Introduce some common types

include/types.hrl

@@ -0,0 +1,22 @@
+%% Copyright (c) 2013-2019 EMQ Technologies Co., Ltd. All Rights Reserved.
+%%
+%% Licensed under the Apache License, Version 2.0 (the "License");
+%% you may not use this file except in compliance with the License.
+%% You may obtain a copy of the License at
+%%
+%%     http://www.apache.org/licenses/LICENSE-2.0
+%%
+%% Unless required by applicable law or agreed to in writing, software
+%% distributed under the License is distributed on an "AS IS" BASIS,
+%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+%% See the License for the specific language governing permissions and
+%% limitations under the License.
+
+-type(maybe(T) :: undefined | T).
+
+-type(startlink_ret() :: {ok, pid()} | ignore | {error, term()}).
+
+-type(ok_or_error(Reason) :: ok | {error, Reason}).
+
+-type(ok_or_error(Value, Reason) :: {ok, Value} | {error, Reason}).
+