From: Frank Manola <fmanola@mitre.org>

Date: Mon, 14 Oct 2002 20:59:22 -0400

Message-ID: <3DAB686A.3010208@mitre.org>

To: Jeremy Carroll <jjc@hplb.hpl.hp.com>

CC: w3c-rdfcore-wg@w3.org

Date: Mon, 14 Oct 2002 20:59:22 -0400

Message-ID: <3DAB686A.3010208@mitre.org>

To: Jeremy Carroll <jjc@hplb.hpl.hp.com>

CC: w3c-rdfcore-wg@w3.org

Jeremy-- I seem to recall this "graph vs. triples" business being mentioned during the last teleconference, but maybe I'm wrong, and in any case I don't remember what happened. Speaking as Primer editor, consider this a vote for option 2. I'd hate to have to rewrite all the text that describes the drawn graphs. Besides, I think the graph model is helpful, and having multiple ways of looking at the same thing is often useful in tutorial material. At a more fundamental level, I think part of our problem in trying to match up graph concepts and triple (or relation) concepts is that we didn't go far enough in any of our material in describing the connection between a graph-theoretic type of model and a relational type of model (we're using triples to represent the tuples in a binary relation, where we name the relation in each tuple). It might be helpful to look at John Sowa's mathematical background article at http://users.bestweb.net/~sowa/misc/mathw.htm, in particular section 6 on "representing relations by graphs". In particular, he starts off by saying "Graphs and dyadic relations are mathematical structures that look different, but they can represent the same information in logically equivalent ways. Historically, graphs are more closely associated with geometric properties that can be seen from diagrams, and relations are associated with more abstract mathematics and logic. But every dyadic relation can be represented as a graph, and every graph defines a dyadic relation." So far this is more-or-less conventional graph theory (where a graph is defined by a set of vertices and a relation defined on the set of vertices). The particularly important part follows, namely: "Although every dyadic relation can be represented by a graph, some extensions are necessary to represent multiple relations in the same graph. A common technique is to label the arcs of a graph with the names of the relations they represent. Informally, a labeled graph can be viewed as a set of graphs * one for each relation * overlaid on top of one another with the labels showing which relation each arc has been derived from." It's this business about representing multiple relations in the same graph (which is what we're trying to do), that's been somewhat missing from a lot of our graph-theoretic discussions. I think something like this could help clarify the relationship between a graph-theoretic approach (which conventionally deals only with *one* relation), a model that consists of multiple relations having sets of pairs as extensions [that's if you don't discuss classes as being 1-ary "relations"], and the triple model (which is another way of writing the relational model). --Frank Jeremy Carroll wrote: > > > I have slowly been having a conversation with Pat about the mismatch > between the abstract syntax document's use of labelled nodes within a > graph, and the model theory which uses literals and uris directly within > triples. > > Pat seems to have argued me round to his position (I am not quite sure > how :) ). > > I had wanted to know from the other editors (Dave (syntax and ntriples), > Frank (primer) and DanBri (vocab)) as to what difficulties two possible > versions of the abstract syntax would cause. I would also appreciate the > series editor's input here since it seems to be about the global style > of our recommendation. > > Version 1 - labels out, triples as way forward. > > this would follow recent model theory documents in describing the RDF > data model as a set of triples; an extreme version would not even use > the graph language at all, or would define an "RDF graph" as a set of > triples. > With this language other docs should not refer to the "label on the > subject node" or the "URI label" or similar; we may choose to revisit > use of words like arc or edge and replace them all with triple. > > Version 2 - mix and match > > This would have the version 1 text, to connect with the model theory, > and sketch the isomorphism to directed graphs with labelled nodes and > labelled edges. This would allow a promiscuous use of any graph > theoretic or triple oriented language in all the other documents. > > Pros and Cons: > > Version 1, clearer, less work for me, more work for other editors. > Readers get consistent terminolog. > > Version 2, less work overall, readers get exposure to the variety of > terminology used in RDF community to describe the same thing. > > > Comments? > > Jeremy > > > > -- Frank Manola The MITRE Corporation 202 Burlington Road, MS A345 Bedford, MA 01730-1420 mailto:fmanola@mitre.org voice: 781-271-8147 FAX: 781-271-875Received on Monday, 14 October 2002 20:43:31 EDT

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