[[!meta  title="eXtensible Style Language Transforms"]]

Often data is stored in
<abbr title="eXtensible Markup Language">XML</abbr> files must be
massaged into other formats (e.g. [[DocBook to roff|DocBook 5]]).
There are well developed procedures for defining such transformations
([XSLT][]) and a number of tools to apply them (e.g. [xsltproc][]).

Besides the [W3 tutorial][w3], there is also a nice
[introduction][intro] by Paul Grosso and Norman Walsh.  I've copied a
simple [[example|chapter]] from this intro and also included a
[[slightly more complicated setup|code]] for generating online help
for a list of macros.

XSLT is also useful for standardizing XML content.  For example, I was
recently trying to compare to [[Gramps]] XML files, to see what had
changed between two database backups.  Unfortunately, the backup XML
was not sorted by `id`, so there were many diff chunks due to node
shuffling that didn't represent any useful information.  With the
following XSLT:

    <?xml version="1.0"?>
    <xsl:stylesheet version="1.0"
                    xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    	<xsl:output method="xml" indent="yes"/>
    	<xsl:strip-space elements="*"/>
    	<!-- identity transform -->
    	<xsl:template match="@*|node()">
    		<xsl:copy>
    			<xsl:apply-templates select="@*|node()"/>
    		</xsl:copy>
    	</xsl:template>
    	<!-- sort node children by their `id` attributes -->
    	<xsl:template match="node()">
    		<xsl:copy>
    			<xsl:apply-templates select="@*"/>
    			<xsl:for-each select="node()">
    				<xsl:sort select="@id" order="ascending"/>
    				<xsl:apply-templates select="."/>
    			</xsl:for-each>
    		</xsl:copy>
    	</xsl:template>
    </xsl:stylesheet>

With the above saved as `sort-by-id.xsl`, you can sort `some.xml` using

    $ xsltproc --nonet --novalid sort-by-id.xsl some.xml

You can compare two [[Gramps]] XML files with

    $ diff -u <(zcat a.gramps | xsltproc --nonet --novalid sort-by-id.xsl -)
              <(zcat b.gramps | xsltproc --nonet --novalid sort-by-id.xsl -) | less

Jesper Tverskov has a nice page about [the identity template and
related tricks][identity] if you want more examples of quasi-copy
transforms.

[XSLT]: http://www.w3.org/TR/xslt
[w3]: http://www.w3schools.com/xsl/
[intro]: http://nwalsh.com/docs/tutorials/xsl/xsl/slides.html
[xsltproc]: http://www.xmlsoft.org/XSLT/
[identity]: http://www.xmlplease.com/xsltidentity

[[!tag tags/tools]]
[[!tag tags/web]]