100 lines
3.4 KiB
XML
100 lines
3.4 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>This chapter describes how to extend and change Nixpkgs packages using
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overlays. Overlays are used to add layers in the fix-point used by Nixpkgs
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to compose the set of all packages.</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>The set of overlays is looked for in the following places. The
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first one present is considered, and all the rest are ignored:
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<orderedlist>
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<listitem>
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<para>As an argument of the imported attribute set. When importing Nixpkgs,
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the <varname>overlays</varname> attribute argument can be set to a list of
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functions, which is described in <xref linkend="sec-overlays-layout"/>.</para>
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</listitem>
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<listitem>
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<para>In the directory pointed by the environment variable
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<varname>NIXPKGS_OVERLAYS</varname>.
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</listitem>
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<listitem>
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<para>In the directory <filename>~/.nixpkgs/overlays/</filename>.
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</listitem>
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</orderedlist>
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</para>
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<para>For the second and third option, the directory should contain Nix expressions defining the
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overlays. Each overlay can be a file, a directory containing a
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<filename>default.nix</filename>, or a symlink to one of those. The expressions should follow
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the syntax described in <xref linkend="sec-overlays-layout"/>.</para>
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<para>The order of the overlay layers can influence the recipe of packages if multiple layers override
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the same recipe. In the case where overlays are loaded from a directory, these are loaded in
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alphabetical order.</para>
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<para>To install an overlay using the last option, you can clone the overlay's repository and add
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a symbolic link to in the <filename>~/.nixpkgs/overlays/</filename> directory.</para>
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</section>
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<!--============================================================-->
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<section xml:id="sec-overlays-layout">
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<title>Overlays Layout</title>
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<para>Overlays are expressed as Nix functions which accept 2 arguments and return a set of
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packages</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>The first argument, usually named <varname>self</varname>, corresponds to the final package
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set. You should use this set for the dependencies of all packages specified in your
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overlay. For example, all the dependencies of <varname>rr</varname> in the example above come
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from <varname>self</varname>, as well as the overriden dependencies used in the
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<varname>boost</varname> override.</para>
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<para>The second argument, usually named <varname>super</varname>,
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corresponds to the result of the evaluation of the previous stages of
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Nixpkgs. It does not contain any of the packages added by the current
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overlay nor any of the following overlays. This set should be used either
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to refer to packages you wish to override, or to access functions defined
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in Nixpkgs. For example, the original recipe of <varname>boost</varname>
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in the above example, comes from <varname>super</varname>, as well as the
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<varname>callPackage</varname> function.</para>
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<para>The value returned by this function should be a set similar to
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<filename>pkgs/top-level/all-packages.nix</filename>, which contains
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overridden and/or new packages.</para>
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</section>
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</chapter>
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