Subject: Re: General rule for designing XPath expressions to return items in document order?
From: Michael Kay <mike@xxxxxxxxxxxx>
Date: Fri, 10 Jan 2014 12:01:23 +0000
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On 10 Jan 2014, at 10:56, Costello, Roger L. <costello@xxxxxxxxx> wrote:
> David wrote:
>
> I haven't checked the streaming rules in detail
> but I would expect //head to be streamable.
> (You might not be able to access any of the
> child nodes in a streamable way but for example
> count(//head) ought to be able to count all the
> head in the document in a single pass.
>
> Michael responded:
>
> That's a fair summary. //head is "consuming"
> (it reads the input stream), and "crawling"
> (it accesses all the nodes in the subtree). When
> you get an expression that is consuming and
> crawling, you are allowed to do "inspection"
> operations on the result, for example count()
>
> But Michael doesn't that contradict section 19.1 of the XSLT 3.0
specification:
>
> For example <xsl:value-of select="//head"/> will
> still fail the streamability tests, because of the
> possibility that one head element is a child of
> another. This problem can be remedied by
> writing <xsl:value-of select="//head/text()"/>.
>
>
No, there's no contradiction:
(1) <xsl:value-of select="//head"/> isn't allowed for the same reasons that
//head/data() isn't allowed: xsl:value-of is an "absorption" operation.
(2) The fact that <xsl:value-of select="//head/text()"/> is allowed doesn't
contradict anything I said. It's allowed because
a) Under section 19.1, it gets turned into //text()[parent::head] or
equivalent
b) Unlike //head, //text() is "striding" - the nodes it selects are
non-overlapping (because text nodes never overlap). So absorption operations
like data(), string(), and <xsl:value-of> are allowed.
I suspect you're making the mistake of thinking that //head/text() is very
similar to //head/string(). But that's only true when every <head> element has
a single text node child.
Michael Kay
Saxonica
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