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   Re: [xml-dev] XQuery types was Re: [xml-dev] Yet another plea for XUpdat

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> > > > At the beginning of this thread Dare asserted that a weakness of
> > > > XML-Query is that it's semantics is too weak to allow 
> > > static analysis 
> > > > of the correctness of updates.
> > > > 
> > > > What's interesting is that this assertion is in fact untrue, but
> > > > people seem to be accepting it and instead arguing 
> > whether that's a 
> > > > fatal flaw or not (and what the relevant definition of "type" is).
> > 
> > Please prove this assertion untrue. XML type systems (especially with
> > W3C XML Schema) are based on constraints. Constraints are runtime
> > issues....
> There's a misunderstanding here. No static analysis of any program can
> classify programs unambiguously as correct or incorrect; if that were the
> case it would never be necessary to execute a program. The purpose of static
> checking is to reject as many incorrect programs as possible before
> executing them. An interesting design choice is the extent to which we
> reject the programs that might or might not be correct, depending on the
> input data. Typically we solve this by distinguishing structural
> constraints, which can be checked statically, from value-based constraints,
> which can't: but it's a fuzzy boundary.
> Static analysis potentially allows one to catch mistakes like the following
> common one:
> <xsl:for-each select="item">
>   <xsl:value-of select="item">
> and this is surely a Good Thing.

Dynamic analysis could allow precisely the same thing, if one had access to the content model, which could be a DTD or RELAX NG as well as XSDL.  This has been the crux of my point.  Rather than giving the XML "programmer" *generic* facilities for things such as correctness through constraint checking and even functional dispatch based based on type analysis, XPath 2.0/XSLT 2.0 gives us a highly limited yet complex system wedded to a single, flawed schema definition language.

Uche Ogbuji                                    Fourthought, Inc.
http://uche.ogbuji.net    http://4Suite.org    http://fourthought.com
Track chair, XML/Web Services One (San Jose, Boston): http://www.xmlconference.com/
DAML Reference - http://www.xml.com/pub/a/2002/05/01/damlref.html
RDF Query using Versa - http://www-106.ibm.com/developerworks/xml/library/x-think10/index.html
XML, The Model Driven Architecture, and RDF @ XML Europe - http://www.xmleurope.com/2002/kttrack.asp#themodel


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