Higher-Order XSL

hoxsl is a library for XSLT 2.0, written in pure XSLT, introducing various types of higher-order logic; this includes higher-order functions and XSLT templates that take XSLT as input and produce XSLT as output.

The system is fully tested---see the test cases for additional documentation as this project gets under way.

Higher-Order Functions

Higher-order functions are a part of XSLT 3.0, but implementations that support them (such as Saxon) hide it behind proprietary versions of their software. Others still may wish to continue using XSLT 2.0.

There are various approaches/kluges for this problem in earlier versions of XSLT; this implementation is motivated by Dimitre Novatchev's work on higher-order functions in FXSL.

For example, consider an implementation of a filter function that accepts a node set and a predicate:

  <xsl:function name="my:filter" as="xs:element()*">
    <xsl:param name="nodes" as="xs:element()*" />
    <xsl:param name="pred" />

    <xsl:for-each select="$nodes">
      <xsl:if test="f:apply( $pred, . )">
        <xsl:sequence select="." />
      </xsl:if>
    </xsl:for-each>
  </xsl:function>


  <xsl:function name="my:pred" as="xs:boolean">
    <xsl:param name="node" as="element()" />

    <xsl:sequence select="$node/@foo = 'true'" />
  </xsl:function>

We could then apply a filter using this predicate like so:

  <sequence select="my:filter( $nodes, my:pred() )" />

hoxsl takes this a step further by providing a stylesheet to generate the boilerplate necessary for functions to be able to be applied using f:apply, as shown above. Applying tranform/apply-gen.xsl to the XSL stylesheet containing the above function definitions would produce output that can be directly imported (as a stylesheet); no additional work is needed. This can be included as part of a build process, and the output included within a distribution.

License

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.