<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>
