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

From: Graham Klyne <GK@Dial.pipex.com>
Date: Tue, 30 May 2000 20:15:17 +0100
Message-Id: <>
To: michaelm@netsol.com
Cc: Tim Berners-Lee <timbl@w3.org>, xml-uri@w3.org
At 02:27 PM 5/30/00 -0400, Michael Mealling wrote:
>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...

I considered that, but I find this position very hard to reconcile with the 
statements I was querying...

(a)  name X 'different' name Y =!=> resource X 'different' resource Y
(b)  URI:resource mapping is 1:1

I would have thought that if (b) were true,
then one would be entitled to conclude the converse of (a):
(c)  name X 'different' name Y ==> resource X 'different' resource Y

especially for the very strong notion of 'sameness' of resources that you 
describe, and assuming that
      same(A,B) <=> NOT( different(A,B) )

>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....

[And also the tv: scheme whose RFC appeared today ;-)]

It seems to me that these present just the same problem as a relative 
URI.  Why should they be treated any differently?


Graham Klyne
Received on Wednesday, 31 May 2000 10:23:06 UTC

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