XSLT, like m4 macros, is a language for describing transformations of a text stream. But it does much more than simple macro substitution; it describes transformations of XML data, including query and report generation. It is the language used to write XML stylesheets. For practical applications, see the description of XML document processing in Chapter18. XSLT is described by a World Wide Web Consortium standard and has several open-source implementations.

XSLT and m4 macros are both purely declarative and Turing-complete, but XSLT supports only recursions and not loops. It is quite complex, certainly the most difficult language to master of any in this chapter's case studies — and probably the most difficult of any language mentioned in this book.^[84]

Despite its complexity, XSLT really is a minilanguage. It shares important (though not universal) characteristics of the breed:

<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="https://www.w3.org/1999/XSL/Transform" 
                version="1.0">
  <xsl:output method="xml"/>
  <xsl:template match="*">
    <xsl:element name="{name()}">
      <xsl:for-each select="@*">
        <xsl:element name="{name()}">
          <xsl:value-of select="."/>
        </xsl:element>
      </xsl:for-each>
      <xsl:apply-templates select="*|text()"/>
    </xsl:element>
  </xsl:template> 
</xsl:stylesheet>

The program in Example8.3 transforms an XML document so that each attribute of every element is transformed into a new tag pair directly enclosed by that element, with the attribute value as the tag pair's content.

We've included a glance at XSLT here partly to illustrate the point that ‘declarative’ does not imply either ‘simple’ or ‘weak’, and mostly because if you have to work with XML documents, you will someday have to face the challenge that is XSLT.

XSLT: Mastering XML Transformations [Tidwell] is a good introduction to the language. A brief tutorial with examples is available on the Web.^[85]