xslt - How to call named templates based on a variable?

Question

I don't know if it's possible, but I'm wondering how to do it...

Let's say we have the following XSL:

<xsl:template name="foo">
  Bla bla bla
</xsl:template>
...
<xsl:template name="bar">
  Bla bla bla
</xsl:template>
...
<xsl:template match="/">
  <xsl:if test="$templateName='foo'">
    <xsl:call-template name="foo"/>
  </xsl:if>
  <xsl:if test="$templateName='bar'">
    <xsl:call-template name="bar"/>
  </xsl:if>
</xsl:template>

Is it possible to change the XSL to read something like...

<xsl:template match="/">
  <xsl:call-template name="$templateName"/>
</xsl:template>

Answer 1

It's not possible exactly as you describe, but if you want to be able to choose a template at run-time based on some value you set elsewhere, there is a trick to do that. The idea is to have your named template also match a node with a corresponding name in a distinct mode (so that it doesn't mess up your normal transformation), and then match on that. For example:

<xsl:stylesheet ... xmlns:t="urn:templates">

  <!-- Any compliant XSLT processor must allow and ignore any elements 
       not from XSLT namespace that are immediate children of root element -->
  <t:templates>
    <t:foo/>
    <t:bar/>
  </t:templates>

  <!-- document('') is the executing XSLT stylesheet -->     
  <xsl:variable name="templates" select="document('')//t:templates" />

  <xsl:template name="foo" match="t:foo" mode="call-template">
    Bla bla bla
  </xsl:template>

  <xsl:template name="bar" match="t:foo" mode="call-template">
    Bla bla bla
  </xsl:template>

  <xsl:template match="/">
    <xsl:variable name="template-name" select="..." />
    <xsl:apply-templates select="$templates/t:*[local-name() = $template-name]"
                         mode="call-template"/>
  </xsl:template>

Note that you can use <xsl:with-param> in <xsl:apply-templates>, so you can do everything with this that you could do with a plain <xsl:call-template>.

Also, the code above is somewhat lengthier than you might need because it tries to avoid using any XSLT extensions. If your processor supports exslt:node-set(), then you can just generate nodes directly using <xsl:element>, and use node-set() to convert the resulting tree fragment to a plain node to match against, without the need for document('') hack.

For more information, see FXSL - it's a functional programming library for XSLT that is based on this concept.