On Fri, Aug 15, 2014 at 08:56:59AM -0000, Michael Kay
mike(_at_)saxonica(_dot_)com
scripsit:
It's common enough and we've all experienced it: you write a complex
expression that looks right, and it returns nothing, and you have no
clue why.
I'd suggest the following strategy:
1. Add something like
<debug><xsl:copy-of select="X"/></debug>
where X is the offending expression
2. If <debug/> comes out empty, successively remove predicates and
axis steps from X starting at the right-hand end, until you get some
output.
3. When you get some output, the last step you removed was the one
that was wrong.
An alternative, and I have trouble remembering to do this even when I've
hit the staring-and-staring stage, is to not try to write the complex
expression in one go. Write it in steps with tests for "something came
back!" between the steps.
Possibly write the XPath in steps in an XML database tool against known
input, if you have both of those things.
-- Graydon
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