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<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:
<orderedlist>
<listitem>
<para>First, if an <varname>overlays</varname> argument to the nixpkgs function itself is given,
then that is used. This can be passed explicitly when importing nipxkgs, for example
<literal>import <nixpkgs> { overlays = [ overlay1 overlay2 ] }</literal>.</para>
<para>On a NixOS system the value of the <literal>nixpkgs.overlays</literal> option, if present,
is passed to the system Nixpkgs in this way. Note that this does not affect the overlays for
non-NixOS operations (e.g. <literal>nix-env</literal>), which are looked up independently.</para>
</listitem>
<listitem>
<para>Otherwise, if the Nix path entry <literal><nixpkgs-overlays></literal> exists and is a
directory, then the result is the set of overlays found in that directory, ordered lexicographically.</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><nixpkgs-overlays>.</literal></para>
</listitem>
<listitem>
<para>Otherwise, if <filename>~/.config/nixpkgs/overlays/</filename> exists and is a directory, then
the result is the set of overlays found in that directory, ordered lexicographically.</para>
</listitem>
</orderedlist>
</para>
<para>For the second and third options overlays can be provided as files,
directories containing a <filename>default.nix</filename>, or symlinks to one of those.</para>
<para>The last option provides a convenient way to install an overlay from a repository,
by cloning the overlay's repository and adding a symbolic link to it in
<filename>~/.config/nixpkgs/overlays/</filename>.</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>
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