Adding new overlays

Overlays provide a way to extend and modify the Nixpkgs package set.

Say you wanted the latest version of picom from the next branch, you would use an overlay to modify the existing definition of picom to point to the next branch.

In overlays/picom.nix:

final: prev: {
  picom = prev.picom.overrideAttrs (old: {
    src = prev.fetchFromGitHub {
      owner = "yshui";
      repo = old.pname;
      rev = "468d8a0879d619f3028ea371c1b3ad34c6ae2183";
      sha256 = "sha256-HY4xMh33ycWzlK3bo7jrQm3uVn4toO56u8qnYKmdxW0=";
    };
  });
}

and my configuration will "automagically" import and use this overlay.

To learn more about overlays see the NixOS Wiki.