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Re: Anonymous resource names -versus- variables

From: Dan Connolly <connolly@w3.org>
Date: Wed, 03 May 2000 17:01:27 -0500
Message-ID: <3910A1B7.31BCB9A0@w3.org>
To: www-rdf-interest@w3.org

Dan Brickley wrote:
> 
> Jan W writes...
> >       http://www.swi.psy.uva.nl/projects/SWI-Prolog/packages/sgml/online.html
> >
> > P.s.s.        What about a standard or at least reserved naming schema for
> >         anonymous resources?  I'm now using <Type>__<N>, with is nice
> >       for debugging. We also see genid<N>, but none of this is
> >       reserved!
> 
> I've been thinking about this a fair bit lately.

Me too. My thoughts haven't jelled, but see the
"<!-- @@hmm... what subject to use for an anonymous node?" comment
in
http://www.w3.org/XML/2000/04rdf-parse/rdfp.xsl
$Id: rdfp.xsl,v 1.4 2000/04/29 06:44:12 connolly Exp $


I was inspired when somebody (TimBL, I think) said that RDF already
has existential quantification (in addition to conjunctions:
multiple properties are generally considered to be 'and'ed together).

So...


> If I write (assume some namespace declarations with the RDF namespace
> imported as 'web:', and that top level is a typedNode)...
> 
> <Wordnet:Photo>
>  <image:depicts>
>         <Wordnet:Person>
>         <abc:personalMbox web:resource="mailto:danbri@w3.org"/>
>         </Wordnet:Person>
>  </image:depicts>
>  <ecom:buyOnline web:resource="http://example.com/shoppingcart/checkout?items=photo323423"/>
>  <dc:description>...etc</dc:description>
> </Wordnet:Photo>
> 
> In this serialized RDF, I'm mentioning some resource of rdf:type

Exactly! "For some resource of rdf:type..." is, by another
name, \exists X ...


> My strawman suggestion is (a) that we adopt a convention of using URIs of
> the form var:2342534647647476456456 for such situations, and (b) that such
> identifiers are serving as named placeholders much like variables, and
> that this analogy might be worth exploring further.

These variables should be seen as skolem constants (i.e.
0-adic skolem functions). If you
have any uinversally quantified variables surrounding the
anonymous nodes, you have to capture those as arguments to the
skolem function.

"When an existential quantifier is being eliminated from a fact that
contains free variables, or when the existential quantifier occurs in
the
scope of a universal quantifier, the quantified variable must be
replaced by a term involving a Skolem function applied to the free
and universally quantified variables. For example, the fact \A x (x <
bigger(x)) can be obtained by eliminating the existential quantifier
from \A x \E y (x < y) and replacing y by bigger(x). See the fix
command and the generalization proof method. 
"
	-- LP, the Larch Prover -- Skolem constants and functions
	http://www.sds.lcs.mit.edu/spd/larch/LP/glossary/skolem.html


(courtesy of google...
http://www.google.com/search?q=skolem+existential)

-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Wednesday, 3 May 2000 18:01:32 GMT

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