test.nix

(import <nixpkgs/lib>).evalModules {
  modules = [
    ({ lib, config, ... }: {


      options.foo = lib.mkOption {
        type = lib.types.int;
      };
      options.bar = lib.mkOption {
        default = { };
        type = lib.types.attrsOf (lib.types.submodule {
          options.baz = lib.mkOption {
            type = lib.types.str;
            default = "don't know";
          };
        });
      };

      config = {
        foo = 10;

        #bar.x = {
        #  baz = lib.mkIf (config.foo > 5) "foo is greater than 5";
        #};
        #bar = lib.mkIf (config.foo > 5) {
        #  x.baz = "foo is greater than 5";
        #};
        bar = lib.mkMerge [
          {
            x.baz = "other module";
          }
          (lib.mkIf (config.foo > 5) {
            x.baz = lib.mkDefault "foo is greater than 5";
          })
          #(lib.mkIf (config.foo <= 5) {
          #  x.baz = lib.mkDefault "foo is less than 5";
          #})
        ];
      };

    })
  ];
}