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RE: motivation for bNodes/existentials in RDF; note for parsers

From: Jeremy Carroll <jjc@hplb.hpl.hp.com>
Date: Thu, 4 Apr 2002 20:17:09 +0100
To: "Massimo Marchiori" <massimo@w3.org>, "Pat Hayes" <phayes@ai.uwf.edu>
Cc: <w3c-rdfcore-wg@w3.org>, "Lynn AndreaStein" <las@olin.edu>
Message-ID: <JAEBJCLMIFLKLOJGMELDCEJLCDAA.jjc@hplb.hpl.hp.com>
From our charter:
The deliverables [...] are
+ update the RDF Model and Syntax Specification (as one, two or more
documents) clarifying the model and fixing issues with syntax

> In fact, I profoundly disagree here. If there's something where
> M&S is very
> vague is just anonymous nodes. Even, reading the spec carefully,
> one will see that, in fact, this vagueness can be somehow justified,
> as anonymous nodes are there seen just as accessories, and not as a
> fundamental component of M&S: a facility to avoid having to assign
> names, but just a facility.

The definitive statement in M&S is (para 41)
The sentence above does not give a name to that resource; it is anonymous,
so in the diagram below we represent it with an empty oval

The RDF Core WG is chartered to clarify that statement, and the amount of
angst in the RDF community that that statement caused justifies quite some

"clarifying the model" in our charter primarily reflects the need to clarify
whether the anonymous node is blank or labelled. The (normative) diagrams in
M&S show pictures of graphs with blank nodes. We have decided that the use
of the word Model in M&S for the data model was confusing, and have, fairly
systematically, referred to the same concept as graph or (mainly in the
primer as data model). We have clarified the second part of the quoted
sentence from para 41 (and the picture below) by making it clear that the
graph includes nodes that are blank as well as those that are labelled.

The first part of the sentence indicates that there is a resource
(corresponding to the blank node). We have clarified that by providing the
model theory. We have of course provided a much clearer spec than perhaps
the charter writers had in mind, but it is merely a clarification. It is not
an "RDF Logic" or "RDF Query", contrast it with N3 and see how much more
restrained in its goals it is.

The model theory is a crucial part of our key chartered task of clarifying
and rearticulating. It is not intended as merely a proprosal, but the heart
of the work of
clarifying the data model, in that it clarifies the most awkward part of the
data model the blank node. An existential reading is not the beginnings of a
logic but merely an articulation in the clearest possible language (i.e. the
language of logic) how the resource referred to by a blank node and the
resource referred to by a labelled node may or may not be the same.

A second role of the model theory is in support of RDF schema.
You will note that so far the only work we have published on schema is in
the model theory.

"The RDF Core WG is chartered to complete the work on RDF vocabulary
description present in the RDF Schema Candidate Recommendation."

I personally find that the schema closure rules within the model theory are
an extremely clear and precise way of moving the RDF vocabulary description
work forward.

The ability to clarify these rules further by use of entailment tests both
with and without the schema closure rules active is a further advantage
provided by the model theory that is clearly within our principle charter
objective - clarity.

The ability to produce entailment tests which is furnished by the model
theory is further within charter considering the explicit deliverable:
"publish a set of machine-processable test cases corresponding to technical
issues addressed by the WG"

Many of the technical issues we have addressed is about what does such and
such an RDF graph mean, and the clarity that is gained by converting these
issues into entailment tests is highly advantegeous.

The model theory holds the work of this wg together. If it is not normative
we have not fulfilled our charter of providing clarity.

>    Then, we can debate at length on how the
> spec should have been clearer on this (and where the architectural
> pitfalls are). We can even decide M&S was not smart and that we need to
> redefine and introduce first-class anonymous nodes (not a bad
> idea), but it
> should be clear the distinction on what *we* are saying, and
> what's written
> in the normative spec (the only normative thing we have so far).

You appear to be saying that:
   M&S is unclear,
   hence the only clarification of M&S is that it is unclear.
   (Anything more precise is adding something that wasn't in M&S).

Logically this is correct; but it defeats the charter.

Given that the WG is chartered to clarify, in those places where M&S is
unclear I believe it is our duty within the charter to make the best
decision we can to resolve the lack of clarity.

> So, now the higher level of criticism can be better understood.
> If MT is just a proposal, fine (even more: good! as, it helps to
> provide a possible good starting point for a next RDF Logic/Query wg).
> If on the other hand, it aims to define now the normative RDF Logic,
> then I think it's truly beyond scope.
> Its formal status is unclear now, so better clarify it.

Received on Thursday, 4 April 2002 14:17:20 EST

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