with import <nixpkgs/lib>;
evalModules {
  modules = [{

    options.foo = mkOption {
      default = {};
      type = with types;
        let
          sub = submodule {
            freeformType = t;
            options.bar = mkOption {
              type = str;
              default = "";
            };
          };
          t = attrsOf (either int sub);
        in t;
    };

    config.foo.bla.bla = {};
  }];
}