W3C home > Mailing lists > Public > www-rdf-interest@w3.org > November 2002

Re: why arcs never be merged?

From: Graham Klyne <GK@ninebynine.org>
Date: Fri, 29 Nov 2002 12:26:27 +0000
Message-Id: <>
To: jeffzhang726@yahoo.com.cn
Cc: "www-rdf-interest@w3.org" <www-rdf-interest@w3.org>

At 12:02 PM 11/29/02 -0800, JeffZhang wrote:

>Dear all,
>   When two graphs merged, arcs are never merged,why? What the difference 
> between a single
>assertion and duplicated assertions?

Do you mean the distinction between these graphs:

    ex:foo ex:property ex:bar .


    ex:foo ex:property ex:bar .
    ex:foo ex:property ex:bar .


Semantically, they are the same in that each entails the other.  How they 
are treated syntactically is rather up to an application.

Or do you mean:

    ex:foo ex:property ex:bar .
    ex:foo ex:property ex:bif .


These are two distinct statements -- what would it mean to merge the 
arcs?  Each arc is, in effect, the combination of subject, object and 
predicate:  no arc exists separately from a statement.  The URI that labels 
an arc may, however, be a separate node in the graph, and the presence of 
multiple arcs with a given URI don't mean that the URI node can appear 
multiple times.

A slightly different way of looking at this is that the property URI is a 
kind of type label, and an arc is an instance of that type.

>   The literals,which were not merged in previous specificaions, are 
> merged in this version.

That's down to a semantic change in the nature of literals, a big 
discussion quite different from the matter of arcs.

>Would the same change happen to arcs?

There is no discussion that I've heard that calls to change the role of arcs.


Graham Klyne
Received on Friday, 29 November 2002 07:50:44 UTC

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