RE: debugging techniques [was:qualitative decline of x

> If people would like to recount their debugging experiences 
> and methods,
> I'll collate them for the faq?
> 

Here's a selection, of varying degrees of obviousness.

General-purpose:

  Add <xsl:message> to output progress messages


  Add <xsl:comment> to put messages into the result tree indicating
where output nodes came from


  Use <xsl:comment><xsl:value-of
select="system-property('xsl:vendor')"/></xsl:comment> so you are in no
doubt about which XSLT processor you are using


  To output the values of variables, using attribute value templates is
quicker than using xsl:value-of, e.g. write <debug x="{$x}" y="{$y}"/>


  If you want to leave debug lines in the stylesheet that can be
switched on and off at will, make them into extension elements:

  <xsl:stylesheet xmlns:debug="debug.uri"
    extension-element-prefixes="debug">
  ...

      <debug:debug x="{$x}" y="{$y}"><xsl:fallback/></debug:debug>

  The <debug:debug> element will be ignored, but values will be output
if "debug" is removed from the "extension-element-prefixes" attribute.


  To see what's in a temporary tree (or result tree fragment), copy it
to the output:

  <debug tree="$x">
    <xsl:copy-of select="$x"/>
  </debug>


  To add tracing to all template rules, add a debug module:

  <xsl:import href="real-stylesheet.xsl"/>
  <xsl:template match="*">
    <xsl:message>Processing <xsl:value-of
select="name()"/></xsl:message>
    <xsl:apply-imports/>
    <xsl:message>Finished processing <xsl:value-of
select="name()"/></xsl:message>
  </xsl:template>

  (You can also use xsl:comment instead of xsl:message)


  To deal with character-encoding problems: use a hex editor!


  If the source (or result) XML is badly laid out and difficult to
understand, run it through an indentation transform:

  <xsl:strip-space elements="*"/>
  <xsl:output method="xml" indent="yes"/>
  <xsl:template match="/">
    <xsl:copy-of select="."/>
  </xsl:template>  


  If you don't understand the error messages from your XSLT processor
(or if you are beginning to suspect that it has a bug), run the
transformation through another XSLT processor to see if it sheds any
light on the matter.


Saxon-specific:

  Use -T on the command line to switch on tracing

  Use saxon:path() to output an XPath expression that identifies the
context node

  Use saxon:line-number() to output the line number of the context node
in the source file (having switched on numbering using -l on the command
line)

  In 7.x only: add type declarations on variables and parameters, e.g.
  <xsl:param name="x" as="xs:integer"/> to trap type errors.


MSXML3-specific:

  Get your transformation working in standalone mode first, before you
integrate it into the browser. Write the version used in the browser as
a customisation layer (using xsl:import) of the standalone version, and
when debugging is necessary, use the standalone version. Try to avoid
debugging in the browser.



Michael Kay
Software AG
home: Michael.H.Kay@xxxxxxxxxxxx
work: Michael.Kay@xxxxxxxxxxxxxx 


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