pyrox.dev
Nixpkgs manual: expand documentation for overlays
authored by Michael Peyton Jones and committed by Nicolas B. Pierron d60b2886 3fde7111
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doc/overlays.xml
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doc/overlays.xml
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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 to by the Nix search path entry
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<literal><nixpkgs-overlays></literal>.</para>
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</listitem>
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<listitem>
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<para>In the directory <filename>~/.config/nixpkgs/overlays/</filename>.</para>
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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 options, 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, they 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 it in <filename>~/.config/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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</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 overridden 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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···
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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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<para>Nixpkgs can be configured with a list of overlays, which are
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applied in order. This means that the order of the overlays can be significant
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if multiple layers override the same package.</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 list of overlays is determined as follows:
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<orderedlist>
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<listitem>
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<para>First, if an <varname>overlays</varname> argument to the nixpkgs function itself is given,
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then that is used. This can be passed explicitly when importing nipxkgs, for example
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<literal>import <nixpkgs> { overlays = [ overlay1 overlay2 ] }</literal>.</para>
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<para>On a NixOS system the value of the <literal>nixpkgs.overlays</literal> option, if present,
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is passed to the system Nixpkgs in this way. Note that this does not affect the overlays for
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non-NixOS operations (e.g. <literal>nix-env</literal>), which are looked up independently.</para>
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</listitem>
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<listitem>
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<para>Otherwise, if the Nix path entry <literal><nixpkgs-overlays></literal> exists and is a
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directory, then the result is the set of overlays found in that directory, ordered lexicographically.</para>
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<para>See the section on <literal>NIX_PATH</literal> in the Nix manual for more details on how to
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set a value for <literal><nixpkgs-overlays>.</literal></para>
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</listitem>
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<listitem>
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<para>Otherwise, if <filename>~/.config/nixpkgs/overlays/</filename> exists and is a directory, then
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the result is the set of overlays found in that directory, ordered lexicographically.</para>
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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 options overlays can be provided as files,
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directories containing a <filename>default.nix</filename>, or symlinks to one of those.</para>
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<para>The last option provides a convenient way to install an overlay from a repository,
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by cloning the overlay's repository and adding a symbolic link to it in
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<filename>~/.config/nixpkgs/overlays/</filename>.</para>
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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>Overlays are Nix functions which accept two arguments,
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conventionally called <varname>self</varname> and <varname>super</varname>,
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and return a set of packages. For example, the following is a valid overlay.</para>
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<programlisting>
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self: super:
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}
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</programlisting>
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<para>The first argument (<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 overridden dependencies used in the
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<varname>boost</varname> override.</para>
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<para>The second argument (<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>, containing
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overridden and/or new packages.</para>
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<para>Overlays are similar to other methods for customizing Nixpkgs, in particular
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the <literal>packageOverrides</literal> attribute described in <xref linkend="sec-modify-via-packageOverrides"/>.
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Indeed, <literal>packageOverrides</literal> acts as an overlay with only the
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<varname>super</varname> argument. It is therefore appropriate for basic use,
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but overlays are more powerful and easier to distribute.</para>
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</section>
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