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196 lines
6.7 KiB
XML
196 lines
6.7 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xml:id="chap-overlays">
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<title>Overlays</title>
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<para>
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This chapter describes how to extend and change Nixpkgs using overlays.
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Overlays are used to add layers in the fixed-point used by Nixpkgs to compose
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the set of all packages.
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</para>
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<para>
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Nixpkgs can be configured with a list of overlays, which are applied in
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order. This means that the order of the overlays can be significant if
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multiple layers override the same package.
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</para>
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<!--============================================================-->
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<section xml:id="sec-overlays-install">
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<title>Installing overlays</title>
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<para>
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The list of overlays can be set either explicitly in a Nix expression, or
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through <literal><nixpkgs-overlays></literal> or user configuration
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files.
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</para>
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<section xml:id="sec-overlays-argument">
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<title>Set overlays in NixOS or Nix expressions</title>
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<para>
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On a NixOS system the value of the <literal>nixpkgs.overlays</literal>
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option, if present, is passed to the system Nixpkgs directly as an
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argument. Note that this does not affect the overlays for non-NixOS
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operations (e.g. <literal>nix-env</literal>), which are
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<link xlink:href="#sec-overlays-lookup">looked</link> up independently.
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</para>
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<para>
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The list of overlays can be passed explicitly when importing nixpkgs, for
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example <literal>import <nixpkgs> { overlays = [ overlay1 overlay2 ];
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}</literal>.
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</para>
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<para>
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Further overlays can be added by calling the <literal>pkgs.extend</literal>
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or <literal>pkgs.appendOverlays</literal>, although it is often preferable
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to avoid these functions, because they recompute the Nixpkgs fixpoint,
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which is somewhat expensive to do.
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</para>
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</section>
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<section xml:id="sec-overlays-lookup">
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<title>Install overlays via configuration lookup</title>
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<para>
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The list of overlays is determined as follows.
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</para>
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<para>
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<orderedlist>
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<listitem>
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<para>
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First, if an
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<link xlink:href="#sec-overlays-argument"><varname>overlays</varname>
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argument</link> to the Nixpkgs function itself is given, then that is
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used and no path lookup will be performed.
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</para>
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</listitem>
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<listitem>
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<para>
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Otherwise, if the Nix path entry
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<literal><nixpkgs-overlays></literal> exists, we look for overlays at
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that path, as described below.
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</para>
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<para>
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See the section on <literal>NIX_PATH</literal> in the Nix manual for
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more details on how to set a value for
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<literal><nixpkgs-overlays>.</literal>
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</para>
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</listitem>
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<listitem>
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<para>
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If one of <filename>~/.config/nixpkgs/overlays.nix</filename> and
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<filename>~/.config/nixpkgs/overlays/</filename> exists, then we look
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for overlays at that path, as described below. It is an error if both
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exist.
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</para>
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</listitem>
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</orderedlist>
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</para>
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<para>
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If we are looking for overlays at a path, then there are two cases:
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<itemizedlist>
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<listitem>
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<para>
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If the path is a file, then the file is imported as a Nix expression and
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used as the list of overlays.
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</para>
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</listitem>
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<listitem>
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<para>
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If the path is a directory, then we take the content of the directory,
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order it lexicographically, and attempt to interpret each as an overlay
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by:
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<itemizedlist>
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<listitem>
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<para>
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Importing the file, if it is a <literal>.nix</literal> file.
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</para>
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</listitem>
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<listitem>
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<para>
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Importing a top-level <filename>default.nix</filename> file, if it is
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a directory.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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Because overlays that are set in NixOS configuration do not affect
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non-NixOS operations such as <literal>nix-env</literal>, the
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<filename>overlays.nix</filename> option provides a convenient way to use
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the same overlays for a NixOS system configuration and user configuration:
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the same file can be used as <filename>overlays.nix</filename> and imported
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as the value of <literal>nixpkgs.overlays</literal>.
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</para>
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<!-- TODO: Example of sharing overlays between NixOS configuration
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and configuration lookup. Also reference the example
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from the sec-overlays-argument paragraph about NixOS.
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-->
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</section>
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</section>
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<!--============================================================-->
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<section xml:id="sec-overlays-definition">
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<title>Defining overlays</title>
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<para>
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Overlays are Nix functions which accept two arguments, conventionally called
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<varname>self</varname> and <varname>super</varname>, and return a set of
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packages. For example, the following is a valid overlay.
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</para>
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<programlisting>
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self: super:
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{
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boost = super.boost.override {
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python = self.python3;
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};
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rr = super.callPackage ./pkgs/rr {
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stdenv = self.stdenv_32bit;
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};
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}
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</programlisting>
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<para>
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The first argument (<varname>self</varname>) corresponds to the final
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package set. You should use this set for the dependencies of all packages
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specified in your overlay. For example, all the dependencies of
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<varname>rr</varname> in the example above come from
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<varname>self</varname>, as well as the overridden dependencies used in the
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<varname>boost</varname> override.
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</para>
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<para>
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The second argument (<varname>super</varname>) corresponds to the result of
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the evaluation of the previous stages of Nixpkgs. It does not contain any of
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the packages added by the current overlay, nor any of the following
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overlays. This set should be used either to refer to packages you wish to
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override, or to access functions defined in Nixpkgs. For example, the
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original recipe of <varname>boost</varname> in the above example, comes from
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<varname>super</varname>, as well as the <varname>callPackage</varname>
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function.
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</para>
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<para>
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The value returned by this function should be a set similar to
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<filename>pkgs/top-level/all-packages.nix</filename>, containing overridden
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and/or new packages.
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</para>
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<para>
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Overlays are similar to other methods for customizing Nixpkgs, in particular
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the <literal>packageOverrides</literal> attribute described in
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<xref linkend="sec-modify-via-packageOverrides"/>. Indeed,
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<literal>packageOverrides</literal> acts as an overlay with only the
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<varname>super</varname> argument. It is therefore appropriate for basic
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use, but overlays are more powerful and easier to distribute.
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</para>
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</section>
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</chapter>
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