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   re: Why namespaces?

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  • From: David Megginson <david@megginson.com>
  • To: XML-Dev Mailing list <xml-dev@ic.ac.uk>
  • Date: Tue, 31 Aug 1999 14:27:04 -0400 (EDT)

Paul Prescod writes:

 > David Megginson wrote:
 > > 
 > > Sure -- that's why you have a DOCTYPE declaration at the top, so that
 > > validating processors can check the HTML against a DTD.
 > This raises a major question about the utility of namespaces in a
 > vocabulary that cannot be mixed with other vocabularies. What does
 > the namespace add that you can't get from the doctype?

Who said that they cannot be mixed?  XHTML documents cannot (yet)
contain markup from other Namespaces, but anyone else designing a
document type is free to include HTML markup in theirs (say, for
documentation in a schema).

So, the answer is that Namespaces give us non-local identity.  With
Namespaces (or Architectural Forms, or something similar) I can look
at an arbitrary document and answer questions like

  "Is this an HTML <cite> element?"

An ability to answer this kind of question is the minimal foundation
for processing, searching, and querying large heterogenous sets of XML
documents (as on the Web, for example).  Without it, we're sunk.

In fact, without a global naming scheme, I cannot even reliably answer
the following question for an arbitrary XML document (say, text/xml):

  "Is this an HTML document?"

The DOCTYPE declaration doesn't help at all here, as Eliot has
repeatedly (and convincingly) pointed out -- DTDs are for
guided-authoring and for validation, not for recognition.

All the best,


David Megginson                 david@megginson.com

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