Inheritance in XML

[Jon Bosak]

I'm generally not able to track discussions like this, fascinating
though they may be, and I make it a firm principle not to become
involved in them, so don't expect any further comments from me
regarding this one.  But catching up on my email backlog just now I
see so much good energy being wasted that I can't pass by without
contributing a couple of items of information that may save some
wheel-spinning out there.

First, allow me to vent just a little bit about a common
misunderstanding.

[Matthew Gertner:]

| In last month's Wired, XML made it into the "hype list" with the
| comment that we crazy XML types are kidding ourselves because XML will
| never fly without well-defined semantics.

This gets the "No Shit, Sherlock" award for excellence in trade press
reporting.  XML was very carefully designed to have no built-in
semantics whatsoever.  So considered in isolation, an XML document is
found to have... no semantics!  What an insight!

And we can go further: to give semantics to this thing that was
designed to have no semantics we have to have... it's coming to me,
wait a minute... yes!  We have to supply something else that *does*
provide the semantics!  Wow!  Pulitzer prize time for sure.

Here are some examples of things that can provide semantics for XML
documents:

* Scripts or programs.  Especially Java programs.  :-)

* Prose descriptions (if you said "DTDs" you are confused, but
understandably so; a lot of good people have been confused about this
before you).  The namespace specification provides a standard way to
associate prose descriptions and other bearers of semantic information
with classes of XML documents.

* Stylesheeets.  Especially XSL stylesheets, which are even as we
speak being defined by a very active W3C XSL WG.  This is why you will
want to look carefully at the first XSL working draft expected out in
July, because XSL will provide what is intended to be the most
powerful standardized high-level way to associate presentational
semantics with XML documents in publishing environments.  Watch this
space:

   http://www.w3.org/Style/XSL

So people who think that there is something missing from XML are by
and large simply unaware that it was not intended to be used by itself
and that the other pieces are on their way.  (There's XLink, too.)
This has all been made abundantly clear in every W3C statement about
the XML activity for the last year and a half, but it's to be expected
that a lot of folks just won't bother to pay attention to stuff like
that.

Now let's turn to the chief concern of this thread.  After a number of
excellent observations about the need for a schema language for XML
documents and the considerations that have to go into the
specification of such a thing, Matthew asks the following question:

| More tricky than any of these technical issues is the question of
| what, if anything, could be done to promote a mechanism of this
| sort. Obviously this would require a change to the XML spec as
| well as modification to all existing tools which process DTDs, so
| it's a pretty big deal. I wonder if anyone besides me thinks that
| a simple mechanism like this would make sense.  If so, is there
| any room in the XML standards process to discuss a change of this
| type at some point in the future (certainly not for XML 1.0)?

The answer is, Yes, there are other people who think that it would
make sense to design an XML schema mechanism to handle issues like
what has been called "inheritance" in this discussion (not to mention
good old-fashioned data typing).  The workings of a W3C committee can
be made public only at the discretion of the chair of the committee,
so I will put on my official XML WG Chairman hat and reveal unto ye
that the XML WG has officially requested that the job of defining a
schema language for XML documents be added to its charter.  If
approved by the W3C Director, this work would certainly involve a
consideration of most of the issues raised in this discussion and
would include a close look not only at XML Data but also at other
proposed solutions to the same problem.

Jon

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