1. Using Extension Elements and Extension Functions with XSLT and XPath
Roger L. Costello
XML Technologies

2. Extension Elements
The XSL processor understands how to process xsl:template, xsl:apply-templates, xsl:if, xsl:for-each, etc
That is, it understands the vocabulary in the XSL namespace
XSL Processor implementers oftentimes provide additional elements that you may use in your stylesheet
These extension elements will belong to a namespace defined by the implementer

3. Example Extension Element: instruct the xsl processor to output to another file
Many of the xsl processor implementers provide an extension element that instructs the xsl processor to output the contents of the element to another file.
Thus, your stylesheet can generate multiple output files!
XSL
Processor
XML
XSL

4. Vendor-specific
Each implementor gives the extension element a different name:
saxon calls it: output
xalan calls it: write

5. How to use an extension element
1. Declare the namespace that the extension element belongs to:
saxon:
xmlns:saxon="
xalan:
xmlns:xalan="
2. Indicate that any element that is namespace qualified by the prefix
is an extension element, i.e., it has a specific meaning and should be
processed using the implementer's code:
extension-element-prefixes="saxon"
extension-element-prefixes="xalan"
3. Use the extension element:
<saxon:output href="...">
-- anything in here will go to the file specified ---
</saxon:output>
<xalan:write file="...">
</xalan:write>

6. Problem
Write a stylesheet which outputs the platinum members in one file, the gold members in another file, and the third file is an index to the other two files.

7. platinum.xml XSL FitnessCenter.xml Processor new-FitnessCenter.xml
<PlatinumMembers href="platinum.xml"/>
<GoldMembers href="gold.xml"/>
FitnessCenter.xml
new-FitnessCenter.xml
gold.xml
FitnessCenter.xsl

8. <xsl:copy-of select="xpath"/>
This element instructs an xsl processor to copy to the output file the element selected by xpath, plus all its descendents.
<xsl:template match="Member">
<xsl:copy-of select="."/>
</xsl:template>
This instructs the xsl processor to copy everything from <Member> to </Member>
i.e., the Member element and all its descendents.

9. See extension-example01
<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="
xmlns:saxon="
extension-element-prefixes="saxon"
version="1.0">
<xsl:output method="xml"/>
<xsl:template match="FitnessCenter">
<FitnessCenter>
<PlatinumMembers href="platinum.xml"/>
<saxon:output href="platinum.xml">
<PlatinumMembers>
<xsl:for-each
<xsl:copy-of select="."/>
</xsl:for-each>
</PlatinumMembers>
</saxon:output>
<GoldMembers href="gold.xml"/>
<saxon:output href="gold.xml">
<GoldMembers>
<xsl:for-each
</GoldMembers>
</FitnessCenter>
</xsl:template>
</xsl:stylesheet>
See extension-example01

10. Don’t forget extension-element-prefixes
The extension-element-prefixes is used to tell the xsl processor, "whenever you encounter an element with any of these prefixes listed here you are to treat it as an extension element, and process it using the implementer's code"
If you fail to do so the xsl processor will simply output the element literally (see extension-example02)

11. Extension Functions
We have seen some of the functions that XSL provides: substring(), contains(), substring-before, etc.
Many xsl processor implementers provide additional functions. You signify that a function is an extension function by namespace qualifying it.

12. Dynamic (run-time) Evaluation
Many xsl processor implementers give you an extension function that enables you to dynamically evaluate an expression.
That is, you can generate the expression on the fly, or read it in from an external file.
SAXON provides an extension function called evaluate to do this.

13. XSL
Processor
FitnessCenter.xml
results.xml
checkFitnessCenter.xml
FitnessCenter.xsl
This file contains expressions that are dynamically evaluated against FitnessCenter.xml
Example: provide an xpath expression that ensures that each Member's level attribute is
either Platinum or Gold, and nothing else.

14. Now any references is to elements in checkFitnessCenter.xml
<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="
xmlns:saxon="
extension-element-prefixes="saxon"
version="1.0">
<xsl:output method="xml"/>
<xsl:variable name="tests"
select="document('checkFitnessCenter.xml')"/>
<xsl:template match="/">
<xsl:variable name="here" select="."/>
<FitnessCenter-results>
<xsl:for-each select="$tests//xpath">
<result>
<xsl:variable name="xpath" select="."/>
<xsl:value-of select="$xpath"/>
<xsl:for-each select="$here">
<xsl:choose>
<xsl:when test="saxon:evaluate($xpath)">
<xsl:text> SUCCEEDED</xsl:text>
</xsl:when>
<xsl:otherwise>
<xsl:text> FAILED</xsl:text>
</xsl:otherwise>
</xsl:choose>
</xsl:for-each>
<xsl:text>
</xsl:text>
</result>
</FitnessCenter-results>
</xsl:template>
</xsl:stylesheet>
Now any references is to elements
in checkFitnessCenter.xml
Takes us back to referencing
elements in FitnessCenter.xml