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[Bug 8444] Limited Extensibility

From: <bugzilla@wiggum.w3.org>
Date: Mon, 07 Dec 2009 14:53:31 +0000
To: www-xml-schema-comments@w3.org
Message-Id: <E1NHexv-0003lE-QR@wiggum.w3.org>
http://www.w3.org/Bugs/Public/show_bug.cgi?id=8444





--- Comment #2 from C. M. Sperberg-McQueen <cmsmcq@blackmesatech.com>  2009-12-07 14:53:31 ---
As Michael Kay has observed, the reformulation of the Unique Particle
Attribution in 1.1
(the so-called 'weakened wildcard' change) does make it legal for a wildcard
particle to 
compete with an element particle in a content model, and thus for an optional
element
particle to precede a wildcard in a sequence.

In 1.0 it is not allowed for wildcard and element particles to compete, and so
an optional element particle cannot be followed in a sequence by a wildcard. 
So in 1.0 the solution to the problem needs to take place on a different layer
of the design:  if you don't want first-generation apps to break when they see
content that is not valid against the first-generation schema, then write them
to accept invalid input and do the best they can with it.  (HTML has prospered
fairly well with this recipe.)  Or rewrite the illegal content model with an
equivalent content model that is legal. 

In the example given, the content models desired for the second and third
generation apps (the ones shown, plus an additional repeating wildcard) seem at
first glance to accept the same language as the content model given for the
first-generation app.  The differences thus appear to have at most a rhetorical
or documentary function.  Am I missing something?

The Unique Particle Attribution constraint certainly makes some regular
languages inexpressible, and I continue to believe that it should never have
been made part of XSD and that it should be eliminated at the first
opportunity.  But the simple wildcard problems described here and in some other
discussions of versioning don't seem to involve languages that cannot be
expressed in XSD 1.0 content models; they involve, as far as I can see, only a
dissatisfaction with the expression prescribed for those languages by XSD 1.0.


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Received on Monday, 7 December 2009 14:53:34 GMT

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