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Re: Determining what a URI identifies

From: Tim Berners-Lee <timbl@w3.org>
Date: Wed, 11 Dec 2002 22:53:21 -0500
Cc: www-tag@w3.org
To: Paul Prescod <paul@prescod.net>
Message-Id: <41FF88B1-0D85-11D7-8A22-000393914268@w3.org>


On Monday, Nov 4, 2002, at 15:21 US/Eastern, Paul Prescod wrote:

> Tim Berners-Lee wrote:
>> ...
>> So when RDF talks about  <#joe> as having
>> a contact:mailbox of  <mailto:joe@example.com>
>> an RDF processor which is aware of the URI spec
>> and the spec of mailto:  knows that the object
>> is an email mailbox according to the email specs.
>
> You want the processor to infer the type of an object from the syntax 
> of its URI? What happened to opacity of URIs?
>

Good point!

Its a great general rule not to look inside a URI when you can
do what you can without doing so.

However, the opacity gradually reveals more and more information
as more and more specifications are used, one  after the other
in the chain of normative reference which starts with the URI spec.

The principle of opacity suggests that you do not
in an application put constraints (or interpretations)
on the stuff in a URI or you will limit the other specs it can be
used with and thus not leverage the whole power of the web.

Its important, for example, not to assume that anything whose
URI ends "html" is an HTML document - it may had an md5
URI - or some as yet uninvented URI scheme, or the server may
use the last few characters for something else.

So principle of opacity does not say "the characters in a URI
are quite arbitrary, put anything there."  You can't use
a mailto: URI to identify anything other than an RFC822 mailbox
because the specs say how mailboxes are identified by these
address@domain.things and how these can be used as
URIs with the "mailto:" prefix.

Tim

>  Paul Prescod
Received on Wednesday, 11 December 2002 22:53:10 GMT

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