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Unfortunately schema was never intended to perform that role!
Not the least because it has no context driven mechanisms.
Apart from that - way too many people think it can do this 'magic' because it
has been over sold - well beyond the original requirements the W3C started
We started off with a DTD - simple mission to describe the structure
permutations of an XML instance. XSD then subsumed that role. Snag is neither
is able to deliver fully. It's all to easy to create an XML instance, or set
of instances, that look perfectly reasonable and straightforward that is darn
hard to then describe in schema.
I'm reminded of the situation in England in the 1500's - when Latin was still
the official legal language of law - but everyone uses English as the working
language. The solution beckons ; -)
Quoting "Cox, Bruce" <Bruce.Cox@USPTO.GOV>:
> In my world, attorneys speak "business rules" and IT folk speak "data
> constraints". Often, their intention and extension are identical. A
> really good schema is the membrane where these two sets touch each
> other, that is, it is equally successful from both points of view.
> Bruce B. Cox
> -----Original Message-----
> From: Thomas B. Passin [mailto:firstname.lastname@example.org]
> Sent: Thursday, August 19, 2004 4:59 PM
> To: email@example.com
> Subject: Re: [xml-dev] Are people really using Identity constraints
> specified in XML schema?
> Roger L. Costello wrote:
> > - The value of the <minimum-age> must be an integer. This is a
> > constraint on the data. It will not change over time.
> Ha! What happens when the government decides that some relevant age is
> 67.5 years instead of 67?
> > Therefore, an XML Schema should simply constrain <minimum-age> to be
> > an integer. Higher level applications should implement the business
> > rule that <minimum-age> be further constrained to 16.
> > How would you characterize the distinction between "business rules"
> > and "constraints on data"?
> A tricky, tricky issue - what is or is not a "business rule". I suspect
> that in practice most constraints that are not business rules are in
> place for supposed programming reasons, or by force of habit.
> In one project I work on, we have a data type that is a union of 1) an
> enumeration of strings, 2) a string that follows a certain regex
> pattern, and 3) an integer constrained to a certain range. No, don't
> bother to ask - it's one of those multi-agency reconciliations.
> Thomas B. Passin
> Explorer's Guide to the Semantic Web (Manning Books)
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