Restructuring the Nix manual

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+<section xmlns="http://docbook.org/ns/docbook"
+      xmlns:xlink="http://www.w3.org/1999/xlink"
+      xmlns:xi="http://www.w3.org/2001/XInclude"
+      version="5.0"
+      xml:id='sec-expression-syntax'>
+
+<title>Expression Syntax</title>
+
+<example xml:id='ex-hello-nix'><title>Nix expression for GNU Hello
+(<filename>default.nix</filename>)</title>
+<programlisting>
+{ stdenv, fetchurl, perl }: <co xml:id='ex-hello-nix-co-1' />
+
+stdenv.mkDerivation { <co xml:id='ex-hello-nix-co-2' />
+  name = "hello-2.1.1"; <co xml:id='ex-hello-nix-co-3' />
+  builder = ./builder.sh; <co xml:id='ex-hello-nix-co-4' />
+  src = fetchurl { <co xml:id='ex-hello-nix-co-5' />
+    url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz;
+    md5 = "70c9ccf9fac07f762c24f2df2290784d";
+  };
+  inherit perl; <co xml:id='ex-hello-nix-co-6' />
+}</programlisting>
+</example>
+
+<para><xref linkend='ex-hello-nix' /> shows a Nix expression for GNU
+Hello.  It's actually already in the Nix Packages collection in
+<filename>pkgs/applications/misc/hello/ex-1/default.nix</filename>.
+It is customary to place each package in a separate directory and call
+the single Nix expression in that directory
+<filename>default.nix</filename>.  The file has the following elements
+(referenced from the figure by number):
+
+<calloutlist>
+
+  <callout arearefs='ex-hello-nix-co-1'>
+
+    <para>This states that the expression is a
+    <emphasis>function</emphasis> that expects to be called with three
+    arguments: <varname>stdenv</varname>, <varname>fetchurl</varname>,
+    and <varname>perl</varname>.  They are needed to build Hello, but
+    we don't know how to build them here; that's why they are function
+    arguments.  <varname>stdenv</varname> is a package that is used
+    by almost all Nix Packages packages; it provides a
+    <quote>standard</quote> environment consisting of the things you
+    would expect in a basic Unix environment: a C/C++ compiler (GCC,
+    to be precise), the Bash shell, fundamental Unix tools such as
+    <command>cp</command>, <command>grep</command>,
+    <command>tar</command>, etc.  <varname>fetchurl</varname> is a
+    function that downloads files.  <varname>perl</varname> is the
+    Perl interpreter.</para>
+
+    <para>Nix functions generally have the form <literal>{ x, y, ...,
+    z }: e</literal> where <varname>x</varname>, <varname>y</varname>,
+    etc. are the names of the expected arguments, and where
+    <replaceable>e</replaceable> is the body of the function.  So
+    here, the entire remainder of the file is the body of the
+    function; when given the required arguments, the body should
+    describe how to build an instance of the Hello package.</para>
+
+  </callout>
+
+  <callout arearefs='ex-hello-nix-co-2'>
+
+    <para>So we have to build a package.  Building something from
+    other stuff is called a <emphasis>derivation</emphasis> in Nix (as
+    opposed to sources, which are built by humans instead of
+    computers).  We perform a derivation by calling
+    <varname>stdenv.mkDerivation</varname>.
+    <varname>mkDerivation</varname> is a function provided by
+    <varname>stdenv</varname> that builds a package from a set of
+    <emphasis>attributes</emphasis>.  A set is just a list of
+    key/value pairs where each key is a string and each value is an
+    arbitrary Nix expression.  They take the general form <literal>{
+    <replaceable>name1</replaceable> =
+    <replaceable>expr1</replaceable>; <replaceable>...</replaceable>
+    <replaceable>nameN</replaceable> =
+    <replaceable>exprN</replaceable>; }</literal>.</para>
+
+  </callout>
+
+  <callout arearefs='ex-hello-nix-co-3'>
+
+    <para>The attribute <varname>name</varname> specifies the symbolic
+    name and version of the package.  Nix doesn't really care about
+    these things, but they are used by for instance <command>nix-env
+    -q</command> to show a <quote>human-readable</quote> name for
+    packages.  This attribute is required by
+    <varname>mkDerivation</varname>.</para>
+
+  </callout>
+
+  <callout arearefs='ex-hello-nix-co-4'>
+
+    <para>The attribute <varname>builder</varname> specifies the
+    builder.  This attribute can sometimes be omitted, in which case
+    <varname>mkDerivation</varname> will fill in a default builder
+    (which does a <literal>configure; make; make install</literal>, in
+    essence).  Hello is sufficiently simple that the default builder
+    would suffice, but in this case, we will show an actual builder
+    for educational purposes.  The value
+    <command>./builder.sh</command> refers to the shell script shown
+    in <xref linkend='ex-hello-builder' />, discussed below.</para>
+
+  </callout>
+
+  <callout arearefs='ex-hello-nix-co-5'>
+
+    <para>The builder has to know what the sources of the package
+    are.  Here, the attribute <varname>src</varname> is bound to the
+    result of a call to the <command>fetchurl</command> function.
+    Given a URL and an MD5 hash of the expected contents of the file
+    at that URL, this function builds a derivation that downloads the
+    file and checks its hash.  So the sources are a dependency that
+    like all other dependencies is built before Hello itself is
+    built.</para>
+
+    <para>Instead of <varname>src</varname> any other name could have
+    been used, and in fact there can be any number of sources (bound
+    to different attributes).  However, <varname>src</varname> is
+    customary, and it's also expected by the default builder (which we
+    don't use in this example).</para>
+
+  </callout>
+
+  <callout arearefs='ex-hello-nix-co-6'>
+
+    <para>Since the derivation requires Perl, we have to pass the
+    value of the <varname>perl</varname> function argument to the
+    builder.  All attributes in the set are actually passed as
+    environment variables to the builder, so declaring an attribute
+
+    <programlisting>
+perl = perl;</programlisting>
+
+    will do the trick: it binds an attribute <varname>perl</varname>
+    to the function argument which also happens to be called
+    <varname>perl</varname>.  However, it looks a bit silly, so there
+    is a shorter syntax.  The <literal>inherit</literal> keyword
+    causes the specified attributes to be bound to whatever variables
+    with the same name happen to be in scope.</para>
+
+  </callout>
+
+</calloutlist>
+
+</para>
+
+</section>
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