RDF Model Theory Working Draft: Comment

From the W3C Working Draft, dated 25 September 2001 [1]:

> A graph can be viewed as a set of triples corresponding to its
> edges; the correspondence between an ntripleDoc and an RDF graph
> is that the graph has one node for each uriref, bNode or literal
> identifier in the document, and one edge for each triple, directed
> from the node corresponding to the subject to the node
> corresponding to the object.  Nodes corresponding to urirefs and
> literals are labeled with the corresponding URI or literal, and
> arcs are labeled with the property URI from the corresponding
> triple. Notice that this requires that all occurrences of a
> particular uriref or bNode identifier in an N-triples document be
> mapped to a single node in the corresponding graph.

This correspondence does not seem to have any support beyond its mere
assertion, as if it were a matter more of belief than fact.  While a
graph can be viewed as a set of triples corresponding to its edges, a
set of triples need not correspond to the edges of a graph.

The problem is this: If an RDF graph has "one node for each uriref,
bNode or literal identifier", then the general representation of a
statement will need three nodes - not two - because all three of the
triple's terms are *nodal*: the arc from subject to object allegedly
corresponding to the triple is being labelled with a term that is
already supposed to be (the label of) a node.  In short, how can the
label of an arc be a URI (reference) that by definition has been
mapped to a node already?

The pretty picture in the RDFMS recommendation [2, Fig 11] would seem
to be plain and simple wrong.  This difficulty was pointed out by Paul
Walton in Mar 1998 [3], on this list.  There was no response, and the
same difficulty has persisted, through more than one bout of shifts to
apparently more resonant terminology, from the Working Drafts to the
Recommendation to this latest incarnation as Model Theory.

Is this list the official place to be ignored?

[1] http://www.w3.org/TR/rdf-mt/
[2] http://www.w3.org/TR/REC-rdf-syntax/
[3] http://lists.w3.org/Archives/Public/www-rdf-comments/1998JanMar/0002.html

[I would claim, on the basis of my "gestalt" of RDF, that a ntriple
should be a node itself.  The "basic" RDF graph representation would
have four nodes, for the triple and its three terms; the arcs would be
labelled by "subject", "verb" and "object" - or their equivalents du
jour; anonymous nodes would just be missing outbound subject arcs;
nodes with only inbound arcs would be the natural places to locate all
atomic/primitive terms (literals, urirefs, symbols, whatever); and
"reification" would seem to be unnecessary.  What this means for Model
Theory I haven't the faintest idea.]


Arjun

Received on Saturday, 6 October 2001 15:44:25 UTC