nixos-config/modules/users.nix

# This enables home-manager specific configs and an easier modularization for user-specific configurations.
{ inputs, config, options, lib, ... }:

let
  cfg = config.modules.users;
  userModules = lib.filesToAttr ../users;

  users = lib.attrNames userModules;
  nonexistentUsers = lib.filter (name: !lib.elem name users) cfg.users;

  mkUser = user: path:
  let
    defaultConfig = {
      home.username = user;
      home.homeDirectory = "/home/${user}";
    };
  in
    {
    users.users.${user} = {
      isNormalUser = true;
      extraGroups = [ "wheel" ];
    };

    home-manager.users.${user} = defaultConfig // import path;
  };
in
{
  options.modules.users = {
    users = lib.mkOption {
      default = [];
      type = with lib.types; listOf str;
      description = "A list of users from the `./users` directory to be included in the NixOS config.";
    };
  };

  # FIXME: Recursion error when using `lib.getUsers cfg.users`.
  # Time to study how Nix modules really work.
  # The assertion is basically enough for this case.
  imports = [
    # home-manager to enable user-specific config.
    inputs.home-manager.nixosModules.home-manager

    # The global configuration for the home-manager module.
    {
      home-manager.useUserPackages = true;
      home-manager.useGlobalPkgs = true;
    }
  ] ++ (lib.mapAttrsToList mkUser userModules);

  config = {
    assertions = [{
      assertion = (builtins.length nonexistentUsers) < 1;
      message = "${lib.concatMapStringsSep ", " (u: "'${u}'") nonexistentUsers} is not found in the `./users` directory.";
    }];
  };
}