lichtblau's blog

Gary King is confused because [XSLT] seems so ridiculously verbose. Others have already suggested mad higher-order function tricks using XSLT 2.0.

My solution: Macros. If XSLT lacks an element to do what you want (creating a text node with a newline, in Gary's case), just invent the feature you need and send your XSLT stylesheet through another XSLT stylesheet to implement it.

BR

Say you have demo.xsl which wants to use

 <x:br/>

to emit a newline. (In this example, `x' is simply the namespace for our extensions.) Write an additional stylesheet macros.xsl and send the original demo.xsl through the macro stylesheet to generate the actual XSLT source code. A macro template for <x:br> would be as simple as:

 <xsl:template match="x:br">
 <_xsl:text><xsl:text>&#10;</xsl:text></_xsl:text>
 </xsl:template>

In the macro stylesheet, xsl is the namespace of the "macro definition" and _xsl is the namespace of the "macro expansion". (If you care about details, the trick is to use xsl:namespace-alias to make the XSLT processor believe they are different namespaces.)

DOTIMES

For a more interesting example of macro use, suppose we want to repeat our code count times. Doing this kind of iteration involves a recursive template call, which we want to hide. We will define a macro <x:dotimes> that can be used like this:

 <x:dotimes var="i" count="3">
 <xsl:value-of select="$i"/>
 </x:dotimes>

Our macro stylesheet replaces each use of <x:dotimes> with a template call, and adds a recursive template as a top-level element:

 <xsl:template match="xsl:stylesheet">
 <xsl:copy>
 <xsl:apply-templates select="@*|node()"/>
 <xsl:for-each select="//x:dotimes">
 <_xsl:template name="x:dotimes_{generate-id()}">
 ... recursive template definition here ...
 </_xsl:template>
 </xsl:for-each>
 </xsl:copy>
 </xsl:template>
 
 <xsl:template match="x:dotimes">
 <_xsl:call-template name="x:dotimes_{generate-id()}">
 ... parameters elided for brevity ...
 </_xsl:call-template>
 </xsl:template>

Download the full macros.xsl and demo.xsl to try the example. To run it with xsltproc, use the Makefile in the same directory.

XOM is a DOM alternative written in Java and for Java -- in contrast to DOM, which feels wrong in almost every language.

Key phrases:

• "Comatose lists"
• This is a cathedral, not a bazaar
• There's exactly one way to do it
• The Wrong Side of 80/20

Lots of good ideas waiting to be stolen. Stay tuned for a Common Lisp adaptation.