RE: [xml-dev] Rick Jelliffe quotable quote on the purpose of schemas

["Paul Spencer" <xml-dev-list@boynings.co.uk>]

I agree totally with Rick Marshall, but for slightly different reasons. Very
often, the data being validated has been generated by some computer system
that has extracted data from a database. When receiving the data, a schema
will usually be used to validate both the structure of the document (which
has been created by the software) and the data types (that rely on the
database content). These are completely separate issues and should be
treated separately. I might choose to reject the document if the structure
is incorrect, but process those parts that I can if the data does not meet
datatype constraints, while quarantining the rest for some manual
inspection.

Regards

Paul Spencer



> -----Original Message-----
> From: Rick Marshall [mailto:rjm@zenucom.com]
> Sent: 11 December 2006 23:34
> To: xml-dev@lists.xml.org
> Subject: Re: [xml-dev] Rick Jelliffe quotable quote on the purpose of
> schemas
>
>
> Hi
>
> I want to throw my hat in the ring here from my own experience at
> constructing these things (yes i know - in the database, not document
> world...).
>
> However, the point is that the discussion will continue until we
> recognise the fundamental difference between "structure" and "constraint".
>
> For my money I'd be happy if it took 2 documents to provide a valid
> description of what an xml document could contain. The first simply and
> only outlines structures. In effect it could easily be a prototype xml
> document with one of all possible structures, possibly with some sort of
> ellipsis to indicate recursive inclusion - or we could stick with dtd's.
>
> The second would be an XPath type document that lists restraints similar
> to XSD.
>
>  From my perspective (and the commercial view) this means I could have
> eg some sort of generic description of say a purchase order and then a
> constraint based instance that was my requirements - what must be
> included, optional, values limits, etc.
>
> This was the basic approach in the ANSI EDI standards and worked very
> well to build what must remain the most successful B2B system.
>
> This is a long way of saying that I think trying to get one tool to do 2
> fundamentally different things is always going to less than optimal.
>
> Regards
> Rick
>
>
> Michael Kay wrote:
> >> "The flaw with grammars is that they only allow to constraint
> >> content models in a declarative manner
> >>
> >
> > There's nothing wrong with constraints being declarative - in fact, they
> > definitely should be - the flaw is with the expressive power of
> a grammar as
> > a contraint language.
> >
> > This is all well known. And in fact, XML Schema itself has some
> > extra-grammatical constraints, namely uniqueness and referential
> > constraints. But there's still a camp that believes (wrongly,
> in my view)
> > that the expressive power of a schema language should be restricted for
> > performance reasons.
> >
> > Michael Kay
> > http://www.saxonica.com/
> >
> >
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