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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;
-    sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
-  };
-  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 a SHA-256 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>