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Re: nodes and node labels [was New document: revised version for WG review]

From: Stephen Petschulat/CanWest/IBM <spetschu@ca.ibm.com>
Date: Tue, 30 Jul 2002 07:11:29 -0700
To: Frank Manola <fmanola@mitre.org>
Cc: w3c-rdfcore-wg@w3.org
Message-ID: <OF641FD5D4.28E947C0-ON88256C06.004D6404@mkm.can.ibm.com>

> a.  if, in the current model theory, some nodes *are* URIrefs, rather
> than being *labeled with* URIrefs, why shouldn't arcs *be* URIrefs,
> rather than being *labeled with* URI refs?

Arcs in a mathematical graph are defined in terms of their relationship
between two nodes so there is no way for them to exist independently of the
two nodes. Hence you can only label them with a URIref, they can't *be* a
URIref since by definition they are an ordered pair of nodes (or URIrefs in
this case).


- steve

Stephen Petschulat

                      Frank Manola                                                                                                      
                      <fmanola@mitre.org        To:       Jeremy Carroll <jjc@hplb.hpl.hp.com>                                          
                      >                         cc:       w3c-rdfcore-wg@w3.org                                                         
                      Sent by:                  Subject:  Re: nodes and node labels [was New document: revised version for WG review]   
                      07/29/2002 05:09                                                                                                  

Jeremy Carroll wrote:

> I wanted to draw the WG attention, particularly Pat, to a mathematical
> with the graph syntax.
> I do not believe that this has any substantive content, but is merely
> aesthetic!

Aesthetic possibly, but it also has implications on apparent
consistency.  I'm explaining some of this stuff in the Primer as well,
and it isn't going to help the understandability of our specs if we
appear to be inconsistent in how we talk about truly basic stuff like
the various parts of the graph (and whether things are "labels" or not).
  I don't care if the difference is trivial in "substantive content",
the mere *appearance* of a difference creates a problem.  I think having
such differences ought to be rigorously avoided, unless it's absolutely
God-awful necessary.  In case you're wondering, the Primer currently
uses the description from the "older" version of the model theory;  but
I'll be happy to change it to gain consistency, once I make sure what I
need to be consistent with!  (my default is the model theory)

> In the very first version of the model theory, the RDF graph was
> as having nodes some of which had labels, and the labels were URI refs or
> strings.
> In the most recent version, some of the nodes are URI refs and some of
> nodes are strings (and none of the nodes are labelled).

A couple of minor questions about this:

a.  if, in the current model theory, some nodes *are* URIrefs, rather
than being *labeled with* URIrefs, why shouldn't arcs *be* URIrefs,
rather than being *labeled with* URI refs?

b.  if some nodes are URIrefs and some are strings (and we say nothing
about them being labeled any more), why are we so insistent to say that
blank nodes are unlabeled, and that node identifiers are *not* labels?
Apparently, at the moment, no nodes have labels.  (This node identifier
business, by the way, is one of the things I'm trying to explain in the
Primer that I don't think is helped by inconsistency among the documents).


> In the new document draft, we have reverted to the earlier version using
> explicit node labels.
> The motivations for this include:
> - tidying a graph is an explicit operation rather than implicit by
> mathematical construction
> - it easier to modify the exact tidyness specification (if the WG changes
> its mind about whether literals are tidy or not - uriref nodes don't seem
> doubt).
> - implementations will almost all use nodes with explicit labels.
> The first point is the decisive one. From a mathematical point of view
> latest model theory treatment in which the URI refs and strings *are* the
> nodes is extremely elegant. Tidiness just falls out and no text needs to
> spent on it.
> However, in practice any implementor needs to be aware of tidiness as an
> operation which has to be coded, and any user needs some understanding of
> tidiness. Thus, I felt that Pat's earlier treatment, while a little more
> clunky, is clearer for our intended audience: implementators, users, web
> architects - rather than mathematicians.
> Although I raise this issue now, I would hope that any debate, if debate
> needed, can be postponed until after the first WD. I don't think this is
> crucial issue either way.

Well, I think debate is needed, not so much about which explanation is
better, but about whether we ought to use one, and not two (or, heaven
help us, more than two!).  And I think *this* debate ought to happen
ASAP.  It seems to me that if using the basic concepts from the model
theory causes problems for non-mathematicians, we need to explain things
better, not introduce apparent inconsistencies about whether nodes *are*
URIs or are labeled by them (and I suspect that the non-mathematicians
will be the first to notice, and have a problem with such inconsistencies).


PS:  You guys did say this new document was going to *help* with
something, right?  :-)

Frank Manola                   The MITRE Corporation
202 Burlington Road, MS A345   Bedford, MA 01730-1420
mailto:fmanola@mitre.org       voice: 781-271-8147   FAX: 781-271-875
Received on Tuesday, 30 July 2002 10:11:35 UTC

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