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99 lines
3.1 KiB
XML
99 lines
3.1 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 content using
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overlays. Overlays are used to add layers in the fix-point used by Nixpkgs
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to bind the dependencies 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 are looked for in the following order, only 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 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 would be describe in <xref linkend="sec-overlays-layout"/>.</para>
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</listitem>
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<listitem>
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<para>As a directory pointed by the environment variable named
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<varname>NIXPKGS_OVERLAYS</varname>. This directory can contain symbolic
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links to Nix expressions.
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</listitem>
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<listitem>
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<para>As the directory located at
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<filename>~/.nixpkgs/overlays/</filename>. This directory can contain
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symbolic links to Nix expressions.
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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 contains either
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directories providing a default.nix expression, or files, or symbolic links
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to the entry Nix expression of the overlay. These Nix expressions are
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following the syntax described in <xref
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linkend="sec-overlays-layout"/>.</para>
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<para>To install an overlay, using the last option. Clone the repository of
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the overlay, and add a symbolic link to it in the
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<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>An overlay is a Nix expression, which is a function which accepts 2
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arguments.</para>
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<programlisting>
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self: super:
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{
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foo = super.foo.override { ... };
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bar = import ./pkgs/bar {
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inherit (self) stdenv fetchurl;
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inherit (self) ninja crawl dwarf-fortress;
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};
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}
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</programlisting>
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<para>The first argument, usualy named <varname>self</varname>, corresponds
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to the final package set. You should use this set to inherit all the
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dependencies needed by your package expression.</para>
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<para>The second argument, usualy 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 is used in to override
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existing packages, either by changing their dependencies or their
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recipes.</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 either
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extra packages defined by the overlay, or which overwrite Nixpkgs packages
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with other custom defaults. This is similar to <xref
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linkend="sec-modify-via-packageOverrides"/>.</para>
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</section>
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</chapter>
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