Re: [xsl] testing preceding sibling always evaluates true

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Subject: Re: testing preceding sibling always evaluates true
From: Michael Kay <mike@xxxxxxxxxxxx>
Date: Wed, 10 Aug 2011 10:13:34 +0100

On 10/08/2011 04:15, Hank Ratzesberger wrote:

Thank you Michael.

I just received the 4th edition (it really is hardbound!)
so I could check if anything had changed, but on p. 602 :

 There are three kinds of expressions in XPath 2.0 whose result
 is guaranteed to be a sequence of nodes in document order...

  o Any axis step (even and axis step like <preceding-sibling::*> that
    uses a reverse axis delivers its results in forwards document order)

But, reading on... p. 618

 In effect, the predicate operator <[]> has a higher precedence (it
 binds more tightly) than the path operator </>

And the following paragraph is also relevant to this discussion: in a FilterExpr, the items in a sequence are considered in their original order when evaluating the predicate; in an AxisStep, the nodes are considered in the order of the relevant axis.

You're right that this is a highly subtle (=obscure and easily missed) feature of the XPath semantics.

This didn't change between XPath 1.0 and 2.0. Many people would have liked to change it, but backwards-compatibility ruled the day.

Michael Kay
Saxonica

Current Thread
Re: testing preceding sibling always evaluates true Hank Ratzesberger - 10 Aug 2011 03:15:43 -0000 Michael Kay - 10 Aug 2011 09:14:08 -0000 <= Sam Wilmott - 11 Aug 2011 20:08:23 -0000 Sam Wilmott - 11 Aug 2011 20:15:13 -0000

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