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Re: Proposal: 'tag' URIs

From: Sandro Hawke <sandro@w3.org>
Date: Fri, 27 Apr 2001 22:45:24 -0400
Message-Id: <200104280245.WAA01208@hawke.org>
To: michaelm@netsol.com
cc: Tim Kindberg <timothy@hpl.hp.com>, uri@w3.org, sandro@w3.org

> > Hrm.  What if we want a term
> > (tag:sandro@w3.org/1-4-27/you) which, when used in a message, denotes
> > the recipient of the message.  The term denotes a different recipient
> > if the same message is transmitted again, to someone else.  That can
> > be a tag URI.  Can it be a URN?  
> 
> Sure. In this case the URN (or tag) is denoting the recipient container.
> What's actually in that container can be different from second to second.
> Its the old concept of an identifier that identifies the weather that
> currently exists 'today', an identifier that identifies the conditions on
> 4/27/2001 and will always do so, and another that identifies the weather map.

I'm trying to understand how the first kind of weather identifier fits
in with the definition of URN persistence.  The denotation of "the
weather that currently exists 'today'" depends on the time at which
the term is interpretted.  That doesn't sound like a URN.  It's no
more persistent than "http://www.cnn.com".

I asked if you can use a URN to denote the recipient, and you said
"sure", but then you said that it could denote the recipient
*container*.  Isn't that a different thing?  Can [0-9]* denote a
negative number?  "Yes" it can denote a negative number's additive
inverse.  Which is really "No".

I'm not trying to be difficult here.  Usually arguing semantics is the
last refuge of someone who's really upset about something else,
but I'm pretty sure this is a real issue.

In Newtonian mechanics (as I remember it from long ago), if you want
to represent the position of an object which can move along a line,
you can do it like x(t).  The value of "x(t)" itself varies as time (t)
varies, but x (the function), like every other "variable" in
mathematics, does not actually change with time.  If you ever say
x(t)=c you're saying that the object can never move.   But I want to
be able to say x(t)=@@C, where @@C is a true variable, a shorthand for
x(t).    (I just picked @@ because I don't think it's anywhere in
mathematics, and this doesn't belong in mathematics.) 

> > (There's a weirdness here between denoting the recipient and denoting
> > the concept of denoting the recipient.   I'm trying to do the former;
> > URNs could clearly do the latter, which might be sufficient for all
> > applications.)
> 
> Its sufficient because everything, no matter how abstract, can be given an
> identifier, even another identifier. This is what makes things like RDF
> so interesting...

And the reason I want to say x(t)=@@C is because of RDF's "triple"
model, which means I can't use a structure like x(t) as an identifier
(as I could in Lisp, Prolog, etc); I need to use something like @@C.

If I want to say x(t)=y(t) in RDF, I say something like:
   x timevar @@X
   y timevar @@Y
   @@X equal @@Y
but if @@X and @@Y are URNs, then their denotation is "persistent".  I
need @@X and @@Y to vary over all places in space, so @@X might denote
Boston, MA at one moment and Reston, VA at another.  That doesn't
sound like URN "persistence".

I should work this through an example RDF representation of a web page
being edited (where I'll need identifiers for, say, the text of the
page which changes), but I've got to run.

    -- sandro 
Received on Friday, 27 April 2001 22:48:16 UTC

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