[xml-dev] Nested Documents (was: XML 2.0)

[noah_mendelsohn@us.ibm.com]

One thread of this discussion seems to be:  how much can we trim out of 
XML 1.0 to wind up with something yet leaner, clearner and easier to use? 
The other seems to be: if we could add just a bit, such as allowing more 
than one root element, what would it be?   In that latter vein, I'm 
curious that nobody has discussed what I have always thought was one of 
the most questionnable compromises in XML:  it's a hierarchical format 
that can't contain nested instances of itself.  Given XML's SGML roots as 
a system for coding individual documents that may have been sensible, but 
we now see lots of cases in which XML is used as a container format.  SOAP 
is one example, but there are lots of other cases in which it would be 
tempting to carry one document inside another.  The inability to cleanly 
nest XML documents within each other is a major limitation and 
complication, IMO.

So, a question:  what would be the cost/benefit of allowing XML 
declaration(s) nested to arbitrary depth within an XML document?  I think 
the semantics should probably be that nested documents have semantics 
relatively independent of the container, which suggests that namespace 
prefix bindings not be inherited, that there be a separate scope for 
ID/IDREF resolution and conflict detection, etc.  Of course, there are 
significant complexity tradeoffs,  Even if the specfication for the XML 
part of this could be written in a clean and compact way, there would 
obviously be knock-on effects on APIs, XPath, XML Query, XML Schema, etc. 
So, this would be a not at all trivial or entirely compatible change to 
the stack.  Still, I think that a feature like this has merits in 
principle.

I'm not at all convinced that the community is ready for a new XML at all, 
and if it is, I'm not ready to claim that that the benefits of doing this 
now outweigh the attendant cost and confusion.  Still, if we're 
daydreaming about newer and cleaner XMLs, I think this should at least be 
part of the debate.

Noah

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Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142
1-617-693-4036
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