Nixpkgs module

[roberth]

Prototype a more extensive Nixpkgs module in flake-parts before perhaps eventually doing #41

https://twitter.com/ldesgoui/status/1592576468789628928

[srid]

Could we have more than one pkgs as a result? For, eg., native pkgs and a rosetta pkgs (on aarch64-darwin).

[roberth]

Good idea. The root nixpkgs.* options could be permanent aliases for nixpkgsSets.pkgs or something.

[roberth]

For Rosetta, you can already get a pkgs via getSystem and/or withSystem. Those will also give you access to your own packages for that system.
The feature still makes a lot of sense for having different versions of Nixpkgs, but for mixed-system and cross, it'd be better to use and improve the perSystem infrastructure.

[Atry]

I would just use https://github.com/NixOS/nixpkgs/blob/3b4e70e987a42792196294560a233d285b1c1d10/nixos/modules/misc/nixpkgs.nix

    ml-ops.inputs.flake-parts.lib.mkFlake { inherit inputs; } {
      perSystem = { pkgs, system, ... }:
        {
          imports = [
            "${inputs.nixpkgs}/nixos/modules/misc/nixpkgs.nix"
          ];
          nixpkgs = {
            hostPlatform = system;
            
            
            # other options, e.g. 
            # config.allowUnfree = true;
          };
       };
    }

[roberth]

I'd previously opened NixOS/nixpkgs#231940 with the idea to import from that, but I've since realized that Nixpkgs needs an exception, because imports will fetch it eagerly, and that's a problem for such a large dependency, when it might not even be used (e.g. easyOverlay use case; see #147 (comment))

I'm sorry to have gone back and forth on this, but at least we didn't commit to a mistake in the meanwhile.

Aforementioned Nixpkgs makes a number of improvements which we should adopt in the flake-parts Nixpkgs module.

It could look as follows:

• flake-parts.modules.flake.pkgs is a new module
• it declares in perSystem.nixpkgs.<name>:
  • hostPlatform (default: system)
  • buildPlatform (build elsewhere for cross)
  • pkgs (alias for _module.args.pkgs)
  • overlays
  • config
  • source (default: inputs.nixpkgs)
• <name> causes _module.args.<name> to be set
  • if enabled?
• it does not have the legacy options system, localSystem or crossSystem
• the perSystem.pkgs option is an alias for perSystem.nixpkgs.pkgs
  • or should this be the other way around? That way it's more NixOS compatible.
    e.g. perSystem.nixpkgs.config.allowUnfree = true;
    or pkgs.pkgs-unstable.source = inputs.nixpkgs-unstable;

[tie]

It’d also be nice to have readOnlyPkgs module, similar to what we have for NixOS.