Category: XSLT

Inasmuch as I was talking at Balisage about adapting the Saxon-CE event model to XSL-FO, I ended up talking with Lauren Wood and Matt Patterson about the general case for a XSLT 2.0 processor, or a processor for a subset of XSLT 2.0, that could be used in language bindings and web browsers as a replacement for the stuck-at-XSLT-1.0 libXSLT processor.

Since we were going to talk about it more at the XML Summer School anyway, I did a five-minute lightning talk (slides) at the “unconference sessions”. Happily, it got more people at the Summer School trying things. Michael Kay of Saxonica was at the presentation, and not only did he welcome the idea of compiling Saxon to C, Saxonica now state that they’re looking at the possibility.

Inasmuch as use-when is one of the standard attributes in XSLT 2.0 (and later) rather than being on a particular XSLT element, it seldom gets much of a mention; e.g., currently only 568 mentions on the XSL-List according to MarkMail (and some of those are false positives).

use-when (and xsl:use-when on non-XSLT elements) is for “conditional element inclusion” and is very useful for excluding elements that either aren’t currently useful or that will cause errors if acted upon. The use-when in the example below causes the xsl:value-of to be used only when saxon:line-number() is available, thereby avoiding the Saxon extension functions are not available under Saxon-HE message from more recent versions of Saxon HE where saxon:line-number() is no longer available but producing a useful result on older Saxon HE versions where the function is available.

<!-- Leftover intermediate elements. -->
<xsl:template match="t:*">
  <xsl:if test="$debug">
    <xsl:message>
      <xsl:value-of select="t:node-basename(.)" />
      <xsl:value-of
        use-when="function-available('saxon:line-number')"
        select="concat(':', saxon:line-number())"
        xmlns:saxon="http://saxon.sf.net/" />
      <xsl:text> :: </xsl:text>
      <xsl:copy-of select="." />
    </xsl:message>
  </xsl:if>
  <xsl:apply-templates />
</xsl:template>

The use-when expression is evaluated very early in the processing of the stylesheet, and you can’t use variable references in the expression. So don’t be tempted to try:

<xsl:message use-when="$debug">
...
</xsl:message>