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RE: Are duplicate property/value pairs permitted for a resource?

From: Ralph R. Swick <swick@w3.org>
Date: Thu, 08 Apr 1999 15:25:25 -0400
Message-Id: <3.0.5.32.19990408152525.02bf6450@127.0.0.1>
To: Samuel Yang <syang@peoplemoverinc.com>
Cc: <www-rdf-comments@w3.org>, <rdf-dev@mailbase.ac.uk>
At 11:55 AM 4/8/99 -0700, Samuel Yang wrote:
http://lists.w3.org/Archives/Public/www-rdf-comments/1999AprJun/0011

(Thanks to Dan Brickley for fielding this question first.)

>  Two resources with the same URI always refer to the same
>resource.

yes.  This doesn't mean the resource content won't change from
moment-to-moment, of course.

>  But literals, not having URIs, have apparently been left bereft
>of a formal definition of equality.

yes.  This is not an oversight.  The question of identifying
equality in XML text is outside the scope of RDF.

>  However, I don't see any reason not to
>consider two identical literals to be equal.  For instance, under what
>scenarios does "2" not equal "2"?

It does get tricky.  Mostly it depends on your application.
Consider, for example, the following two fragments of XML text:

1:
<span xml:lang="en">2</span>

2:
<span xml:lang="fr">2</span>

Are these the same?  What techniques would your application(s) use
to test equality?  Would those techniques work in the following
two cases:

3:
<rdf:RDF xml:lang="en"
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
  xmlns:dc="http://purl.org/metadata/dublin_core#">
 <rdf:Description about="http://www.dlib.org">
  <dc:Date>1995-01-07</dc:Date>
 </rdf:Description>
</rdf:RDF>

4:
<rdf:RDF xml:lang="fr"
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
  xmlns:dc="http://purl.org/metadata/dublin_core#">
 <rdf:Description about="http://www.dlib.org">
  <dc:Date>1995-01-07</dc:Date>
 </rdf:Description>
</rdf:RDF>

>Practically speaking, it would be a bad idea to keep adding "duplicate"
>statements having the same literal objects,

Indeed it would.  If your application cannot distinguish the literals
then you just have a garbage collection problem.  How you handle it
is mostly up to you :-)

>(3) Delete all the "duplicate" statements (again, how can it do that when
>these statements are not considered equal?)?

Whatever the implementation does, it should present the external
appearance that there is only one instance of any given triple
{x,y,z} for the "same" values of x, y, and z.  Sameness is the
critical question, which the RDF/XML syntax can't yet answer.

I hope (and expect) that either the XML Fragment Working Group
or the XML Information Set Working Group will be able to clarify
this question for the specific syntax in time.

-Ralph Swick
 W3C/MIT
Received on Thursday, 8 April 1999 15:25:12 GMT

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