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[Bug 29983] [XSLT30] Scanning expressions and function calls

From: <bugzilla@jessica.w3.org>
Date: Sun, 06 Nov 2016 14:33:53 +0000
To: public-qt-comments@w3.org
Message-ID: <bug-29983-523-fqEUsuiggl@http.www.w3.org/Bugs/Public/>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=29983

--- Comment #1 from Michael Kay <mike@saxonica.com> ---
My mental model for scanning expressions is that they are expressions that can
be evaluated by considering each descendant node individually and testing it
against a pattern. Anything that involves evaluating a user-defined function
that selects which nodes should match the pattern doesn't fit into that model
at all.

Remember that all this started with the //section/footer problem, with a a
source tree like

<section id="A">
  <section id="B">
    <footer of="B"/>
  </section>
  <footer of="A"/>
</section>

Here the nested-loop evaluation strategy for the path expression yields nodes
that are not in document order (the child of the second section PRECEDES the
child of the first section). The only streamable strategy for evaluating this
is to consider the footer nodes in document order and test each one against the
pattern //section/footer. If we replaced "//section" here by a function that
selects the section elements then I would have no idea how to devise a
streamable execution strategy.

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