XSLT: Template Engine for XML

XSLT (eXtensible Stylesheet Language Transformations) transforms XML documents into other formats (HTML, other XML, plain text, etc.).

Analogy: XSLT is to XML what template engines (Jinja, Handlebars, JSX) are to HTML in web frameworks.

Remark: XSLT and XQuery overlap significantly, but XSLT is template-driven (declare patterns, the processor applies them) while XQuery is expression-driven (explicitly select and construct). XSLT is better suited for document transformation, while XQuery is better suited for data querying.

Remark: XML starts to look like Typedown's idea. XSLT in Typedown would be for static site generation.

Part of the XSL family.

Basics ​

An XSLT stylesheet is itself an XML document. No new syntax, just new elements in the xsl: namespace.

Two types of content in a stylesheet:

• XSLT elements (xsl: prefix): Instructions the processor executes.

• Literal result elements (everything else): Copied directly to the output.

The processor distinguishes them by namespace: xsl: = instruction, anything else = output.

Declaring a Stylesheet ​

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <!-- templates go here -->
</xsl:stylesheet>

<xsl:stylesheet> and <xsl:transform> are synonymous.

Linking to XML ​

<?xml-stylesheet type="text/xsl" href="catalog.xsl"?>

One XML file can be transformed by many different stylesheets.

How It Works ​

A template is a rule: "When you encounter this pattern, produce this output".

1. The XSLT processor walks the XML tree.

2. At each node, it checks if any template's match attribute matches.

3. If so, it executes that template.

If multiple templates match, the most specific wins (e.g. match="catalog/cd/title" beats match="title"). If no template matches, built-in defaults pass through to children or output text.

<xsl:apply-templates> is like a recursive function call: "Process these children, find the right template for each". This is how templates chain together.

XSL Elements ​

Three types of attributes:

• match (XPath pattern): Declares which nodes trigger this template. The processor decides when to use it.

• select (XPath): Actively selects which nodes to operate on.

• test (boolean XPath): Evaluates to true/false for conditionals.

Examples ​

All examples use this XML:

<catalog>
  <cd><title>Empire Burlesque</title><artist>Bob Dylan</artist><price>10.90</price></cd>
  <cd><title>Hide your heart</title><artist>Bonnie Tyler</artist><price>9.90</price></cd>
  <cd><title>Greatest Hits</title><artist>Dolly Parton</artist><price>9.90</price></cd>
</catalog>

template XML Element ​

Declares a rule: When a node matches the match pattern, produce this output.

<xsl:template match="/">
  <h1>Catalog</h1>
</xsl:template>

Output: <h1>Catalog</h1>

value-of XML Element ​

Outputs the text value of the selected node. Without a loop, selects the first match only.

<xsl:template match="/">
  <p><xsl:value-of select="catalog/cd/title"/></p>
</xsl:template>

Output: <p>Empire Burlesque</p>

for-each XML Element ​

Loops over all nodes matching the select expression. Can filter with predicates: select="catalog/cd[artist='Bob Dylan']".

<xsl:for-each select="catalog/cd">
  <p><xsl:value-of select="title"/></p>
</xsl:for-each>

Output: <p>Empire Burlesque</p><p>Hide your heart</p><p>Greatest Hits</p>

sort XML Element ​

Sorts the enclosing for-each loop by the value selected in select. Place as the first child of for-each.

<xsl:for-each select="catalog/cd">
  <xsl:sort select="artist"/>
  <p><xsl:value-of select="artist"/></p>
</xsl:for-each>

Output: <p>Bob Dylan</p><p>Bonnie Tyler</p><p>Dolly Parton</p>

if XML Element ​

Conditional output. Only renders content when the test expression evaluates to true.

<xsl:for-each select="catalog/cd">
  <xsl:if test="price &gt; 10">
    <p><xsl:value-of select="title"/></p>
  </xsl:if>
</xsl:for-each>

Output: <p>Empire Burlesque</p>

choose/when/otherwise XML Elements ​

Switch/case branching. when branches are tested in order, otherwise is the default fallback.

<xsl:for-each select="catalog/cd">
  <xsl:choose>
    <xsl:when test="price &gt; 10">
      <p style="color:red"><xsl:value-of select="title"/></p>
    </xsl:when>
    <xsl:otherwise>
      <p><xsl:value-of select="title"/></p>
    </xsl:otherwise>
  </xsl:choose>
</xsl:for-each>

Output: Empire Burlesque in red, others in default.

apply-templates XML Element ​

Delegates processing to other templates. The processor finds the right template for each matched child. The modular alternative to for-each: Each node type has its own template.

<xsl:template match="/">
  <xsl:apply-templates select="catalog/cd"/>
</xsl:template>

<xsl:template match="cd">
  <p><xsl:apply-templates select="title"/> by <xsl:apply-templates select="artist"/></p>
</xsl:template>

<xsl:template match="title"><b><xsl:value-of select="."/></b></xsl:template>
<xsl:template match="artist"><i><xsl:value-of select="."/></i></xsl:template>

Output: <p><b>Empire Burlesque</b> by <i>Bob Dylan</i></p> (for each CD)