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Re: When are two URIs equivalent?

From: Eric van der Vlist <vdv@dyomedea.com>
Date: Tue, 23 May 2000 16:02:24 +0200
Message-ID: <392A8F70.60BD2670@dyomedea.com>
To: xml-uri@w3.org

Thanks for your explanation about the URIs these beasts are quite new to
me ;) 

Beyond the question from Josef, I think there is the issue of
"absolutizing" before comparison in "option 2" :

>> References "writ large". If an author writes a relative URI, applications
>> see it only in its absolutized form, after being combined with the base URI
>> in effect at that point in the document. Note that a single document may
>> have multiple base URIs, due to the use of external entities; the XBASE
>> proposal raises the same possibility.

If I understand it correctly, the "absolutization" algorithm would have
to be implemented by the XML tools at a level to be defined.

Since, Josef said :

>> I've seen many people assuming that we know and that it's only a
>> question whether to absolutize or not, but looking up RFC2396, I
>> find that "equivalence" depends on the schema used (section 6)
>> and rules for absolutizing URIs independent of their schema
>> (section 5). 

The question seems therefore to use a "scheme dependent" (and extensible
as new schemes can be added) algorithm embedded in the XML tools.

My 0,02 Euros.


Tim Berners-Lee wrote:
> -----Original Message-----
> From: Josef Dietl <josef@mozquito.com>
> To: Eric van der Vlist <vdv@dyomedea.com>
> Cc: Tim Berners-Lee <timbl@w3.org>; xml-uri@w3.org <xml-uri@w3.org>
> Date: Tuesday, May 23, 2000 6:03 AM
> Subject: When are two URIs equivalent?
> >Eric,
> >
> >ok, you caught me - I meant to say "scheme".
> >
> >Still: would somebody mind telling me when two URIs are the same?
> A URI is a (syntax constrained) string used to identify something.
> A resource is that which, being in general abstract, is identified by the
> URI.
> Two URIs are the same when they compare character for character.
> When two URIs are the same they identify the same resource.
> (NB. There are many cases in which the resources identified by two
> different URIs are the same. Software is not required to know
> all (or for xml well-formedness checking, any)  these cases. They include
> knowledge that the hex %nn encoding for non-reserved charecter
> is an arbitrary choice; the knowledge that if the scheme is HTTP or
> FTP then the domian name part is not case sensistive. They include
> information obtained from a name server returning a "Found" response
> to an HTTP request. They include metata gained from a third party.
> There is no defiitive list of these. Some of them are a function of the
> URI scheme.)
>                          u1 = u2  =>  R1 = R2
> but not the reverse implication does not hold.  On the left hand side,
> "=" means string equality; on the right hand side, "=" stands for
> equivalence for any operation at all.
> For example, u1 may be the absolutized URI from the namespace name
> of a namespace in an xml document, in which case R1 is the namespace.
> u2 may be the absolutized URi from an XSLT style sheet, and R2
> the namespace which h stylesheet is giving a particular style to.
> The name for a string returned in the body of a successful HTTP GET request
> is an HTTP "entity body".
> (Yes, the XML, HTTP and URI communities have to learn a certian amount
> of each others' jargon).
> Tim BL

Eric van der Vlist       Dyomedea                    http://dyomedea.com
http://xmlfr.org         http://4xt.org              http://ducotede.com
Received on Tuesday, 23 May 2000 10:01:11 UTC

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