content: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You 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.
   */
  
  package org.apache.spark.sql.catalyst.expressions;
  
  import org.apache.spark.annotation.DeveloperApi;
  
  import java.lang.annotation.Retention;
  import java.lang.annotation.RetentionPolicy;
  
  /**
   * ::DeveloperApi::
  
   * A function description type which can be recognized by FunctionRegistry, and will be used to
   * show the usage of the function in human language.
   *
   * `usage()` will be used for the function usage in brief way.
   * `extended()` will be used for the function usage in verbose way, suppose
   *              an example will be provided.
   *
   *  And we can refer the function name by `_FUNC_`, in `usage` and `extended`, as it's
   *  registered in `FunctionRegistry`.
   */
  @DeveloperApi
  @Retention(RetentionPolicy.RUNTIME)
  public @interface ExpressionDescription {
      String usage() default "_FUNC_ is undocumented";
      String extended() default "No example for _FUNC_.";
  }
  