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"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>
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<figure>
<title>The Yaws logo</title>
<mediaobject>
<imageobject>
<imagedata fileref="yaws_pb.gif" format="GIF" scale="50%"/>
</imageobject>
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</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
"<", ">" 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">  </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)->"  ";
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>
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