XSLT 3.0 Dynamic select with variable in apply-templates? - xslt

Question

I want to apply templates to a set of nodes where part of the select path is a variable. I'm using Saxon-HE 9.8 (awesome lib!)
I'm trying to achieve the following
<variable name="x" select="string('baz')"/>
<xsl:apply-templates select="foo/bar/$x"/>
This doesn't seem to work. Is there a syntax that will allow me to dynamically construct the select XPath for this apply-templates instruction? Or, is there another technique for dynamically achieving this effect? I even tried pushing this down to my <xsl:template match=foo/bar/$x> but no luck.
My motivation here is in my application the variable value is coming from a separate configuration file. Based on the configuration I need to run templates matching specific path segments driven by config strings...

Answer 1

If your variables are always going to be a simple string value expressing the name of an element, then one option would be to match a little more generically on an element and then use the string variable in a predicate to filter for a match of the element name:
<xsl:apply-templates select="foo/bar/*[local-name() = $x]"/>
With Saxon-PE or Saxon-EE, you could leverage xsl:evaluate and do something like this:
<xsl:variable name="var" as="node()*">
<xsl:evaluate xpath="concat('foo/bar/',$x)" context-item="."/>
</xsl:variable>
<xsl:apply-templates select="$var"/>

Answer 2

If you declare a static parameter <xsl:param name="x" static="yes" as="xs:string" select="'baz'"/> for the value and then use a shadow attribute in the form of _select="foo/bar/{$x}" you can even construct the path dynamically, but only when compiling the XSLT.
In a static parameter you can of course pull in a configuration file and use values from it:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
version="3.0">
<xsl:param name="config-uri" static="yes" as="xs:string" select="'https://martin-honnen.github.io/xslt/2018/config-example1.xml'"/>
<xsl:param name="config-doc" static="yes" as="document-node()" select="doc($config-uri)"/>
<xsl:mode on-no-match="shallow-copy"/>
<xsl:template match="item[#type = 'foo']">
<xsl:copy>
<xsl:value-of _select="{$config-doc/map/from[#key = 'foo']}"/>
</xsl:copy>
</xsl:template>
<xsl:template match="item[#type = 'bar']">
<xsl:copy>
<xsl:value-of _select="{$config-doc/map/from[#key = 'bar']}"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
https://xsltfiddle.liberty-development.net/6qVRKvX/1
Another option I didn't mention in my first answer but that is also a viable way with Saxon 9.8 or any other XSLT 3 processor is the use of XSLT to create XSLT and then to use the transform function (https://www.w3.org/TR/xpath-functions/#func-transform) to run that generated XSLT. That approach has the advantage that it works with Saxon 9.8 HE where xsl:evaluate is not supported:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:axsl="http://www.w3.org/1999/XSL/Transform-alias"
exclude-result-prefixes="axsl"
version="3.0">
<xsl:param name="config-uri" as="xs:string" select="'https://martin-honnen.github.io/xslt/2018/config-example1.xml'"/>
<xsl:param name="config-doc" as="document-node()" select="doc($config-uri)"/>
<xsl:namespace-alias stylesheet-prefix="axsl" result-prefix="xsl"/>
<xsl:variable name="generated-xslt">
<axsl:stylesheet version="3.0">
<axsl:mode on-no-match="shallow-copy"/>
<xsl:for-each select="$config-doc/map/from">
<axsl:template match="item[#type = '{#key}']">
<axsl:copy>
<axsl:value-of select="{.}"/>
</axsl:copy>
</axsl:template>
</xsl:for-each>
</axsl:stylesheet>
</xsl:variable>
<xsl:mode on-no-match="shallow-copy"/>
<xsl:template match="/">
<xsl:sequence
select="transform(map {
'source-node' : .,
'stylesheet-node' : $generated-xslt
})?output"/>
</xsl:template>
</xsl:stylesheet>
https://xsltfiddle.liberty-development.net/6qVRKvX/2