nixpkgs/lib/tests/modules/functionTo/submodule-options.nix

{ lib, config, options, ... }:
let
  inherit (lib) types;
in
{
  imports = [

    # fun.<function-body>.a
    ({ ... }: {
      options = {
        fun = lib.mkOption {
          type = types.functionTo (types.submodule {
            options.a = lib.mkOption { default = "a"; };
          });
        };
      };
    })

    # fun.<function-body>.b
    ({ ... }: {
      options = {
        fun = lib.mkOption {
          type = types.functionTo (types.submodule {
            options.b = lib.mkOption { default = "b"; };
          });
        };
      };
    })
  ];

  options = {
    result = lib.mkOption
      {
        type = types.str;
        default = lib.concatStringsSep " " (lib.attrValues (config.fun (throw "shouldn't use input param")));
      };

    optionsResult = lib.mkOption
      {
        type = types.str;
        default = lib.concatStringsSep " "
          (lib.concatLists
            (lib.mapAttrsToList
              (k: v:
                if k == "_module"
                then [ ]
                else [ (lib.showOption v.loc) ]
              )
              (
                (options.fun.type.getSubOptions [ "fun" ])
              )
            )
          );
      };
  };

  config.fun = lib.mkMerge
    [
      (input: { b = "bee"; })
    ];
}