W3C home > Mailing lists > Public > xml-uri@w3.org > May 2000

Re: Injective Quality (Was: Re: URIs quack like a duck)

From: Michael Mealling <michael@bailey.dscga.com>
Date: Tue, 30 May 2000 14:27:06 -0400
To: Graham Klyne <GK@Dial.pipex.com>
Cc: Tim Berners-Lee <timbl@w3.org>, xml-uri@w3.org
Message-ID: <20000530142706.J14789@bailey.dscga.com>
On Tue, May 30, 2000 at 06:08:10PM +0100, Graham Klyne wrote:
> At 09:50 AM 5/30/00 -0400, Tim Berners-Lee wrote:
> > >
> > >  "If the name X and the name Y are different,
> > >   then we know the resource identified by X is
> > >   different from the resource identified by Y"
> >
> >
> >I disagree. In fact, I would say that you can't make a system
> >which scales globally in a decentralized way with that
> >tautology.
> >
> >[[Two problems with vocabulary, by the way:
> >(1) My definition of a resource is that exactly identified by a URI and so
> >URIs and resources are in 1:1 mapping.
> 
> These two statements seem to be in contradiction:  if X and Y are different 
> names (URIs) that identify the same resource, then URIs and resources are 
> not in 1:1 mapping.  What am I missing?

Because it depends on how you define 'same'. The URI architecture
defines 'same' to be an equality function that is solely dependent
on what the URI tells you. I.e. its a universe where the URI is
the only datatype or semantic you are allowed to use. Its 1:1 because 'n' is 
specifically defined to not exist. I.e. a binding of a URI to its
resource is an identify function....

Now, if you include the entire human universe of 'sameness' such as bit
equality, copyright, version control, legal jurisdiction, intellectual
property, etc, then no, it isn't a 1:1 mapping. Its a 1:n mapping...

> > >  "If the name X and the name Y are the same,
> > >   then we know the resource identified by X and
> > >   the resource identified by Y are the same."
> >
> >
> >This we do have.
> 
> I have seen this asserted a few times, but keep on asking myself:  what 
> about the 'file:' URI scheme?  (I hear word of other schemes that may 
> similarly depend on local context for their interpretation.)

Correct. 'file' and 'news' both depend on a local context such as
"access to your local filesystem" or "access to your locally defined NNTP
server". These schemes define some out of band item that, combined with
the URI, allows for uniqueness. I.e. the tuple of "my locality of reference"
and the "file:/home/michael/.profile" URI combined gives me my
uniqueness and thus my ability to compare any X and Y in the space.
Yes, these locality dependent URIs cause problems from time to time.
Does it invalidate the entire space? IMHO, not really....

-MM

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Received on Tuesday, 30 May 2000 14:38:10 UTC

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