Trouble understanding overlays

Help
1

Okay, I read a couple of sources about overlays + posted on here, but still have no idea how overlays work.

Here is my nixos configuration folder structure

├── flake.lock
├── flake.nix
├── home
│   ├── default.nix
│   ├── overlays
│   │   ├── default.nix
│   │   └── steam.nix
│   └── programs
│       ├── common.nix
│       ├── default.nix
│       ├── texlive.nix
│       └── tmux.nix
├── hosts
│   └── my-host
│       ├── default.nix
│       ├── hardware-configuration.nix
│       └── power.nix
└── modules
    ├── awesome.nix
    └── system.nix

I tried, for the sake of understanding, to implement the steam overlay from Overlays | NixOS & Flakes Book. Here’s how I did it:

  1. this is the content of the file home/overlays/default.nix 
{ config, pkgs, lib, ... }:
{
  nixpkgs.overlays = [
    # Overlay: Define overlays in other files
    # The content of ./overlays/overlay3/default.nix is the same as above:
    # `(final: prev: { xxx = prev.xxx.override { ... }; })`
    (import ./steam.nix)
  ];
}
  1. this is the content of home/overlays/steam.nix (taken from Overlays | NixOS & Flakes Book)
(final: prev: {
  steam-over = prev.steam.override {
    pname = "steam-over";
    extraPkgs = pkgs: with pkgs; [
      keyutils
      libkrb5
      libpng
      libpulseaudio
      libvorbis
      stdenv.cc.cc.lib
      xorg.libXcursor
      xorg.libXi
      xorg.libXinerama
      xorg.libXScrnSaver
    ];
    extraProfile = "export GDK_SCALE=2";
  };
})
  1. I let home-manager know the include the overlays, via imports in home/default.nix 
{ config, pkgs, ...}:
{
  imports = [
    ./programs
    ./overlays
  ];
  home = {
    username = "some-username";
    homeDirectory = "/home/some-username";

    stateVersion = "23.05";
  };

  # workaround to fix manpages not building propertly
  manual.manpages.enable = false;

  # Let home-manager manage itself
  programs.home-manager.enable = true;
}
  1. then I try to install the new overlay, which i names steam-over via including it in the file home/programs/common.nix 
{ pkgs, ...} : {
  home.packages = with pkgs; [
    steam-over
  ];
}

However, when I try to rebuild and switch, I get an error:


error:
       … while calling the 'head' builtin

         at /nix/store/18v62ywqkpymp7asswy8d3239zii9pcd-source/lib/attrsets.nix:922:11:

          921|         || pred here (elemAt values 1) (head values) then
          922|           head values
             |           ^
          923|         else

       … while evaluating the attribute 'value'

         at /nix/store/18v62ywqkpymp7asswy8d3239zii9pcd-source/lib/modules.nix:807:9:

          806|     in warnDeprecation opt //
          807|       { value = builtins.addErrorContext "while evaluating the option `${showOption loc}':" value;
             |         ^
          808|         inherit (res.defsFinal') highestPrio;

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: undefined variable 'steam-over'

       at /nix/store/64lj8f9zmkshrznk66lfbsk9lkxfwsxp-source/home/programs/common.nix:103:5:

          103|     steam-over
             |     ^
          104|
2

You don’t define overlay as presented by the given link. I propose you to remove pname attribute and to rename steam-over to steam in steps 2 and 4. You should obtain the expect steam. Also an overlay is not the attribute set after the (final: prev: { . It is the all code which is an anonymous function with final and prev argument which refers to attribute set all packages (the final one the previous during evaluation iterations)