4.flake-parts/site/src/module-arguments.md

Module Arguments

The module system allows modules and submodules to be defined using plain attribute sets, or functions that return attribute sets. When a module is a function, various attributes may be passed to it.

Top-level Module Arguments

Top-level refers to the module passed to mkFlake, or any of the modules imported into it using imports.

The standard module system arguments are available in all modules and submodules. These are chiefly config, options, lib.

getSystem

A function from system string to the config of the appropriate perSystem.

moduleWithSystem

A function that brings the perSystem module arguments. This allows a module to reference the defining flake without introducing global variables.

{ moduleWithSystem, ... }:
{
  nixosModules.default = moduleWithSystem (
    perSystem@{ config }:  # NOTE: only explicit params will be in perSystem
    nixos@{ ... }:
    {
      services.foo.package = perSystem.config.packages.foo;
      imports = [ ./nixos-foo.nix ];
    }
  );
}

withSystem

Enter the scope of a system. Worked example:

{ withSystem, ... }:
{
  # perSystem = ...;

  nixosConfigurations.foo = withSystem "x86_64-linux" (ctx@{ pkgs, ... }:
    pkgs.nixos ({ config, lib, packages, pkgs, ... }: {
      _module.args.packages = ctx.config.packages;
      imports = [ ./nixos-configuration.nix ];
      services.nginx.enable = true;
      environment.systemPackages = [
        packages.hello
      ];
    }));
}