How to make changes to _module.args.pkgs available to hosts?

[z0al]

If I make changes to pkgs e.g. adding overlay as shown below:

{
  perSystem = { pkgs, system, ... }: {
    _module.args.pkgs = import nixpkgs { 
		inherit system; 
		overlays = [ myOverlay ]; };
  	};
}

These are not picked up automatically by lib.nixosSystem or lib.darwinSystem and I get errors when trying to use something from the overlay.

The only way I could get it to work is to use withSystem and pass pkgs in specialArgs but it feels rather hacky:

 withSystem "x86_64-linux" ({ system, pkgs, ... }:
	inputs.lib.nixosSystem {
		inherit system;
		specialArgs = {
			inherit pkgs inputs; # <-- this
		};
        # ... rest of config omitted 
	}

Is that the correct way to do things?

[roberth]

IIRC that allows nixpkgs.* to be set by modules, which means that some definitions that NixOS modules want to set will be ignored silently.
Ideally NixOS would detect specialArgs.pkgs and do this for you, but I'd recommend to import nixpkgs.nixosModules.readOnlyPkgs when you do that.

If you do want NixOS modules to further change your Nixpkgs package set, and specifically config, you could pass the overlay into nixpkgs.overlays. If you only need NixOS modules to set nixpkgs.overlays and not nixpkgs.config, you could set nixpkgs.pkgs + nixpkgs.overlays, but that's no more efficient than just passing the overlays without nixpkgs.pkgs.

This is way too confusing :(

Things to be improved

• NixOS could detect specialArgs.pkgs. Maybe not fail like readOnlyPkgs, but warn instead?
• Here, we should have a Nixpkgs module #74

[z0al]

Thanks for the detailed answer.

I imported nixpkgs.nixosModules.readOnlyPkgs as you recommended. But yeah, this is indeed super confusing.

Here, we should have a #74

Sounds exciting.