noticing that this is such a fundamental architectural decision >that a complete and careful pro/con analysis is due. The above three "pro" >reasons >all have good counterarguments, I'd be interested in hearing them. >so I'll wait to see for the full official >motivation-and-reasoning behind such a milestone decision. > >-M For me, the fundamental motivation for not skolemizing blank nodes concerns entailment. Consider an RDF graph G1 and another graph G2 got from G1 by erasing some of the node labels, ie replacing urirefs with blank nodes. Does G1 entail G2? Seems to me that the answer has to be 'yes' in order to capture the intended meaning of blank nodes as described in the M&S. If blank nodes are hidden skolemizations, however, the answer is 'no'. So skolemized graphs do not adequately support the proper RDF entailment conditions. It would be fine for blank nodes to have some kind of implementation-dependent hidden labels - in effect, that is what the bnode labels in Ntriples are - but the critical point is that these labels are not treated like urirefs; they have no global scope, and are not meaningful outside the graph, and are not treated semantically as names. RDF could be redefined without blank nodes, of course, but it would be a different, and much weaker, language. In effect, it would be a simple positive propositional logic, with no quantification. Skolemized nodes are not blank, so the whole concept of anonymous nodes would be eliminated from the language, rendering it simpler in both its syntax and its semantics. I agree that this is a fundamental architectural decision, but it seems to me that the M&S has already clearly made the decision: the language does contain blank nodes. Given that decision, skolemization is not an option. The milestone decision was taken long ago, and all we are doing is stating it more precisely. Pat Hayes -- --------------------------------------------------------------------- IHMC (850)434 8903 home 40 South Alcaniz St. (850)202 4416 office Pensacola, FL 32501 (850)202 4440 fax phayes@ai.uwf.edu http://www.coginst.uwf.edu/~phayes
Received on Wednesday, 3 April 2002 13:15:15 UTC