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

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Jeff Rafter wrote:

>>>    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:
> /foo/bar[baz2="test"]/baz3
> 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.

IMO we should at least allow abbreviated positional predicates:


Going further, I think we could come up with a formula that allows us to 
decide for most predicates whether or not they can be decided with N or 
less SAX event look-ahead.

At least I think it's worth trying to concoct such a formula, and if no 
one beats me to it, I'll give it a whirl when I get a chance.

Uche Ogbuji                                    Fourthought, Inc.
http://uche.ogbuji.net    http://4Suite.org    http://fourthought.com
Use CSS to display XML - http://www.ibm.com/developerworks/edu/x-dw-x-xmlcss-i.html
Full XML Indexes with Gnosis - http://www.xml.com/pub/a/2004/12/08/py-xml.html
Be humble, not imperial (in design) - http://www.adtmag.com/article.asp?id=10286
UBL 1.0 - http://www-106.ibm.com/developerworks/xml/library/x-think28.html
Use Universal Feed Parser to tame RSS - http://www.ibm.com/developerworks/xml/library/x-tipufp.html
Default and error handling in XSLT lookup tables - http://www.ibm.com/developerworks/xml/library/x-tiplook.html
A survey of XML standards - http://www-106.ibm.com/developerworks/xml/library/x-stand4/
The State of Python-XML in 2004 - http://www.xml.com/pub/a/2004/10/13/py-xml.html


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