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7<title>NixOS overlay notes</title>
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20<h1 class="title">NixOS overlay notes</h1>
21</header><p>
22<a href="https://nixos.org">NixOS</a> has a concept of <code>overlays</code> since a while (<a href="https://nixos.org/nix-dev/2016-December/022386.html">around 2016</a>). The
23official manual has a chapter on it : <a href="https://nixos.org/nixpkgs/manual/#chap-overlays">Overlays</a>.
24</p>
25
26<blockquote>
27<p>
28Overlays are used to add layers in the fix-point used by Nixpkgs to
29compose the set of all packages.
30</p>
31</blockquote>
32
33<p>
34There is two distinct element to overlays:
35</p>
36
37<ul class="org-ul">
38<li>How to define overlays ?</li>
39<li>Where <code>nix</code> (and NixOS) looks for it ?</li>
40</ul>
41
42<section id="outline-container-h:56de4f99-53be-49fd-b969-a419d8e9273f" class="outline-2">
43<h2 id="h:56de4f99-53be-49fd-b969-a419d8e9273f">Define an overlay</h2>
44<div class="outline-text-2" id="text-h:56de4f99-53be-49fd-b969-a419d8e9273f">
45<p>
46According to the documentation, « Overlays are Nix functions which
47accept two arguments, conventionally called self and super, and return
48a set of packages ». The documentation gives an example :
49</p>
50
51<div class="org-src-container">
52<pre class="src src-nix">self: super:
53
54{
55 <span class="org-nix-attribute">boost</span> = super.boost.override {
56 <span class="org-nix-attribute">python</span> = self.python3;
57 };
58 <span class="org-nix-attribute">rr</span> = super.callPackage <span class="org-nix-constant">./pkgs/rr</span> {
59 <span class="org-nix-attribute">stdenv</span> = self.stdenv_32bit;
60 };
61}
62</pre>
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64
65<p>
66I used to have a personal channels on github : <a href="https://github.com/vdemeester/sbrpkgs/">sbrpkgs</a>. Let’s convert
67this repository to an overlay by updating the <code>default.nix</code> file.
68</p>
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