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

// Copyright 2014 Google Inc. 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.Preconditions;
import com.google.devtools.build.lib.events.Location;

import java.io.Serializable;
import java.util.Collection;

/**
 * Class representing an LValue.
 * It appears in assignment, for loop and comprehensions, e.g.
 *    lvalue = 2
 *    [for lvalue in exp]
 *    for lvalue in exp: pass
 * An LValue can be a simple variable or something more complex like a tuple.
 */
public class LValue implements Serializable {
  private final Expression expr;

  public LValue(Expression expr) {
    this.expr = expr;
  }

  public Expression getExpression() {
    return expr;
  }

  /**
   * Assign a value to an LValue and update the environment.
   */
  public void assign(Environment env, Location loc, Object result)
      throws EvalException, InterruptedException {
    assign(env, loc, expr, result);
  }

  private static void assign(Environment env, Location loc, Expression lvalue, Object result)
      throws EvalException, InterruptedException {
    if (lvalue instanceof Identifier) {
      assign(env, loc, (Identifier) lvalue, result);
      return;
    }

    if (lvalue instanceof ListLiteral) {
      ListLiteral variables = (ListLiteral) lvalue;
      Collection<?> rvalue = EvalUtils.toCollection(result, loc);
      int len = variables.getElements().size();
      if (len != rvalue.size()) {
        throw new EvalException(loc, String.format(
            "lvalue has length %d, but rvalue has has length %d", len, rvalue.size()));
      }
      int i = 0;
      for (Object o : rvalue) {
        assign(env, loc, variables.getElements().get(i), o);
        i++;
      }
      return;
    }

    throw new EvalException(loc,
        "can only assign to variables and tuples, not to '" + lvalue + "'");
  }

  /**
   * Assign value to a single variable.
   */
  private static void assign(Environment env, Location loc, Identifier ident, Object result)
      throws EvalException, InterruptedException {
    Preconditions.checkNotNull(result, "trying to assign null to %s", ident);

    if (env.isSkylark()) {
      // The variable may have been referenced successfully if a global variable
      // with the same name exists. In this case an Exception needs to be thrown.
      if (env.isKnownGlobalVariable(ident.getName())) {
        throw new EvalException(
            loc,
            String.format(
                "Variable '%s' is referenced before assignment. "
                    + "The variable is defined in the global scope.",
                ident.getName()));
      }
    }
    env.update(ident.getName(), result);
  }

  void validate(ValidationEnvironment env, Location loc) throws EvalException {
    validate(env, loc, expr);
  }

  private static void validate(ValidationEnvironment env, Location loc, Expression expr)
      throws EvalException {
    if (expr instanceof Identifier) {
      Identifier ident = (Identifier) expr;
      env.declare(ident.getName(), loc);
      return;
    }
    if (expr instanceof ListLiteral) {
      for (Expression e : ((ListLiteral) expr).getElements()) {
        validate(env, loc, e);
      }
      return;
    }
    throw new EvalException(loc,
        "can only assign to variables and tuples, not to '" + expr + "'");
  }

  @Override
  public String toString() {
    return expr.toString();
  }
}