Overlays: Difference between revisions

<blockquote>Overlays are Nix functions which accept two arguments, conventionally called final and prev, and return a set of packages. ... Overlays are similar to other methods for customizing Nixpkgs, in particular the packageOverrides ... Indeed, packageOverrides acts as an overlay with only the prev argument. It is therefore appropriate for basic use, but overlays are more powerful and easier to distribute.</blockquote>

The data flow around overlays, especially regarding <tt>prev</tt> and <tt>final</tt> arguments can be a bit confusing if you are not familiar with how overlays work. This graph shows the data flow:

[[File:Dram-overlay-self-super.png]]

As you can see, <tt>final</tt> is the same for every stage, but <tt>prev</tt> comes from only the stage before. So when you define an attribute <tt>foo</tt> in the set to override it, within that overlay <tt>final.foo</tt> will be it's version, and <tt>prev.foo</tt> will be the non-overriden version. This is why you see patterns like <tt>foo = prev.foo.override { ... }</tt>.

The names <tt>final</tt> and <tt>prev</tt> might remind you of inheritance in object-oriented languages. In fact, overlays are exactly the same thing as subclasses, with regards to overriding and calling methods. This data flow is also how objects know which method to call. This is probably why the two arguments got their names, too.

And <syntaxhighlight lang="nix">final: prev: firefox = final.firefox.override { ... };</syntaxhighlight> would cause infinite recursion.

Then, add the following contents to <tt>/etc/nixos/overlays-compat/overlays.nix</tt><ref>Based on [https://gitlab.com/samueldr/nixos-configuration/blob/3febd83b15210282d6435932944d426cd0a9e0ca/modules/overlays-compat/overlays.nix [[User:samueldr|@samueldr]]'s configuration: overlays-compat]</ref>:

       # sha256 = "0000000000000000000000000000000000000000000000000000";

       # wanted: sha256:0000000000000000000000000000000000000000000000000000

       # got:    sha256:173gxk0ymiw94glyjzjizp8bv8g72gwkjhacigd1an09jshdrjb4

       sha256 = "173gxk0ymiw94glyjzjizp8bv8g72gwkjhacigd1an09jshdrjb4";

         sha256 = "07sx98d422589gxr8wflfpkdd0k44kbagxl3b51i56ky2wfix7rc";

   gnome = prev.gnome.overrideScope' (gfinal: gprev: {

           sha256 = "m6PKjVxhGVuzsMBVA82UyJ6Cb1s6SMI0eRooa+F2MY8=";

           sha256 = "XBMD0chaV6GGg3R9/rQnsBejXspomVZz/a4Bvv/AHCA=";

           sha256 = "Pm6PTmsL2bW9JAHD1u0oUEqD1PCIErOlcuqlwvP593I=";

=== Overriding a package inside an attribute set ===

   vimPlugins = prev.vimPlugins // {

     indent-blankline-nvim-lua =

      prev.pkgs.callPackage ../packages/indent-blankline-nvim-lua { };

   };

rec {

     packageOverrides = final: prev: {

       my_stuff = prev.buildPythonPackage rec {

         src = prev.fetchPypi {

           sha256 = "0bp9bjqy1n6ij1zb86wz9lqa1dhla8qr1d7w2kxyn7jbj56sbmcw";

   pythonPackages = python.pkgs;

   my_stuff = pythonPackages.buildPythonPackage rec {

       sha256 = "0bp9bjqy1n6ij1zb86wz9lqa1dhla8qr1d7w2kxyn7jbj56sbmcw";

Here is an example of an R packages overlay, in which it can be seen how to provide different versions of packages then those available in the current R version. It should be noticed that in the case of R and Python the argument to <code>override</code> is named differently. Names of these can be find using <code>nix repl</code> and evaluating e.g. <code>python.override.__functionArgs</code>.

           sha256 = "1i0s1f7hla91yw1fdx0rn7c18dp6jwmg2mlww8dix1kk7qbxfjww";

           sha256 = "0ky6sq6n8px3b70s10hy99sccf3vcjjpdhamql5dr7i9igsf8nqy";

it is not possible to just override <code>cargoSha256</code>, instead cargoDeps has to be overriden

 

       sha256 = "sha256-w0hpyFXxltmOpbBKNQ2tfKRWELQzStc/ho1EcNyYaWc=";