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Re: Feedback: #rdfms-identity-anon-resources

From: Frank Manola <fmanola@mitre.org>
Date: Thu, 12 Jul 2001 15:42:54 -0400
Message-ID: <3B4DFDBE.CC90A31A@mitre.org>
To: Graham Klyne <GK@ninebynine.org>
CC: RDF core WG <w3c-rdfcore-wg@w3.org>
Graham Klyne wrote:
> 
> My proposal has gratifyingly generated some responses.

I notice you didn't say "has generated some gratifying responses" :-)

snip
> 
> In summary, the arguments _for_ anonymous resources in the model/abstract
> syntax are:
> (a) difficulty of creating globally unique genids
> (b) capturing semantics needed to express queries
> 
> Are there more?

Actually, I think the main (and fundamental) argument for anonymous
resources in the model/abstract syntax is to support, in the
model/abstract syntax, the capability described in the current M&S for
talking about resources that don't have URIs (that you know of),
e.g.(P41):

"The individual whose name is Ora Lassila, email <lassila@w3.org>, is
the creator of http://www.w3.org/Home/Lassila." 

the graph for which shows a resource *without a URI*.

That is, the current M&S says we can express this in RDF.  I interpret
this as meaning that the model/abstract syntax must be capable of
expressing this, not just the XML serialization.  It seems to me, then,
that the choice is whether to (a) explicitly support anonymous resources
as something other than resources with generated URIs;  (b) clarify this
part of the spec by saying we really have to generate a URI in this case
(and have the graph show a *generated* URI, not *no* URI);  (c) delete
this capability from the spec.  This isn't strictly about either (a) the
difficulty of creating globally unique genids (although this may be an
obstacle to doing my case (b) above) or (b) capturing semantics needed
to express queries.  [Note:  my case (a) doesn't preclude generating
internal identifiers;  it just precludes them being considered the same
as URIs (e.g., they might only have local scope);  if we're going to
insist that they be generated *URIs*, that's my case (b).]

I agree (and have said so before) with a translation into logic (which
Dan has referenced) that views these as existentially quantified
variables, i.e., the above example becomes:

"There exists a [resource] x such that name(x, Ora Lassila) and
email(x,lassila@w3.org) and creator(http://www.w3.org/Home/Lassila, x)" 

[I *don't* agree that this has anything to do with queries;  the
interpretation of an existentially quantified variable in a logical
assertion, as above, isn't the same as the interpretation of a variable
to be bound in a query expression, in my opinion].

> 
> I find myself wondering if support for some kind of scoped identifier
> wouldn't address both of the goals raised.  This would be in contrast to
> URIs which are, by design, globally scoped and bound to specific
> resources.  I don't know if the semantics could play out simply yet
> sufficiently flexible to handle these goals.  I think this, too, would be a
> clear extension to M&S.

I think this reference to scope is an important point, since variables
in logic aren't usually interpreted as having universal scope (or as
being quite the same as explicit names).  If we take the logical
interpretation

"There exists a [resource] x such that name(x, Ora Lassila) and
email(x,lassila@w3.org) and creator(http://www.w3.org/Home/Lassila, x)" 

and look at just the variable "x", it certainly isn't intended to have
any universal scope (as a URI is intended to have).  You can easily
imagine several people independently making the same assertion, but
using different variables.  It happens in logic all the time (e.g., in
different knowledge bases).  From the variables alone, it wouldn't be a
good idea to draw any conclusions about whether these people were
talking about the same thing, or different things (as you would need to
do in merging the RDF from these different sources).  You'd have to make
that judgement on other grounds.  

(It might help to consider a more ambiguous example:

"There exists a [resource] x such that name(x, John Smith) and age(x,
24) and sex(x, male)"

Suppose you saw this at two different Web sites.  Would you be very
certain they were talking about the same guy?  Suppose you saw this at
one Web site, and 

"There exists a [resource] y such that name(y, John Smith) and age(y,
24) and sex(y, male)"

at another.  Would you be very certain they were talking about
*different* guys?)

The variables in these cases can be considered as generated identifiers
(since in the original formulation of the RDF we didn't state them
explicitly), but they aren't URIs.  The case for a distinct
representation for anonymous variables is at least partly based on the
desire to distinguish between things that need have only local
identifiers and things that need to have globally unique identifiers.  

--Frank

-- 
Frank Manola                   The MITRE Corporation
202 Burlington Road, MS A345   Bedford, MA 01730-1420
mailto:fmanola@mitre.org       voice: 781-271-8147   FAX: 781-271-8752
Received on Thursday, 12 July 2001 15:44:02 EDT

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