<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xml:id="chap-overlays">

<title>Overlays</title>

<para>This chapter describes how to extend and change Nixpkgs packages using
overlays. Overlays are used to add layers in the fix-point used by Nixpkgs
to compose the set of all packages.</para>

<para>Nixpkgs can be configured with a list of overlays, which are
applied in order. This means that the order of the overlays can be significant
if multiple layers override the same package.</para>

<!--============================================================-->

<section xml:id="sec-overlays-install">
<title>Installing overlays</title>

<para>The list of overlays is determined as follows.</para>

<para>If the <varname>overlays</varname> argument is not provided explicitly, we look for overlays in a path. The path
is determined as follows:

<orderedlist>

  <listitem>
    <para>First, if an <varname>overlays</varname> argument to the nixpkgs function itself is given,
    then that is used.</para>

    <para>This can be passed explicitly when importing nipxkgs, for example 
    <literal>import &lt;nixpkgs> { overlays = [ overlay1 overlay2 ]; }</literal>.</para>
  </listitem>

  <listitem>
    <para>Otherwise, if the Nix path entry <literal>&lt;nixpkgs-overlays></literal> exists, we look for overlays
    at that path, as described below.</para>

    <para>See the section on <literal>NIX_PATH</literal> in the Nix manual for more details on how to 
    set a value for <literal>&lt;nixpkgs-overlays>.</literal></para>
  </listitem>

  <listitem>
    <para>If one of <filename>~/.config/nixpkgs/overlays.nix</filename> and
    <filename>~/.config/nixpkgs/overlays/</filename> exists, then we look for overlays at that path, as
    described below. It is an error if both exist.</para>
  </listitem>

</orderedlist>
</para>

<para>If we are looking for overlays at a path, then there are two cases:
<itemizedlist>
  <listitem>
    <para>If the path is a file, then the file is imported as a Nix expression and used as the list of
    overlays.</para>
  </listitem>

  <listitem>
    <para>If the path is a directory, then we take the content of the directory, order it
    lexicographically, and attempt to interpret each as an overlay by:
    <itemizedlist>
      <listitem>
        <para>Importing the file, if it is a <literal>.nix</literal> file.</para>
      </listitem>
      <listitem>
        <para>Importing a top-level <filename>default.nix</filename> file, if it is a directory.</para>
      </listitem>
    </itemizedlist>
    </para>
  </listitem>
</itemizedlist>
</para>

<para>On a NixOS system the value of the <literal>nixpkgs.overlays</literal> option, if present, 
is passed to the system Nixpkgs directly as an argument. Note that this does not affect the overlays for
non-NixOS operations (e.g. <literal>nix-env</literal>), which are looked up independently.</para>

<para>The <filename>overlays.nix</filename> option therefore provides a convenient way to use the same
overlays for a NixOS system configuration and user configuration: the same file can be used
as <filename>overlays.nix</filename> and imported as the value of <literal>nixpkgs.overlays</literal>.</para>

</section>

<!--============================================================-->

<section xml:id="sec-overlays-definition">
<title>Defining overlays</title>

<para>Overlays are Nix functions which accept two arguments, 
conventionally called <varname>self</varname> and <varname>super</varname>, 
and return a set of packages. For example, the following is a valid overlay.</para>

<programlisting>
self: super:

{
  boost = super.boost.override {
    python = self.python3;
  };
  rr = super.callPackage ./pkgs/rr {
    stdenv = self.stdenv_32bit;
  };
}
</programlisting>

<para>The first argument (<varname>self</varname>) corresponds to the final package
set. You should use this set for the dependencies of all packages specified in your
overlay. For example, all the dependencies of <varname>rr</varname> in the example above come
from <varname>self</varname>, as well as the overridden dependencies used in the
<varname>boost</varname> override.</para>

<para>The second argument (<varname>super</varname>)
corresponds to the result of the evaluation of the previous stages of
Nixpkgs. It does not contain any of the packages added by the current
overlay, nor any of the following overlays. This set should be used either
to refer to packages you wish to override, or to access functions defined
in Nixpkgs. For example, the original recipe of <varname>boost</varname>
in the above example, comes from <varname>super</varname>, as well as the
<varname>callPackage</varname> function.</para>

<para>The value returned by this function should be a set similar to
<filename>pkgs/top-level/all-packages.nix</filename>, containing
overridden and/or new packages.</para>

<para>Overlays are similar to other methods for customizing Nixpkgs, in particular
the <literal>packageOverrides</literal> attribute described in <xref linkend="sec-modify-via-packageOverrides"/>.
Indeed, <literal>packageOverrides</literal> acts as an overlay with only the 
<varname>super</varname> argument. It is therefore appropriate for basic use, 
but overlays are more powerful and easier to distribute.</para>

</section>

</chapter>
