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Nasty XPath expressions

  • To: xml-dev@l...
  • Subject: Nasty XPath expressions
  • From: Kelvin Yeow <whyeow@c...>
  • Date: Sat, 15 Dec 2001 01:53:24 -0800 (PST)

xalan2 xpath
Bob wrote:
>XPath says that nodes should be returned in 'document order', which
>becomes a non-trivial in the case of some expressions involving
>the union operator.  One developer has a solution, but it is of
>O(tree-size) complexity.  Do that against a 30mb document, and
>suddenly XPath isn't nearly as attractive.

Hi Bob,
If 'document-order' is required, 

   $foo/bar | $cheese/melty

certainly means more work. To the best of my knowledge, Xalan2 
for Java seems to follow this 'requirement' even for expressions 
involving the union operator. In the case of '$foo/bar | $cheese/melty' 
does the order of the returned node-set matters to the user? I can
understand if a predicate is used  (example from the spec):

preceding::foo[1] != (preceding::foo)[1]

Evan wrote:
>Actually, XPath nowhere says that nodes must be returned in 
> document order. Node-sets are sets; they have no order.

Although, it is not stated explicitly that nodes must be returned in 
document order, if a predicate is used in a location step, the 
ordering becomes relevant. You know, it would be more helpful if this
issue is described under section 3.3 Node-sets.

Ramin wrote:
> '//' just about made me want to hurl
> every time I thought about how much work it generated under the hood.
> Then I would remember how useful it was to end-users and calm down (:-)

It is convenient and it invites abuse. I have had a brief
moment with XPath through XSLT. I often wondered
how other users take advantage of XPath's expressiveness
esp. with other axes such as 'ancestor', 'preceding' or 'following'.

Mike Kay wrote:
> ... <xsl:if test="//a=//b">

This is a nasty XPath expression. It is a valid expression but all 
too convenient for the user. The developers have no choice but to tackle
it head on while the users will be delighted (esp. lazy power users).

I didn't expect such a huge response to the subject, else, I would have
replied earlier. xml-dev rocks! 



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