<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 using overlays.
  Overlays are used to add layers in the fixed-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 can be set either explicitly in a Nix expression, or
   through <literal>&lt;nixpkgs-overlays></literal> or user configuration
   files.
  </para>

  <section xml:id="sec-overlays-argument">
   <title>Set overlays in NixOS or Nix expressions</title>

   <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
    <link xlink:href="#sec-overlays-lookup">looked</link> up independently.
   </para>

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

   <para>
    Further overlays can be added by calling the <literal>pkgs.extend</literal>
    or <literal>pkgs.appendOverlays</literal>, although it is often preferable
    to avoid these functions, because they recompute the Nixpkgs fixpoint,
    which is somewhat expensive to do.
   </para>
  </section>

  <section xml:id="sec-overlays-lookup">
   <title>Install overlays via configuration lookup</title>

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

   <para>
    <orderedlist>
     <listitem>
      <para>
       First, if an
       <link xlink:href="#sec-overlays-argument"><varname>overlays</varname>
       argument</link> to the Nixpkgs function itself is given, then that is
       used and no path lookup will be performed.
      </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>
    Because overlays that are set in NixOS configuration do not affect
    non-NixOS operations such as <literal>nix-env</literal>, the
    <filename>overlays.nix</filename> option 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>

<!-- TODO: Example of sharing overlays between NixOS configuration
     and configuration lookup. Also reference the example
     from the sec-overlays-argument paragraph about NixOS.
 -->
  </section>
 </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>
