let
  module = { lib, ... }:
    with lib;
    with types;

    let
      mySubmodule = types.submodule {
        options.flags = mkOption {
          type = types.listOf types.str;
          default = [];
        };
        options.value = mkOption {
          type = types.nullOr types.str;
        };
      };

      toAttr = value: { inherit value; };
      strToMySubmodule = types.coercedTo types.str toAttr mySubmodule;
    in
    {
      options.my = mkOption {
        type = attrsOf strToMySubmodule;
        default = mapAttrs (_: mkOptionDefault) {
          foo = "bar";
        };
      };

      config.my = mkMerge [
        (mkOptionDefault {
          foo.flags = [ "my-flag"];
        })
        (mapAttrs (_: mkOptionDefault) {
          baz = "test";
        })
      ];
    };
in (import <nixpkgs/lib>).evalModules {
  modules = [ module ];
}