From: Boley, Harold (Harold.Boley@nrc.ca)
Date: 11/05/02
Dan, in the telecon the same issue was brought up. Currently RuleML doesn't have a particular representation for RDF's bNodes. We briefly discussed using anonymous logical variables, empty or gensymed URIs, or an explicit (builtin) element. Looking at the RDF Model Theory edited by Pat, we are still open to various possibilities for "simply indicating the existence of a thing, without using, or saying anything about, the name of that thing" (http://www.w3.org/TR/rdf-mt/#unlabel). It now is clear to me that anonymous logical variables are no solution, since they are universally, not existentially, interpreted in facts. For example, "bNode001 has creator Ora Lassila" cannot be represented as the Prolog fact ("_" indicates an anonymous Prolog variable) has-creator(_,"Ora Lassila"). or the RuleML fact ("<var/>" indicates an anonymous RuleML variable) <fact> <_head> <atom> <_opr> <rel href="http://dublincore.org/documents/dces/index.shtml.rdf#Creator"/> </_opr> <var/> <ind>Ora Lassila</ind> </atom> </_head> </fact> because it would mean that Ora created everything, e.g. making queries like has-creator("http://www.w3.org/People/DanBri","Ora Lassila"). or <query> <_body> <atom> <_opr> <rel href="http://dublincore.org/documents/dces/index.shtml.rdf#Creator"/> </_opr> <ind href="http://www.w3.org/People/DanBri"/> <ind>Ora Lassila</ind> </atom> </_body> </query> wrongly succeed. I now think RDF's bNodes are more akin to some of the many uses of null values in relational databases or, perhaps, closer to RDF, in object-relational databases. A quick Google search revealed a related remark in "Topic Maps and RDF" (http://www.isotopicmaps.org/pipermail/sc34wg3/2002-August/000495.html). Null values are absent from Prolog, but in Dagstuhl discussions with Jens Dietrich (http://www.jbdietrich.de/about.html) we thought they would be important in RuleML anyway (he is working on RuleML extensions for relational databases). So, to leave open the thing created we could assert ("<ind/>" would indicate a RuleML null value of type 'individual') <fact> <_head> <atom> <_opr> <rel href="http://dublincore.org/documents/dces/index.shtml.rdf#Creator"/> </_opr> <ind/> <ind>Ora Lassila</ind> </atom> </_head> </fact> Yet, this would not give us the local existential scope of RDF's bNodes. We hope our analysis of the RDF Model Theory will lead to a good solution for RuleML bNodes as well. Perhaps we should also have a closer look at eigenvariables in Lambda-Prolog (http://www.cse.psu.edu/~dale/lProlog/). Maybe we can even learn something here from the discussion of the "line of identity" in Charles Sanders Peirce's Existential Graphs (http://users.bestweb.net/~sowa/peirce/ms514.htm). Ultimately, we may need explicit (existential) quantifiers for glueing together a conjunction (or a rulebase) of facts. Harold -----Original Message----- From: Dan Brickley [mailto:danbri@w3.org] Sent: November 5, 2002 4:54 PM To: Boley, Harold Cc: 'joint-committee@daml.org' Subject: Re: Slides "RuleML Meets RDF" for today's Joint Committee telecon Harold, One question re: "RDF triples become special binary facts where the relation and first argument must be urirefs, and the second argument can be urirefs or literals" Does RuleML have any particular representation for RDF's bNode (aka 'anonymous' or un-named resource) construction? Dan On Tue, 5 Nov 2002, Boley, Harold wrote: > Hi folks, > > attached are the slides "RuleML Meets RDF" for today's Joint > Committee telecon. > > Please, if possible, have them in PPT 2000 presentation mode > so I can go over them in the 30 minutes allocated by Mike. > > Harold > > > > <<ruleml-rdf.ppt>> >
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