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Nixpkgs manual: expand documentation for overlays

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Michael Peyton Jones 2017-08-09 19:00:30 +01:00 committed by Nicolas B. Pierron
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doc/overlays.xml
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@ -8,59 +8,62 @@
overlays. Overlays are used to add layers in the fix-point used by Nixpkgs overlays. Overlays are used to add layers in the fix-point used by Nixpkgs
to compose the set of all packages.</para> 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"> <section xml:id="sec-overlays-install">
<title>Installing Overlays</title> <title>Installing overlays</title>
<para>The set of overlays is looked for in the following places. The <para>The list of overlays is determined as follows:
first one present is considered, and all the rest are ignored:
<orderedlist> <orderedlist>
<listitem> <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 &lt;nixpkgs> { overlays = [ overlay1 overlay2 ] }</literal>.</para>
<para>As an argument of the imported attribute set. When importing Nixpkgs, <para>On a NixOS system the value of the <literal>nixpkgs.overlays</literal> option, if present,
the <varname>overlays</varname> attribute argument can be set to a list of is passed to the system Nixpkgs in this way. Note that this does not affect the overlays for
functions, which is described in <xref linkend="sec-overlays-layout"/>.</para> non-NixOS operations (e.g. <literal>nix-env</literal>), which are looked up independently.</para>
</listitem> </listitem>
<listitem> <listitem>
<para>Otherwise, if the Nix path entry <literal>&lt;nixpkgs-overlays></literal> exists and is a
directory, then the result is the set of overlays found in that directory, ordered lexicographically.</para>
<para>In the directory pointed to by the Nix search path entry <para>See the section on <literal>NIX_PATH</literal> in the Nix manual for more details on how to
<literal>&lt;nixpkgs-overlays></literal>.</para> set a value for <literal>&lt;nixpkgs-overlays>.</literal></para>
</listitem> </listitem>
<listitem> <listitem>
<para>Otherwise, if <filename>~/.config/nixpkgs/overlays/</filename> exists and is a directory, then
<para>In the directory <filename>~/.config/nixpkgs/overlays/</filename>.</para> the result is the set of overlays found in that directory, ordered lexicographically.</para>
</listitem> </listitem>
</orderedlist> </orderedlist>
</para> </para>
<para>For the second and third options, the directory should contain Nix expressions defining the <para>For the second and third options overlays can be provided as files,
overlays. Each overlay can be a file, a directory containing a directories containing a <filename>default.nix</filename>, or symlinks to one of those.</para>
<filename>default.nix</filename>, or a symlink to one of those. The expressions should follow
the syntax described in <xref linkend="sec-overlays-layout"/>.</para>
<para>The order of the overlay layers can influence the recipe of packages if multiple layers override <para>The last option provides a convenient way to install an overlay from a repository,
the same recipe. In the case where overlays are loaded from a directory, they are loaded in by cloning the overlay's repository and adding a symbolic link to it in
alphabetical order.</para> <filename>~/.config/nixpkgs/overlays/</filename>.</para>
<para>To install an overlay using the last option, you can clone the overlay's repository and add
a symbolic link to it in <filename>~/.config/nixpkgs/overlays/</filename> directory.</para>
</section> </section>
<!--============================================================--> <!--============================================================-->
<section xml:id="sec-overlays-layout"> <section xml:id="sec-overlays-definition">
<title>Overlays Layout</title> <title>Defining overlays</title>
<para>Overlays are expressed as Nix functions which accept 2 arguments and return a set of <para>Overlays are Nix functions which accept two arguments,
packages.</para> 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> <programlisting>
self: super: self: super:
@ -75,25 +78,31 @@ self: super:
} }
</programlisting> </programlisting>
<para>The first argument, usually named <varname>self</varname>, corresponds to the final package <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 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 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 from <varname>self</varname>, as well as the overridden dependencies used in the
<varname>boost</varname> override.</para> <varname>boost</varname> override.</para>
<para>The second argument, usually named <varname>super</varname>, <para>The second argument (<varname>super</varname>)
corresponds to the result of the evaluation of the previous stages of 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 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 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 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 Nixpkgs. For example, the original recipe of <varname>boost</varname>
in the above example, comes from <varname>super</varname>, as well as the in the above example, comes from <varname>super</varname>, as well as the
<varname>callPackage</varname> function.</para> <varname>callPackage</varname> function.</para>
<para>The value returned by this function should be a set similar to <para>The value returned by this function should be a set similar to
<filename>pkgs/top-level/all-packages.nix</filename>, which contains <filename>pkgs/top-level/all-packages.nix</filename>, containing
overridden and/or new packages.</para> 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> </section>
</chapter> </chapter>