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Re: namedNode? in predicate position?

From: Graham Klyne <Graham.Klyne@Baltimore.com>
Date: Wed, 05 Sep 2001 11:23:01 +0100
Message-Id: <5.1.0.14.2.20010905110757.039b0ae0@joy.songbird.com>
To: pat hayes <phayes@ai.uwf.edu>
Cc: Aaron Swartz <aswartz@upclink.com>, w3c-rdfcore-wg@w3.org
At 12:13 PM 9/4/01 -0700, pat hayes wrote:
>>I think that princeNodes should be allowed in the predicate slot... 
>>they're rather useful.
>>
>>While I'm giving unsolicited advice, I think that a DLG is probably not 
>>the best way to represent the RDF model... it's sort of difficult to 
>>picture arrows pointing at other arrows.
>
>If *that* is allowed then the M&S is totally screwed up. Is it legal RDF 
>to have a property be the value of another property??

This is something that I have found intuitively "easy" but difficult to 
explain clearly.

My intuition is that an arc is somehow associated with a property node, but 
that different arcs with the same property label are still different 
arcs.  (The programmer in me might say that an arc is an instance of a type 
described by the property.)  But this is inadequate as a formal explanation...

Turning to the MT document for guidance (28-Aug-01 draft, section 0, 
penultimate para).  We have one node for each uriref, anonNode or qLiteral 
in the document, and one edge for each triple.  But what about the urirefs 
that are used to identify properties?  This seems to be a problem with the 
description as given.

It seems to me that there is a difference in the treatment of URIs used to 
label nodes and URIs used to label arcs, but I'm not sure how that is 
formalized.  I used to think of reification as providing a way to create an 
effect that might have a graph syntax of an arc pointing to/from another arc.

In any case, I think it's certainly possible to have the uri that labels a 
property arc also be used to label a node that is the subject and/or object 
of another property, which some might say is "to have a property be the 
value of another property".

So my points are:

(1) I think the MT text about URIs in graphs may need refining.

(2) My answer to your question is "maybe, yes".

...

Should arcs be allowed to be "princeNodes"?

I note that allowing property arcs to be "princeNodes" is not the same as 
having ordinary nodes be princeNodes.  A URI denotes a node resource, but 
it is an attribute of a property arc.  How does a formalization capture 
this kind of distinction?

I have also found that to describe RDF-schema-based inferences, I have 
wanted to construct expressions in which the RDF property is a 
"variable".  I think the required effect could be achieved by means other 
than "princeNode" arcs, but maybe less intuitive for a developer.

(Hmmm... are we finding out why CGs use bipartite graphs rather than 
labelled arcs?)

#g


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Graham Klyne                    Baltimore Technologies
Strategic Research              Content Security Group
<Graham.Klyne@Baltimore.com>    <http://www.mimesweeper.com>
                                 <http://www.baltimore.com>
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Received on Wednesday, 5 September 2001 07:06:38 EDT

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