xml-dev - RE: Again wit da AND and Repetitions

[Marc.McDonald@Design-Intelligence.com]

	> XML requires deterministic content models. This allows 
	> validators to do their job without having to look more than one
element ahead 
	> or do brute force tree searches.

The validator may not have to look more than 1 element ahead, but it does
need to look <n> elements behind or construct a tree representation for the
pattern due to optional elements:

<!ELEMENT x (a? | b?)>
<!ELEMENT a (c,d,e,f,g)>
<!ELEMENT b (c,d,e,f,h)>

With an input of elements c, d, e, f, h in element x.


Marc B McDonald
Principal Software Scientist
Design Intelligence, Inc
www.design-intelligence.com <http://www.design-intelligence.com> 


	----------
	From:  Kay Michael [SMTP:Michael.Kay@icl.com]
	Sent:  Friday, May 14, 1999 2:11 AM
	To:  xml-dev@ic.ac.uk
	Subject:  RE: Again wit da AND and Repetitions

	> XML requires deterministic content models. This allows 
	> validators to do their job without having to look more than one
element
	ahead 
	> or do brute force tree searches.

	It's not what XML requires that counts, it's what users require.
What you're
	saying is that if you have an integrity constraint that XML parsers
are
	congenitally incapable of enforcing, you had better implement it
yourself,
	the hard way, in the application.

	I don't remember SQL ever adopting the view that the only integrity
	constraints you were allowed to specify were those that could be
evaluated
	in linear time. In fact, the refusal to build implementation-based
	limitations into the language was one of the major reasons for the
success
	of SQL.

	(In implementing GedML I discovered that the integrity constraints
that I
	could specify in the DTD were such a pathetic subset of the total
that I
	might as well do all the validation in the application and ignore
the DTD
	capabilities entirely - especially as I had no way via the SAX API
of
	knowing whether the parser had done any validation or not).

	Mike Kay

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