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

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What you just stated doesn't jibe with any formal
knowledge I have about types and language systems but
since my background in formal computer science theory
is weak I won't attempt to correct your claims. 

So I'll try to explain my point in lay terms. The main
benefit of a type system is so that things can be done
*statically* at compile time instead dynamically at
run time. Validation is a dynamic process. 

Thus a "type system" based on validation isn't really
type system especially to people with a programming or
database background. 

Bottom Line: The XQuery type system although onerous
and complex does not forestall the need for
post-update validation if/when XQuery becomes a DML
and not just a query language. Therefore claims that a
"type system" is necessary before update semantics can
be added to XQuery are ill-considered. 

--- Jonathan Borden <jborden@attbi.com> wrote:
> Dare Obasanjo wrote:
> > Before this turns into a pointless critique of the
> > XQuery formal semantics I'll just steer this
> > conversation back to the original point.
> Let's make this simple.
> There are instances and classes which are sets of
> instances. That's all.
> Pieces of XML are instances, types are classes.
> One can classify an instance using a "classifier".
> The process of "schema
> validation" is simply the process of classifying an
> instance as or not as, a
> member of a class (type).
> So a type is a class, and validation is a process of
> classification.
> In any case, you can't have "validation" without
> some sort of "type" to
> validate against ***
> Jonathan
> *** note that we use the term "type" as a synonym
> for "class" and not as in
> Russell ... A great undergraduate level introduction
> to logic, which
> discusses instances and classes is Tarski
> http://www.oup-usa.org/isbn/019504472X.html . It is
> very simple yet powerful
> stuff.
> >
> > Some members of the XQuery WG feel that a complex
> type
> > system such as XQuery's is necessary for updates
> while
> > I am of the opinion that a post-validation
> requirement
> > is the minimum that should be required.
> >
> > The bottom line is that any XML database product
> that
> > supports XQuery and W3C XML schema plus an update
> > capability will have to support post-update
> validation
> > because the XQuery and W3C XML Schema "type"
> systems
> > leave the door open for too many things to be
> dynamic.
> >
> >
> > By the way here are some quotes from the Formal
> > Semantics document which point out what I mean by
> > validation vs. type system
> >
> > "The XQuery type system is based on XML Schema.
> > Schema defines a notion of validation for XML
> > documents. When doing validation, an XML Schema
> > processor checks if the structure, and the text
> > content of a document verifies the constraints on
> the
> > structure, and the constraints on values,
> specified in
> > the schema. As an example of constraints on the
> > document structure, all book elements might be
> > declared to contain an isbn attribute, and a title
> > element, followed by a sequence of one or more
> author
> > elements. As an example of constraints on values,
> the
> > isbn attribute might be declared as a subset of
> the
> > XML Schema xsd:string values, using a XML Schema
> > pattern facet."
> >
> > Especially with regards to restriction facets,
> there
> > simply is no way for this to be implemented
> without
> > something akin to post-update validation. Identity
> > constraints as well. In fact, when I think about
> even
> > making sure that the results of an update do not
> > result in an invalid content model for an element
> in
> > the document is pretty much another validation
> issue
> > not a type one.
> >
> >
> >

My dungeon cell decor will not feature exposed pipes. 
While they add to the gloomy atmosphere, they are good conductors of vibrations and a lot of prisoners know Morse code.

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