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

From: Clark C. Evans <cce@clarkevans.com>
Date: Mon, 29 May 2000 19:15:09 -0400 (EDT)
To: michaelm@netsol.com
cc: abrahams@acm.org, xml-uri@w3.org
Message-ID: <Pine.LNX.4.10.10005291853340.21852-100000@clarkevans.com>
On Mon, 29 May 2000, Michael Mealling wrote:
> Does urn:isbn:1-56592-487-9 name or locate?
> Before you answer that be aware that there is a standardized process for
> taking that URN, looking it up in an authoritative database and getting
> the exact location of an electronic copy of the resource it names.
> Now, does it name or does it locate?

Our problem isn't this exactly.  I believe the fundamental
question becomes:

  "Is there an injective function from the 
   identifier space onto the resource space?"

Why do we need this injective quality?  Beacuse in our
domain (namespaces), we wish the following conditional 
to be a tautology:

  "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"


The problem with URLs is that, in general, they
lack this injective quality.  In other words, 
I can find N distinct URLs that identify
(resolve to) the same resource.

I am reading into your example above this
injective quality.  Am I correct? 




Just for reference, the "functional" quality gives our
domain this tautology:

  "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."

And, for reference, the "surjective" quality of
the mapping described gives our domain this property:

  "For any resource X, there is at least one
   name X' such that X' identifies X."
Received on Monday, 29 May 2000 19:11:01 UTC

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