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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: XPath and XPattern (was Re: More on tamingSAX)
 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: /foo/bar[2] 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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