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* Explanation of toXML example.

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This commit is contained in:
Eelco Dolstra 2006-10-04 15:20:19 +00:00
parent 0ef3bd3c37
commit 407c9fd520
1 changed files with 44 additions and 8 deletions
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52
doc/manual/writing-nix-expressions.xml
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@ -1606,7 +1606,8 @@ removeAttrs { x = 1; y = 2; z = 3; } ["a" "x" "z"]</screen>
</varlistentry>
<varlistentry><term><function>builtins.toFile</function>
<varlistentry
xml:id='builtin-toFile'><term><function>builtins.toFile</function>
<replaceable>name</replaceable> <replaceable>s</replaceable></term>
<listitem><para>Store the string <replaceable>s</replaceable> in a
@ -1713,6 +1714,40 @@ in foo</programlisting>
builder in a more structured format than plain environment
variables.</para>
<!-- TODO: more formally describe the schema of the XML
representation -->
<para><xref linkend='ex-toxml' /> shows an example where this is
the case. The builder is supposed to generate the configuration
file for a <link xlink:href='http://jetty.mortbay.org/'>Jetty
servlet container</link>. A servlet container contains a number
of servlets (<filename>*.war</filename> files) each exported under
a specific URI prefix. So the servlet configuration is a list of
attribute sets containing the <varname>path</varname> and
<varname>war</varname> of the servlet (<xref
linkend='ex-toxml-co-servlets' />). This kind of information is
difficult to communicate with the normal method of passing
information through an environment variable, which just
concatenates everything together into a string (which might just
work in this case, but wouldn’t work if fields are optional or
contain lists themselves). Instead the Nix expression is
converted to an XML representation with
<function>toXML</function>, which is unambiguous and can easily be
processed with the appropriate tools. For instance, in the
example an XSLT stylesheet (<xref linkend='ex-toxml-co-stylesheet'
/>) is applied to it (<xref linkend='ex-toxml-co-apply' />) to
generate the XML configuration file for the Jetty server. The XML
representation produced from <xref linkend='ex-toxml-co-servlets'
/> by <function>toXML</function> is shown in <xref
linkend='ex-toxml-result' />.</para>
<para>Note that <xref linkend='ex-toxml' /> uses the <function
linkend='builtin-toFile'>toFile</function> built-in to write the
builder and the stylesheet “inline” in the Nix expression. The
path of the stylesheet is spliced into the builder at
<literal>xsltproc ${stylesheet}
<replaceable>...</replaceable></literal>.</para>
<example xml:id='ex-toxml'><title>Passing information to a builder
using <function>toXML</function></title>
@ -1727,10 +1762,11 @@ stdenv.mkDerivation (rec {
builder = builtins.toFile "builder.sh" "
source $stdenv/setup
mkdir $out
echo $servlets | xsltproc ${stylesheet} - > $out/server-conf.xml
echo $servlets | xsltproc ${stylesheet} - > $out/server-conf.xml]]> <co xml:id='ex-toxml-co-apply' /> <![CDATA[
";
stylesheet = builtins.toFile "stylesheet.xsl" "<?xml version='1.0' encoding='UTF-8'?>
stylesheet = builtins.toFile "stylesheet.xsl"]]> <co xml:id='ex-toxml-co-stylesheet' /> <![CDATA[
"<?xml version='1.0' encoding='UTF-8'?>
<xsl:stylesheet xmlns:xsl='http://www.w3.org/1999/XSL/Transform' version='1.0'>
<xsl:template match='/'>
<Configure>
@ -1745,7 +1781,7 @@ stdenv.mkDerivation (rec {
</xsl:stylesheet>
";
servlets = builtins.toXML [
servlets = builtins.toXML []]> <co xml:id='ex-toxml-co-servlets' /> <![CDATA[
{ path = "/bugtracker"; war = jira + "/lib/atlassian-jira.war"; }
{ path = "/wiki"; war = uberwiki + "/uberwiki.war"; }
];
@ -1753,9 +1789,9 @@ stdenv.mkDerivation (rec {
</example>
<para>The string in the attribute <varname>servlets</varname>
evaluates to something like this:
<example xml:id='ex-toxml-result'><title>XML representation produced by
<function>toXML</function></title>
<programlisting><![CDATA[<?xml version='1.0' encoding='utf-8'?>
<expr>
<list>
@ -1778,7 +1814,7 @@ stdenv.mkDerivation (rec {
</list>
</expr>]]></programlisting>
</para>
</example>
</listitem>