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Re: Anonymous resource names -versus- variables

From: Sergey Melnik <melnik@db.stanford.edu>
Date: Thu, 04 May 2000 11:58:10 -0700
Message-ID: <3911C842.A6F601CE@db.stanford.edu>
To: guha@guha.com
CC: Dan Brickley <danbri@w3.org>, Jan Wielemaker <jan@swi.psy.uva.nl>, Dave Reynolds <der@hplb.hpl.hp.com>, www-rdf-interest@w3.org
Guha wrote:
> 
> Yes, Anonymity is not a property of the object, but a property
> of the description of the object. The situation is similar to
> indexicals. I can use "he" to refer to Dan, but that does not mean
> Dan is of type "he" or "indexical".
> 
> guha

Guha, Dan,

although you are making a valid point, it seems that we are approaching
the edge of metaphysics and axiomatizability of RDF. I tend to view RDF
as a logical assembler language (which you can still associate semantics
with), rather than the expression of the semantics itself.

You are arguing that Anonymity is not a property of the object. Is being
an instance of a class really a property of the object? Furthermore, if
you are concerned about the "correctness" of RDF, have a look at
rdf:Seq. Can we seriously claim that, for example, an instance of
rdf:Seq can be a creator of a resource?

To address typing and order, "semantic transparency" wrt rdf:type,
rdf:Seq and ordinals is required. Those can be viewed as building blocks
of a low-level (structural) language.

BTW, as simple as RDF looks like, it can still be split into two layers:

1) Object identity + binary relationships (no predefined vocabulary!)
2) Basic typing (rdf:type), order (rdf:Seq, rdf:_1, etc), n-ary
relationships (currently missing)


Both approaches to anonymity (var:... and rdf:type AnonymousResource)
have differences from the operational (rather than theoretical)
viewpoint, too. Given a generic RDF repository it may be significantly
simpler to ask for (X, rdf:type, rdf:AnonymousResource) than to look at
the syntactic representation of IDs which may require building an index
over strings.

Sergey


> 
> Dan Brickley wrote:
> 
> > On Thu, 4 May 2000, Sergey Melnik wrote:
> >
> > > Instead of associating semantics with URIs one could state it
> > > explicitly:
> > >
> > >       book1 --author--> var:....
> > >
> > > would become
> > >
> > >       book1        --author-->   whatever_URI
> > >       whatever_URI --rdf:type--> rdf:Anonymous
> > >
> >
> > Interesting approach, but I don't think that quite works. We really
> > shouldn't be calling these "anonymous resources" as that leads us to
> > think that the resource itself is intrinsically nameless. That habit leads
> > us to solutions such as yours, which suggest that the anonymity is a
> > property of the resource, rather than of some mention or description of
> > the resource. Since I don't think anonymity is a property of the resource,
> > your proposal expresses exactly what I'm disagreeing with!
> >
> > I argued a similar point last month:
> > http://lists.w3.org/Archives/Public/www-rdf-interest/2000Apr/0022.html
> >
> > excerpt:
> > > Sometimes we don't know the commonly agreed ID for some entity, but
> > > nevertheless write some RDF statements that mention it in passing. When
> > > we send someone an RDF model and lack such URIs (sometimes because of
> > > partial knowledge; sometimes because social process
> > > doesn't exist to give uncontroversial URIs to these entitites) we still
> > > need to represent those entities in some kind of data structure.
> > >
> > > So these are the so-called 'anonymous resources'. I really think this is a
> > > bad name; it suggests that the anonymity is in some way intrinsic to the
> > > resource. That's not the case: rather, some 'mention' of that resource is
> > > anonymous in that we happen not to name the resource on that occasion.
> >
> > I agree with you that associating semantics with special URI schemes for
> > use with RDF isn't a great solution, but feel the analogy with variables
> > makes this worthy of some investigation.
> >
> > RDF's world consists solely of Web-nameable resources (things for which
> > the notion of identity makes sense, and which may/might/can have URIs) and
> > un-nameable (intrinsically anonymous??) literal strings. RDF applications
> > that express queries, inference rules etc. typically add to this some
> > notion of a variable. eg. TimBL's DesignIssues work
> > (eg. http://www.w3.org/DesignIssues/Toolbox.html) makes a stab at doing
> > this.
> >
> > Anyone representing rules/queries over RDF content (and hence
> > using variables) needs to decide whether those data structures will
> > themselves have a representation in the RDF 1.0 Model, or whether they'll be
> > inexpressible in the core model and be represented in a superset of RDF
> > 1.0 that has an explicit notion of variables. As Tim says in the Toolbox
> > doc. (discussing NOT, rather than variables, but general point holds)...
> >
> >         'As RDF has little power at its basic level, anything new has to be
> >         introduced by reification - by describing it in RDF. Hence, to say
> >         "not(node, property, value)", you have to say, for example, "there is
> >         something which is an RDF property and has a subject of A and whose B
> >         property has vale C and is false". '
> >
> > So if I want to represent variables within an RDF context, we either
> > find a hack that appeals (eg. "var:" IDs) or admit that certain things are
> > not elegantly represented in base RDF (although of course they can be
> > "tunnelled through" the RDF triples model, ie. quoted
> > but not understood by most 1.0 processors).
> >
> > I've seen quite a few systems tunnel rules (containing variables) over
> > RDF, eg. Jos De Roo's work (ftp://windsor.agfa.be/outgoing/RCEI/NET/euler/index.html)
> > which lead me to suggest that the anonymous node problem and the
> > tunnelling-data-containing-variables-through-RDF problem are two views of
> > the same issue. In both cases, we need to represent to machines the fact
> > that we don't know the Web names for various entities, but nevertheless
> > know some other things about them. Whether the definition of URI/Resource
> > is broad enough to be used for variables is an interesting
> > (religious?) question. Reading RFC2396 I'm inclined to suggest that this
> > is acceptable:
> >
> > Excerpted from related thread at
> > http://lists.w3.org/Archives/Public/www-rdf-interest/2000Mar/0028.html
> > the RFC tells us...
> >
> > i)      A Uniform Resource Identifier (URI) is a compact string of characters
> >         for identifying an abstract or physical resource.
> >
> > ii)     A resource can be anything that has identity.
> >
> > and in particular this rather cryptic paragraph:
> >
> > iii)
> >
> >          The resource is the conceptual mapping to an entity or set of
> >          entities, not necessarily the entity which corresponds to that
> >          mapping at any particular instance in time.  Thus, a resource
> >          can remain constant even when its content---the entities to
> >          which it currently corresponds---changes over time, provided
> >          that the conceptual mapping is not changed in the process.
> >
> > Now this in particular sounds to me a lot like our current problem...
> >
> > Dan
> >
> > --
> > mailto:danbri@w3.org
Received on Thursday, 4 May 2000 14:47:37 GMT

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