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RE: [xml-dev] Separate data from rules ... is the XML Schema 1.1 <assert> element a step backwards?

> So "structural rules" are those that specify constraints on 
> the structure of XML instances? That is, they specify what 
> elements and attributes can go where.

It's your term, not mine. You can define it how you like. I've been trying
to say that I don't think your classification is particularly useful. If
there is a useful distinction, I'd say it's based on who made the rule:

(a) laws of nature - children have two parents, planes take off before they

(b) rules decided by the business (or more generally, by the user community)
- every customer belongs to one branch office, every employee is over 16,
ISBNs are ten digits long

(c) rules decided by IT system designers - every customer has at most three
phone numbers, tables cannot be nested within tables, messages are limited
to 140 characters

That's nothing to do with the technology used to validate instances, it's to
do with where the rules come from, how they get changed, and what you do
when you find data that doesn't conform to the rules.

To put this another way, classifying rules as "business" or other is
orthogonal to classifying them as "structural" or other. If you want, adopt
the language of compilers to divide rules into "syntax" and "semantics", but
don't imagine that this split is correlated with the split above according
to ownership of the rule.


Michael Kay

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