W3C home > Mailing lists > Public > w3c-rdfcore-wg@w3.org > June 2001

Re: log:forSome (Was: Model-specific identity for anon resources, and its representation: A new issue?

From: Brian McBride <bwm@hplb.hpl.hp.com>
Date: Fri, 15 Jun 2001 19:46:30 +0100
Message-ID: <3B2A5806.FA92DC39@hplb.hpl.hp.com>
To: jos.deroo.jd@belgium.agfa.com
CC: Jan.Grant@bristol.ac.uk, w3c-rdfcore-wg@w3.org
Hi Jos,

With regard to the recent discussion with Jan, I think there might be
some miscommunication.  What I think than Jan is saying is that, given
the model:

  <http://descr1> <http:/prop> _:anon .
  <http://descr2> <http:/prop> _:anon .

it is not possible to express this exactly in the RDF/XML syntax.

The reason is that the only way this can be done is to assign an ID
or a URI to the anonymous resource.  That 'resource' then ceases to 
be anonymous.  In effect, it is no longer a variable.  Try 
translating the above n-triple example to RDF/XML and you will see 
what I mean.

Reading your message again, maybe I get an inkling of what you are
saying.  We are talking here about what is an anonymous resource.
We have been saying that it is an existentially qualified variable.
I have been understanding that to mean that it denotes some specific
resource with certain properties, i.e.

  _:anon <rdf:type> <rdf:Class> .

says there is some specific resource, _:anon, which has type class.
Is that what you would call a 'constant'?  Ah.  This is beginning
to make sense to me.  If it were a constant, then you wouldn't
mind at all that a URI had been assigned to it.   That would not
change its nature.

I think you may be saying that what I thought of as a variable
is not really a variable.  It does not bind to multiple values.
Instead it represents a single value (constant) though I don't
know what the value is.   Hmmm.  Now consider:

   5+x=1

Is x a variable?  No.  Its an unknown, until I solve the equation.
Have we been calling anon resources variables, when we should have
called them unknowns?

If what I previously thought of as a variable, is not really a
variable, how would you explain what a variable is, without
having to refer to concepts defined in a logic layer above
rdfcore?

This is really very interesting.

Have a good weekend.

Brian


jos.deroo.jd@belgium.agfa.com wrote:
> 
> > > I think/hack that both anonymous *terms* can be *unified*
> > > which is NOT the same as equality
> >
> > Then your hack only seems to works when we can unify anonymous
> > terms, or they have some sort of UniqueProperty attached. How do
> > we solve the general case?
> 
> There was some related discussion in the telecon
> this afternoon, and I was kind of unable to explain
> my point...(I'm really hopeless in that respect)
> 
> The thing about anonymous nodes is that they
> are ***variables***
> if they would be constants, we would be able
> to identify them with ***URI constants***
> Now they are actually existentially quantified
> variables, something like: there exists an _:a
> or (using existing vocab): this log:forSome _:a
> So I think we should say that *explicitly*
> is the testresults (and in the model theory)
> (the general case?)
> 
> --
> Jos De Roo, AGFA http://www.agfa.com/w3c/jdroo/
Received on Friday, 15 June 2001 14:48:13 EDT

This archive was generated by hypermail pre-2.1.9 : Wednesday, 3 September 2003 09:37:09 EDT