src/main/java/com/google/devtools/build/lib/syntax/HasBinary.java

// Copyright 2020 The Bazel Authors. 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.

package com.google.devtools.build.lib.syntax;

import javax.annotation.Nullable;

/**
 * A Starlark value that supports binary operators such as {@code x+y}.
 *
 * <p>During evaluation of a Starlark binary operation, if none of the built-in cases match, then
 * the left operand is queried; if it implements HasBinary, its {@link #binaryOp} method is called.
 * If the left operand does not implement HasBinary, or declines to implement the particular
 * operation by returning null, then the right operand is queried for HasBinary and its {@link
 * #binaryOp} method is called. If neither operand defines the operator, evaluation fails.
 *
 * <p>Subclasses should strive for appropriate symmetries in their implementations, such as {@code x
 * * y == y * x}.
 */
// TODO(adonovan): rename BinaryOperand?
public interface HasBinary extends StarlarkValue {

  /**
   * Returns {@code this op that}, if thisLeft, or {@code that op this} otherwise. May return null
   * to indicate that the operation is not supported, or may throw a specific exception.
   */
  @Nullable
  Object binaryOp(TokenKind op, Object that, boolean thisLeft) throws EvalException;
}