{ lib, ... }:
  let
    inherit (lib)
      mkOption
      types
    ;
  in {
    options = {
      valhalla = mkOption {
        type = types.submodule (
          { extendModules, ... }:
            let
              osVariant = extendModules { };
              linuxVariant = osVariant.extendModules { };
              windowsVariant = osVariant.extendModules { };
            in {
              options = {
                linux = mkOption {
                  inherit (linuxVariant) type;
                  description = "The Linux specific configuration.";
                  default = {};
                  visible = "shallow";
                };

                windows = mkOption {
                  inherit (windowsVariant) type;
                  description = "The Windows specific configuration.";
                  default = {};
                  visible = "shallow";
                };
              };
            });

        description = "Configuration for PortValhalla.";
        default = {};
      };
    };
  }