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LC-13 extending content models

From: C. M. Sperberg-McQueen <cmsmcq@acm.org>
Date: Thu, 05 Oct 2000 20:22:42 -0600
Message-Id: <>
To: "Cohen, Aaron M" <aaron.m.cohen@intel.com>
Cc: W3C XML Schema Comments list <www-xml-schema-comments@w3.org>
Dear Aaron Cohen:

The W3C XML Schema Working Group has spent the last several months
working through the comments received from the public on the last-call
draft of the XML Schema specification.  We thank you for the comments
you made on our specification during our last-call comment period, and
want to make sure you know that all comments received during the
last-call comment period have been recorded in our last-call issues
list (http://www.w3.org/2000/05/12-xmlschema-lcissues).

Among other issues, you raised the point registered as issue LC-13,
which asks for confirmation that it is possible, using XML Schema, to
extend elements defined in one module of a complex language by adding
new children, in declarations contained in a second model of the

There are several ways in which schema authors can modify the contents
of elements; which mechanism is appropriate depends on the

For illustration, imagine that I wish to extend the content of element
E (which has complex type T) by allowing it to contain one or more
occurrences of element F.  At least the following mechanisms might
be appropriate:

   - define complex type T2, an extension of type T, and
     define element type E2, with complex type T2; declare
     E2 to be in the substitution group of E (this allows
     you to put E2 elements pretty much wherever E elements
     are allowed, but does involve a different name in the
     start- and end-tags
   - define type T with an ANY wildcard (this allows F, but
     also G and H and ...)
   - define T as being derived from type U.  Make type U
     represent the content model of E as it should be without
     the optional module; make T be the content model of E
     as it should be *with* the optional module.  In the
     optional module, derive T from U appropriately.  If the
     optional module is not included in the schema, include
     an alternate module instead, which derives T from U
     without any changes.
   - define E and T in the normal way.  In the optional module,
     include the schema document in which they are defined,
     and within the inclusion element (named 'redefine', for
     reasons that are about to become clear) include a new
     definition of type T, with the appropriate changes.
     This use of 'redefine' is legal only if the two
     schema documents (the including and the included)
     define constructs for the same namespace, and only
     if the new / variant form of T is derived (in what
     looks like a circularity) from the old form of T.

The first three of these mechanisms have been present in the language
for some time, though their best use in circumstances like the ones
you have in mind has only gradually become clearer.  The last item in
the list, however, reflects a change introduced by the XML Schema WG
in response to comments from you and others, which we believe will
make it much simpler to define markup languages composed of modules
which may be combined according to various simple rules.  We hope
you find it useful.

I apologize for not being in a position, right now, to expound on
these mechanisms, especially the last one, at greater length.  For
further information and discussion, you may find the following papers
useful.  They are unfortunately each in various stages of disarray,
and each is now in need of revision to reflect the changes we have
made in response to last-call comments in XML Schema.  I hope soon to
resume work on a note on using XML Schema to construct modular markup
languages, which may also be of use to you and your WG.


It would be helpful to us to know whether you are satisfied with the
decision taken by the WG on this issue, or wish your dissent from the
WG's decision to be recorded for consideration by the Director of
the W3C.

with best regards,

-C. M. Sperberg-McQueen
  World Wide Web Consortium
  Co-chair, W3C XML Schema WG
Received on Thursday, 5 October 2000 16:58:33 UTC

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