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.calcite.schema;
  
  import java.util.List;
  
  /**
   * Named expression that accepts parameters and returns a result.
   *
   * <p>The application may occur at compile time (for a macro) or at run time
   * (for a regular function). The result may be a relation, and so might any of
   * the parameters.
   *
   * <p>Functions are registered in a {@link Schema}, and may be queried by name
   * ({@link Schema#getFunctions(String)}) then overloads resolved based on
   * parameter types.
   *
   * @see TableMacro
   * @see ScalarFunction
   */
  public interface Function {
    /**
     * Returns the parameters of this function.
     *
     * @return Parameters; never null
     */
    List<FunctionParameter> getParameters();
  }
  