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Re: Clarifying what a URL identifies (Four Uses of a URL)

From: David Booth <dbooth@w3.org>
Date: Tue, 21 Jan 2003 17:40:05 -0500
Message-Id: <>
To: Tim Bray <tbray@textuality.com>
Cc: Michael Mealling <michael@neonym.net>, www-tag@w3.org, "Roy T. Fielding" <fielding@apache.org>, Dan Connolly <connolly@w3.org>, Sandro Hawke <sandro@w3.org>

At 10:59 AM 1/21/2003 -0800, Tim Bray wrote: [1]
 > The Web Architecture has no built-in way to talk about what a Resource 
"is", and seems to get by just fine.

It isn't a problem for the human-oriented Web; it may be a problem for the 
Semantic Web, depending on which approach you take (what I call "different 
names"[2] or "different context" [3]).

At 01:01 PM 1/21/2003 -0800, Tim Bray wrote: [4]
>. . . .  The Web Architecture has a formalism called a "Resource" which is 
>the one thing that corresponds to each URI.  . . . .

I find the word "resource" to be ambiguous.  I understand what I mean by 
saying that a URI denotes a "name" or a "concept" or a "Web location" or a 
"document instance", but I don't understand what it means to say that a URI 
denotes a "resource".

If a URI denotes only one thing (called a "resource"),  then which of those 
four things (name, concept, Web location, or document instance) does 
"http://x.org/love" denote?  If your answer is "it depends", then it seems 
to me that the meaning of a URI is determined by context.  In that case, 
the statement that "a URI corresponds to one resource" seems no more 
helpful than saying "a URI corresponds to one URI".  I.e., it doesn't give 
me any greater understanding of the situation.

On the other hand, if you say that your notion of "resource" always 
corresponds to my notion of "concept" (for example), then I think I 

URLs *are* used in conjunction with denoting (at least) four kinds of 
things.  That's reality.  I say "in conjunction with" because the question 
of whether URLs can *directly* denote more than one of these four kinds of 
things depends on your viewpoint.  If you take the "different names for 
different uses" approach that I described, then a URL denotes only one of 
these four things, and the TAG had better clarify which one it is!  On the 
other hand, if you take the "different context for different uses" 
approach, then the context indicates which of the four things is denoted, 
and the TAG does not necessarily have to say how that context should be 
indicated.  (Though it would be helpful to have standard conventions.)

As far as I can tell, either approach can work for the Semantic Web 
(provided every Semantic Web language clearly indicates how such context 
should be indicated).  Furthermore, the "different context" approach seems 
to subsume the "different names" approach, in the sense that the "different 
names"[2] approach requires everyone to agree to that approach, whereas the 
"different context"[3] approach does not, which is why I favor it thus far.

[1] http://lists.w3.org/Archives/Public/www-tag/2003Jan/0240.html
[4] http://lists.w3.org/Archives/Public/www-tag/2003Jan/0246.html

David Booth
W3C Fellow / Hewlett-Packard
Telephone: +1.617.253.1273
Received on Tuesday, 21 January 2003 17:40:23 UTC

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