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

// Copyright 2014 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 com.google.common.base.Optional;
import com.google.devtools.build.lib.syntax.compiler.DebugInfo;
import com.google.devtools.build.lib.syntax.compiler.LoopLabels;
import com.google.devtools.build.lib.syntax.compiler.VariableScope;

import net.bytebuddy.implementation.bytecode.ByteCodeAppender;

/**
 * Base class for all statements nodes in the AST.
 */
public abstract class Statement extends ASTNode {

  /**
   * Executes the statement in the specified build environment, which may be
   * modified.
   *
   * @throws EvalException if execution of the statement could not be completed.
   * @throws InterruptedException may be thrown in a sub class.
   */
  final void exec(Environment env) throws EvalException, InterruptedException   {
    try {
      doExec(env);
    } catch (EvalException ex) {
      throw maybeTransformException(ex);
    }
  }

  /**
   * Executes the statement.
   *
   * <p>This method is only invoked by the super class {@link Statement} when calling {@link
   * #exec(Environment)}.
   *
   * @throws EvalException if execution of the statement could not be completed.
   * @throws InterruptedException may be thrown in a sub class.
   */
  abstract void doExec(Environment env) throws EvalException, InterruptedException;

  /**
   * Checks the semantics of the Statement using the Environment according to
   * the rules of the Skylark language. The Environment can be used e.g. to check
   * variable type collision, read only variables, detecting recursion, existence of
   * built-in variables, functions, etc.
   *
   * <p>The semantical check should be performed after the Skylark extension is loaded
   * (i.e. is syntactically correct) and before is executed. The point of the semantical check
   * is to make sure (as much as possible) that no error can occur during execution (Skylark
   * programmers get a "compile time" error). It should also check execution branches (e.g. in
   * if statements) that otherwise might never get executed.
   *
   * @throws EvalException if the Statement has a semantical error.
   */
  abstract void validate(ValidationEnvironment env) throws EvalException;

  /**
   * Builds a {@link ByteCodeAppender} that implements this statement.
   *
   * <p>A statement implementation should never require any particular state of the byte code
   * stack and should leave it in the state it was before.
   *
   * @throws EvalException for any error that would have occurred during evaluation of the
   *    function definition that contains this statement, e.g. type errors.
   */
  ByteCodeAppender compile(
      VariableScope scope, Optional<LoopLabels> loopLabels, DebugInfo debugInfo)
      throws EvalException {
    throw new UnsupportedOperationException(this.getClass().getSimpleName() + " unsupported.");
  }
}