W3C home > Mailing lists > Public > uri@w3.org > April 2001

Re: Proposal: 'tag' URIs

From: Sandro Hawke <sandro@w3.org>
Date: Sat, 28 Apr 2001 05:22:14 -0400
Message-Id: <200104280922.FAA01662@hawke.org>
To: michaelm@netsol.com
cc: Tim Kindberg <timothy@hpl.hp.com>, uri@w3.org

You're preaching to the choir saying "URIs can identify anything."

The question is what can URIs do that URNs can not.  Since URNs
supposedly have additional guarantees/constraints, there must be some 
difference.  But you seem to be arguing that URNs can identify
anything, too.

If you can have a URN for "the news at the time you read it," as you
say, then you can certainly have a URN for "the stuff available via
HTTP port 80 at cnn.com, at the time you ask."   So that means HTTP
URLs are also URNs.

   -- sandro

(rest of message unedited)

> On Fri, Apr 27, 2001 at 10:45:24PM -0400, Sandro Hawke wrote:
> > > > 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".
> 
> Its persistently bound to the Resource it identifies. That Resource
> may be a dynamically changing object like CNN or a report that contains
> whatever the current weather is. URIs identify abstract concepts that
> may (or may not) have some physical or network representation. In this
> case the abstract concept is whatever the weather is at any given time.
> The Resource can have a time component. Heck, the Resource can even
> cease to exist, but the URN is still bound to the Resource even if it
> doesn't exist anymore...
> 
> > 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?  
> 
> Yes. But it can do both...
> 
> > Can [0-9]* denote a negative number?  "Yes" it can denote a 
> > negative number's additive inverse.  Which is really "No".
> 
> You're confusing the identifier with the thing you are identifying and,
> in the case where the thing you are identifying is a container, with
> the thing that is in the container. All three can have their own
> seperate identifier if that's what you need...
> 
> > 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.
> 
> It is a real issue since it comes up very often. The first hurdle we
> have to get over is that there is some kind of limitation to what
> URIs or URNs can identify. There isn't. Any URI can be bound to anything
> in the conceivable universe, which includes the concepts of nothingness
> and also the identifier itself. I can assign a URI to ever single character
> in an email message, and then assign one to each word grouping, to each
> paragraph, to the whole thing, to the idea of the message, to the sender,
> the recipient, the recipients pet poodle, etc.
> 
> > 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.) 
> 
> And URIs don't adhere to Newtonian Mechanics. They're to simple. They simple
> identify and there is no limitation to what they can identify and why.
> Or, to actually use your notation, @@C is a URI because you are simply
> saying that @@C is a new variable that represents that function (there's
> a mathematical term for that but I can't remember it). Thus saying @@C +
> @@D would be adding two equations. This is what URIs do inherently. They're
> simply variable names. There are certain classes of these variable names
> that you can do things with on the Internet and find out what they
> currently identify but that's not required in order to just use it the
> way you are here...
> 
> > > > (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 a
> n
> > > 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". 
> 
> Nope. What is persistent is that you can always call use @@X and we'll
> always understand that you always mean x(t), not that x(t) is persistent.
> The persistence semantic of URNs has everything to do with the identifier
> and absolutely nothing to do with the persistence of the thing its identifyin
> g.
> 
> > 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".
> 
> Yes it is. Again, persistence with respect to URNs has nothing to do with
> the Resource and everything to do with the identifier....
> 
> > 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.
> 
> Again, you can assign a URI to any conceivable part of that process. Its
> just a variable....
> 
> -MM
> 
> -- 
> -----------------------------------------------------------------------------
> ---
> Michael Mealling	|      Vote Libertarian!       | www.rwhois.net/michael
> Sr. Research Engineer   |   www.ga.lp.org/gwinnett     | ICQ#:         141988
> 21
> Network Solutions	|          www.lp.org          |  michaelm@netsol.com
Received on Saturday, 28 April 2001 05:24:46 UTC

This archive was generated by hypermail 2.3.1 : Tuesday, 6 January 2015 21:25:02 UTC