* Split overview chapter into a chapter on package management and a

  chapter on writing Nix expressions.

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+<chapter id='chap-writing-nix-expressions'><title>Writing Nix Expressions</title>
+
+<sect1><title>A simple Nix expression</title>
+
+<para>This section shows how to write simple Nix expressions&mdash;the
+things that describe how to build a package.</para>
+
+<example id='ex-hello-nix'><title>Nix expression for GNU Hello</title>
+<programlisting>
+{stdenv, fetchurl, perl}: <co id='ex-hello-nix-co-1' />
+
+derivation { <co id='ex-hello-nix-co-2' />
+  name = "hello-2.1.1"; <co id='ex-hello-nix-co-3' />
+  system = stdenv.system; <co id='ex-hello-nix-co-4' />
+  builder = ./builder.sh; <co id='ex-hello-nix-co-5' />
+  src = fetchurl { <co id='ex-hello-nix-co-6' />
+    url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz;
+    md5 = "70c9ccf9fac07f762c24f2df2290784d";
+  };
+  stdenv = stdenv; <co id='ex-hello-nix-co-7' />
+  perl = perl;
+}</programlisting>
+</example>
+
+<para>A simple Nix expression is shown in <xref linkend='ex-hello-nix'
+/>. It describes how to the build the <ulink
+url='http://www.gnu.org/directory/GNU/hello.html'>GNU Hello
+package</ulink>.  This package has several dependencies.  First, it
+requires a number of other packages, such as a C compiler, standard
+Unix shell tools, and Perl.  Rather than have this Nix expression
+refer to and use specific versions of these packages, it should be
+generic; that is, it should be a <emphasis>function</emphasis> that
+takes the required packages as inputs and yield a build of the GNU
+Hello package as a result.  This Nix expression defines a function
+with three arguments <xref linkend='ex-hello-nix-co-1' />, namely:
+
+  <orderedlist>
+    <listitem><para><varname>stdenv</varname>, which should be a
+    <emphasis>standard environment package</emphasis>.  The standard
+    environment is a set of tools and other components that would be
+    expected in a fairly minimal Unix-like environment: a C compiler
+    and linker, Unix shell tools, and so on.</para></listitem>
+    
+    <listitem><para><varname>fetchurl</varname>, which should be a
+    function that given parameters <varname>url</varname> and
+    <varname>md5</varname>, will fetch a file from the specified
+    location and check that this file has the given MD5 hash code.
+    The hash is required because build operations must be
+    <emphasis>pure</emphasis>: given the same inputs they should
+    always yield the same output.  Since network resources can change
+    at any time, we must in some way guarantee what the result will
+    be.</para></listitem>
+    
+    <listitem><para><varname>perl</varname>, which should be a Perl
+    interpreter.</para></listitem>
+    
+  </orderedlist>
+</para>
+
+<para>The remainder of the file is the body of the function, which
+happens to be a <emphasis>derivation</emphasis> <xref
+linkend='ex-hello-nix-co-2' />, which is the built-in function
+<varname>derivation</varname> applied to a set of attributes that
+encode all the necessary information for building the GNU Hello
+package.</para>
+
+<example><title>Build script (<filename>builder.sh</filename>) for GNU
+Hello</title>
+<programlisting>
+#! /bin/sh
+
+buildinputs="$perl"
+. $stdenv/setup || exit 1
+
+tar xvfz $src || exit 1
+cd hello-* || exit 1
+./configure --prefix=$out || exit 1
+make || exit 1
+make install || exit 1</programlisting>
+</example>
+
+</sect1>
+
+
+<sect1><title>A more complex Nix expression</title>
+
+<example id='ex-svn-nix'><title>Nix expression for Subversion</title>
+<programlisting>
+{ localServer ? false <co id='ex-svn-nix-co-1' />
+, httpServer ? false
+, sslSupport ? false
+, swigBindings ? false
+, stdenv, fetchurl
+, openssl ? null, httpd ? null, db4 ? null, expat, swig ? null
+}:
+
+assert !isNull expat; <co id='ex-svn-nix-co-2' />
+assert localServer -> !isNull db4;
+assert httpServer -> !isNull httpd &amp;&amp; httpd.expat == expat; <co id='ex-svn-nix-co-3' />
+assert sslSupport -> !isNull openssl &amp;&amp; (httpServer -> httpd.openssl == openssl);
+assert swigBindings -> !isNull swig;
+
+derivation {
+  name = "subversion-0.32.1";
+  system = stdenv.system;
+
+  builder = ./builder.sh;
+  src = fetchurl {
+    url = http://svn.collab.net/tarballs/subversion-0.32.1.tar.gz;
+    md5 = "b06717a8ef50db4b5c4d380af00bd901";
+  };
+
+  localServer = localServer;
+  httpServer = httpServer;
+  sslSupport = sslSupport;
+  swigBindings = swigBindings;
+
+  stdenv = stdenv;
+  openssl = if sslSupport then openssl else null; <co id='ex-svn-nix-co-4' />
+  httpd = if httpServer then httpd else null;
+  expat = expat;
+  db4 = if localServer then db4 else null;
+  swig = if swigBindings then swig else null;
+}</programlisting>
+</example>
+
+<para>This example shows several features.  Default parameters <xref
+linkend='ex-svn-nix-co-1'/> can be used to simplify call sites: if an
+argument that has a default is omitted, its default value is
+used.</para>
+
+<para>You can use <emphasis>assertions</emphasis> to test whether
+arguments satisfy certain constraints.  The simple assertion <xref
+linkend='ex-svn-nix-co-2'/> tests whether the <varname>expat</varname>
+argument is not a null value.  The more complex assertion <xref
+linkend='ex-svn-nix-co-3'/> says that if Subversion is built with
+Apache support, then <varname>httpd</varname> (the Apache package)
+must not be null and it must have been built using the same instance
+of the <varname>expat</varname> library as was passed to the
+Subversion expression.  This is since the Subversion code is
+dynamically linked against the Apache code and they both use Expat,
+they must be linked against the same instance&mdash;otherwise a
+conflict might occur.</para>
+
+</sect1>
+
+
+</chapter>