OASIS Mailing List ArchivesView the OASIS mailing list archive below
or browse/search using MarkMail.


Help: OASIS Mailing Lists Help | MarkMail Help



   Re: [xml-dev] XPath and XPattern (was Re: [xml-dev] More on tamingSAX)

[ Lists Home | Date Index | Thread Index ]

>>    Say the test was /foo/bar/[baz2="test"]/baz1
>>    <foo>
>>      <bar>
>>        <baz1/>
>>        <baz2>test</baz2>
>>        <baz3/>
>>      </bar>
>>    </foo>
>>    If your schema stated that the children were (baz1,baz2,baz3)
>>    wouldn't you have enough information to know to surrender when
>>    you reached baz3?

Essentially in order to make a useful case of this (surrendering at 
baz3) you would need to cache all previous events so that they could be 
replayed when the predicate's boolean value was determined. 
Unfortunately that solution doesn't win much against the DOM or other 
tree based models.

The *trick* is to know when you reach baz1 so that you can begin doing 
interesting things with the chain of events as they happen. Now, the 
Schema Assisted Sipping that you mention might broaden the cases that 
predicates would be useful but they are still limited. For example if 
your schema asserted (as you say) that the content model of bar be 
(baz1, baz2, baz3) and that baz2="test" always-- then you could 
"surrender" at the point of baz1 by starting with the assumption of 
validity. If the document turns out to not be valid you could raise some 
special error that says essentially-- hey we assumed this was valid and 
it isn't so we don't know if we reported the right thing or not.

There is still a group of solvable predicates too, like:


Here the predicate's boolean value will be known before the startElement 
for baz3 is reached. The question is: is it worth defining a constraint 
for this in plain language or in grammar, or should predicates be nixed 
entirely? If we aim for defining the constraint then I would suspect 
that we will find a wealth of information on this exact subject on the 
STX list (as Christian mentioned)-- but ultimately it appears that they 
decided no such constraint would appear in their language. In XML 
Schema's stripped down XPath, predicates are not permitted.

All the best,
Jeff Rafter


News | XML in Industry | Calendar | XML Registry
Marketplace | Resources | MyXML.org | Sponsors | Privacy Statement

Copyright 2001 XML.org. This site is hosted by OASIS