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+<?xml version="1.0" encoding="iso-8859-1"?>
+<!DOCTYPE article
+ PUBLIC "-//OASIS//DTD Simplified DocBook XML V1.0//EN"
+ "http://www.oasis-open.org/docbook/xml/simple/1.0/sdocbook.dtd">
+
+<article lang="en" xml:lang="en" >
+ <articleinfo>
+ <title>XSLT like transformations in Erlang </title>
+ <subtitle>User Guide</subtitle>
+ <authorgroup>
+ <author>
+ <firstname>Mikael</firstname>
+ <surname>Karlsson</surname>
+ </author>
+ </authorgroup>
+ <revhistory>
+ <revision>
+ <revnumber>1.0</revnumber><date>2002-10-25</date>
+ <revremark>First Draft</revremark>
+ </revision>
+ <revision>
+ <revnumber>1.1</revnumber><date>2003-02-05</date>
+ <revremark>Moved module xserl to xmerl application, renamed to
+ xmerl_xs</revremark>
+ </revision>
+ </revhistory>
+ <abstract>
+ <para>Erlang has similarities to XSLT since both languages
+ have a functional programming approach. Using the xpath implementation
+ in the existing xmerl application it is possible to write XSLT
+ like transforms in Erlang. One can also combine the
+ transformations with the erlang scripting possibility
+ in the yaws webserver to implement "on the fly" html
+ conversions of xml documents.
+ </para>
+ </abstract>
+ </articleinfo>
+
+
+ <section>
+ <title>Terminology</title>
+ <variablelist>
+ <varlistentry>
+ <term>XML</term>
+ <listitem>
+ <para>Extensible Markup Language</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>XSLT</term>
+ <listitem>
+ <para>Extensible Stylesheet Language: Transformations</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </section>
+ <section>
+ <title>Introduction</title>
+ <para>XSLT stylesheets are often used when transforming XML
+ documents, to other XML documents or (X)HTML for presentation.
+ There are a number of brick-sized books written on the
+ topic. XSLT contains quite many
+ functions and learning them all may take some effort, which
+ could be a reason why the author only has reached a basic level of
+ understanding. This document assumes a basic level of
+ understanding of XSLT.
+ </para>
+ <para>Since XSLT is based on a functional programming approach
+ with pattern matching and recursion it is possible to write
+ similar style sheets in Erlang. At least for basic
+ transforms. XPath which is used in XSLT is also already
+ implemented in the xmerl application written i Erlang. This
+ document describes how to use the XPath implementation together
+ with Erlangs pattern matching and a couple of functions to write
+ XSLT like transforms.</para>
+ <para>This approach is probably easier for an Erlanger but
+ if you need to use real XSLT stylesheets in order to "comply to
+ the standard" there is an adapter available to the Sablotron
+ XSLT package which is written i C++.
+ </para>
+ <para>
+ This document is written in the Simplified Docbook DTD which is
+ a subset of the complete one and converted to xhtml using a
+ stylesheet written in Erlang.
+ </para>
+ </section>
+
+ <section>
+ <title>Tools</title>
+ <section>
+ <title>xmerl</title>
+ <para><ulink url="http://sowap.sourceforge.net/" >xmerl</ulink>
+ is a xml parser written in Erlang</para>
+ <section>
+ <title>xmerl_xpath</title>
+ <para>XPath is in important part of XSLT and is implemented in
+ xmerl</para>
+ </section>
+ <section>
+ <title>xmerl_xs</title>
+ <para>
+ <ulink url="xmerl_xs.yaws" >xmerl_xs</ulink> is a very small
+ module acting as "syntactic sugar" for the XSLT lookalike
+ transforms. It uses xmerl_xpath.
+ </para>
+ </section>
+ </section>
+
+ <section>
+ <title>yaws</title>
+ <para>
+ <ulink url="http://yaws.hyber.org/" >Yaws</ulink>, Yet Another
+ Webserver, is a web server written in Erlang that support dynamic
+ content generation using embedded scripts, also written in Erlang.
+ </para>
+<!--
+ <figure>
+ <title>The Yaws logo</title>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="yaws_pb.gif" format="GIF" scale="50%"/>
+ </imageobject>
+ </mediaobject>
+ </figure>
+-->
+ <para>Yaws is not needed to make the XSLT like transformations, but
+ combining yaws and xmerl it is possible to do transformations
+ of XML documents to HTML in realtime, when clients requests a
+ web page. As an example I am able to edit this document using
+ emacs with psgml tools, save the document and just do a reload
+ in my browser to see the result. The parse/transform time is not
+ visually different compared to loading any other document in the
+ browser.
+ </para>
+ </section>
+
+ </section>
+
+ <section>
+ <title>Transformations</title>
+<para>
+ When xmerl_scan parses an xml string/file it returns a record of:
+</para>
+ <programlisting>
+<![CDATA[
+ -record(xmlElement, {
+ name,
+ parents = [],
+ pos,
+ attributes = [],
+ content = [],
+ language = [],
+ expanded_name = [],
+ nsinfo = [],% {Prefix, Local} | []
+ namespace = #xmlNamespace{}
+ }).
+ ]]>
+</programlisting>
+<para>
+ Were content is a mixed list of yet other xmlElement records and/or
+ xmlText (or other node types).
+</para>
+ <section>
+ <title>xmerl_xs functions</title>
+ <para>
+ Functions used:
+ </para>
+ <variablelist>
+ <varlistentry>
+ <term>xslapply/2</term>
+ <listitem>
+ <para>function to make things look similar
+ to xsl:apply-templates.
+ </para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>value_of/1</term>
+ <listitem>
+ <para>Conatenates all text nodes within a tree.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>select/2</term>
+ <listitem>
+ <para>select(Str, E) extracts nodes from the XML tree using
+ xmerl_xpath.
+ </para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>built_in_rules/2</term>
+ <listitem>
+ <para>The default fallback behaviour, template funs should
+ end with:
+ <computeroutput>template(E)->built_in_rules(fun
+ template/1, E).
+</computeroutput>
+ </para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+<note><para>Text is escaped using xmerl_lib:export_text/1 for
+ "&lt;", "&gt;" and other relevant xml
+ characters when exported. So the value_of/1 and built_in_rules/2
+ functions should be replaced when not exporting to xml or html.
+</para></note>
+ </section>
+
+
+<section><title>Examples</title>
+ <example>
+ <title>Using xslapply</title>
+ <para>original XSLT:</para>
+ <programlisting>
+<![CDATA[
+ <xsl:template match="doc/title">
+ <h1>
+ <xsl:apply-templates/>
+ </h1>
+ </xsl:template>
+ ]]>
+ </programlisting>
+ <para>
+ becomes in Erlang:</para>
+ <programlisting>
+<![CDATA[
+ template(E = #xmlElement{ parents=[{'doc',_}|_], name='title'}) ->
+ ["<h1>",
+ xslapply(fun template/1, E),
+ "</h1>"];
+ ]]>
+ </programlisting>
+
+ </example>
+ <example>
+ <title>Using value_of and select</title>
+ <programlisting>
+<![CDATA[
+ <xsl:template match="title">
+ <div align="center"><h1><xsl:value-of select="." /></h1></div>
+ </xsl:template>
+ ]]>
+ </programlisting>
+ <para>
+ becomes:
+ </para>
+ <programlisting>
+<![CDATA[
+template(E = #xmlElement{name='title'}) ->
+ ["<div align=\"center\"><h1>", value_of(select(".", E)), "</h1></div>"];
+ ]]>
+ </programlisting>
+ </example>
+ <example>
+ <title>Simple xsl stylesheet</title>
+<para>
+ A complete example with the XSLT sheet in the xmerl distribution.
+</para>
+ <programlisting>
+<![CDATA[
+
+<xsl:stylesheet version="1.0"
+ xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
+ xmlns="http://www.w3.org/TR/xhtml1/strict">
+
+ <xsl:strip-space elements="doc chapter section"/>
+ <xsl:output
+ method="xml"
+ indent="yes"
+ encoding="iso-8859-1"
+ />
+
+ <xsl:template match="doc">
+ <html>
+ <head>
+ <title>
+ <xsl:value-of select="title"/>
+ </title>
+ </head>
+ <body>
+ <xsl:apply-templates/>
+ </body>
+ </html>
+ </xsl:template>
+
+ <xsl:template match="doc/title">
+ <h1>
+ <xsl:apply-templates/>
+ </h1>
+ </xsl:template>
+
+ <xsl:template match="chapter/title">
+ <h2>
+ <xsl:apply-templates/>
+ </h2>
+ </xsl:template>
+
+ <xsl:template match="section/title">
+ <h3>
+ <xsl:apply-templates/>
+ </h3>
+ </xsl:template>
+
+ <xsl:template match="para">
+ <p>
+ <xsl:apply-templates/>
+ </p>
+ </xsl:template>
+
+ <xsl:template match="note">
+ <p class="note">
+ <b>NOTE: </b>
+ <xsl:apply-templates/>
+ </p>
+ </xsl:template>
+
+ <xsl:template match="emph">
+ <em>
+ <xsl:apply-templates/>
+ </em>
+ </xsl:template>
+
+</xsl:stylesheet>
+ ]]>
+ </programlisting>
+ </example>
+ <example>
+ <title>Erlang version</title>
+ <para>
+ Erlang transformation of previous example:
+ </para>
+ <programlisting>
+<![CDATA[
+
+-include("xmerl.hrl").
+
+-import(xmerl_xs,
+ [ xslapply/2, value_of/1, select/2, built_in_rules/2 ]).
+
+doctype()->
+ "<!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Transitional//EN\"\
+ \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd \">".
+
+process_xml(Doc)->
+ template(Doc).
+
+template(E = #xmlElement{name='doc'})->
+ [ "<\?xml version=\"1.0\" encoding=\"iso-8859-1\"\?>",
+ doctype(),
+ "<html xmlns=\"http://www.w3.org/1999/xhtml\" >"
+ "<head>"
+ "<title>", value_of(select("title",E)), "</title>"
+ "</head>"
+ "<body>",
+ xslapply( fun template/1, E),
+ "</body>"
+ "</html>" ];
+
+
+template(E = #xmlElement{ parents=[{'doc',_}|_], name='title'}) ->
+ ["<h1>",
+ xslapply( fun template/1, E),
+ "</h1>"];
+
+template(E = #xmlElement{ parents=[{'chapter',_}|_], name='title'}) ->
+ ["<h2>",
+ xslapply( fun template/1, E),
+ "</h2>"];
+
+template(E = #xmlElement{ parents=[{'section',_}|_], name='title'}) ->
+ ["<h3>",
+ xslapply( fun template/1, E),
+ "</h3>"];
+
+template(E = #xmlElement{ name='para'}) ->
+ ["<p>", xslapply( fun template/1, E), "</p>"];
+
+template(E = #xmlElement{ name='note'}) ->
+ ["<p class=\"note\">"
+ "<b>NOTE: </b>",
+ xslapply( fun template/1, E),
+ "</p>"];
+
+template(E = #xmlElement{ name='emph'}) ->
+ ["<em>", xslapply( fun template/1, E), "</em>"];
+
+template(E)->
+ built_in_rules( fun template/1, E).
+ ]]>
+ </programlisting>
+ <para>
+ It is important to end with a call to
+ <computeroutput>xmerl_xs:built_in_rules/2</computeroutput>
+ if you want any text to be written in "push" transforms.
+ That are the ones using a lot <computeroutput>xslapply( fun
+ template/1, E )</computeroutput> instead of
+ <computeroutput>value_of(select("xpath",E))</computeroutput>,
+ which is pull...
+ </para>
+ </example>
+<para>The largest example is the stylesheet to transform this document
+ from the Simplified Docbook XML format to xhtml. The source
+ file is <computeroutput>sdocbook2xhtml.erl</computeroutput>.
+</para>
+</section>
+ <section>
+ <title>Tips and tricks</title>
+ <section>
+ <title>for-each</title>
+ <para>The function for-each is quite common in XSLT stylesheets.
+ It can often be rewritten and replaced by select/1. Since
+ select/1 returns a list of #xmlElements and xslapply/2
+ traverses them it is more or less the same as to loop over all
+ the elements.
+ </para>
+ </section>
+ <section>
+ <title>position()</title>
+ <para>The XSLT position() and #xmlElement.pos are not the
+ same. One has to make an own position in Erlang.</para>
+ <example>
+ <title>Counting positions</title>
+ <programlisting>
+<![CDATA[
+<xsl:template match="stanza">
+ <p><xsl:apply-templates select="line" /></p>
+</xsl:template>
+
+<xsl:template match="line">
+ <xsl:if test="position() mod 2 = 0">&#160;&#160;</xsl:if>
+ <xsl:value-of select="." /><br />
+</xsl:template>
+ ]]>
+ </programlisting>
+<para>Can be written as</para>
+ <programlisting>
+<![CDATA[
+template(E = #xmlElement{name='stanza'}) ->
+ {Lines,LineNo} = lists:mapfoldl(fun template_pos/2, 1, select("line", E)),
+ ["<p>", Lines, "</p>"].
+
+template_pos(E = #xmlElement{name='line'}, P) ->
+ {[indent_line(P rem 2), value_of(E#xmlElement.content), "<br />"], P + 1 }.
+
+indent_line(0)->"&#160;&#160;";
+indent_line(_)->"".
+ ]]>
+ </programlisting>
+ </example>
+ </section>
+ <section>
+ <title>Global tree awareness</title>
+ <para>In XSLT you have "root" access to the top of the tree
+ with XPath, even though you are somewhere deep in your
+ tree.</para>
+ <para>The xslapply/2 function only carries back the child part
+ of the tree to the template fun. But it is quite easy to write
+ template funs that handles both the child and top tree.</para>
+ <example>
+ <title>Passing the root tree</title>
+ <para>The following example piece will prepend the article
+ title to any section title</para>
+ <programlisting>
+<![CDATA[
+template(E = #xmlElement{name='title'}, ETop ) ->
+ ["<h3>", value_of(select("title", ETop))," - ",
+ xslapply( fun(A) -> template(A, ETop) end, E),
+ "</h3>"];
+ ]]>
+ </programlisting>
+ </example>
+ </section>
+ </section>
+
+ </section>
+
+
+ <section>
+ <title>Utility functions</title>
+ <para>
+ The module xmerl_xs contains the functions
+ <computeroutput>mapxml/2, foldxml/3</computeroutput> and
+ <computeroutput> mapfoldxml/3</computeroutput> to traverse
+ <literal>#xmlElement</literal> trees. They can be used in order
+ to build cross-references, see sdocbook2xhtml.erl for instance
+ where <computeroutput>foldxml/3</computeroutput> and
+ <computeroutput> mapfoldxml/3</computeroutput> are used to
+ number chapters, examples and figures and to build the Table of
+ contents for the document.
+ </para>
+ </section>
+
+
+ <section>
+ <title>Future enhancements</title>
+ <para>
+ More wish- than task-list at the moment.
+ </para>
+ <itemizedlist>
+ <listitem>
+ <para>More stylesheets</para>
+ </listitem>
+ <listitem>
+ <para>On the fly exports to PDF for printing and also more
+ "polished" presentations.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </section>
+
+ <section>
+ <title>References</title>
+ <orderedlist>
+ <listitem>
+ <para><ulink url="../xml/xmerl_xs.xml" >XML source
+ file</ulink> for this document.
+ </para>
+ </listitem>
+ <listitem>
+ <para><ulink url="../xs/sdocbook2xhtml.erl" >Erlang style
+ sheet</ulink> used for this document. (Simplified Docbook DTD).</para>
+ </listitem>
+ <listitem>
+ <para><ulink url="http://www.erlang.org/" >Open Source Erlang</ulink>
+ </para>
+ </listitem>
+ </orderedlist>
+
+ </section>
+</article>
+
+<!--
+Local Variables:
+mode: xml
+sgml-indent-step: 2
+sgml-indent-data: t
+sgml-set-face: t
+sgml-insert-missing-element-comment: nil
+End:
+-->
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