// 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,
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package com.google.devtools.build.lib.syntax;
import com.google.common.collect.ImmutableList;
import com.google.devtools.build.lib.events.Location;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
import javax.annotation.Nullable;
/**
* Base class for list and dict comprehension expressions.
*
* <p> A comprehension contains one or more clauses, e.g.
* [a+d for a in b if c for d in e]
* contains three clauses: "for a in b", "if c", "for d in e".
* For and If clauses can happen in any order, except that the first one has to be a For.
*
* <p> The code above can be expanded as:
* <pre>
* for a in b:
* if c:
* for d in e:
* result.append(a+d)
* </pre>
* result is initialized to [] (list) or {} (dict) and is the return value of the whole expression.
*/
public abstract class AbstractComprehension extends Expression {
/**
* The interface implemented by ForClause and (later) IfClause.
* A comprehension consists of one or many Clause.
*/
public interface Clause extends Serializable {
/** Enum for distinguishing clause types. */
public enum Kind {
FOR,
IF
}
/**
* Returns whether this is a For or If clause.
*
* <p>This avoids having to rely on reflection, or on checking whether {@link #getLValue} is
* null.
*/
public abstract Kind getKind();
/**
* The evaluation of the comprehension is based on recursion. Each clause may
* call recursively evalStep (ForClause will call it multiple times, IfClause will
* call it zero or one time) which will evaluate the next clause. To know which clause
* is the next one, we pass a step argument (it represents the index in the clauses
* list). Results are aggregated in the result argument, and are populated by
* evalStep.
*
* @param env environment in which we do the evaluation.
* @param collector the aggregated results of the comprehension.
* @param step the index of the next clause to evaluate.
*/
abstract void eval(Environment env, OutputCollector collector, int step)
throws EvalException, InterruptedException;
abstract void validate(ValidationEnvironment env, Location loc) throws EvalException;
/**
* The LValue defined in Clause, i.e. the loop variables for ForClause and null for
* IfClause. This is needed for SyntaxTreeVisitor.
*/
@Nullable // for the IfClause
public abstract LValue getLValue();
/**
* The Expression defined in Clause, i.e. the collection for ForClause and the
* condition for IfClause. This is needed for SyntaxTreeVisitor.
*/
public abstract Expression getExpression();
}
/**
* A for clause in a comprehension, e.g. "for a in b" in the example above.
*/
public static final class ForClause implements Clause {
private final LValue variables;
private final Expression list;
@Override
public Kind getKind() {
return Kind.FOR;
}
public ForClause(LValue variables, Expression list) {
this.variables = variables;
this.list = list;
}
@Override
public void eval(Environment env, OutputCollector collector, int step)
throws EvalException, InterruptedException {
Object listValueObject = list.eval(env);
Location loc = collector.getLocation();
Iterable<?> listValue = EvalUtils.toIterable(listValueObject, loc);
EvalUtils.lock(listValueObject, loc);
try {
for (Object listElement : listValue) {
variables.assign(env, loc, listElement);
evalStep(env, collector, step);
}
} finally {
EvalUtils.unlock(listValueObject, loc);
}
}
@Override
public void validate(ValidationEnvironment env, Location loc) throws EvalException {
variables.validate(env, loc);
list.validate(env);
}
@Override
public LValue getLValue() {
return variables;
}
@Override
public Expression getExpression() {
return list;
}
@Override
public String toString() {
return Printer.format("for %s in %r", variables.toString(), list);
}
}
/**
* A if clause in a comprehension, e.g. "if c" in the example above.
*/
public static final class IfClause implements Clause {
private final Expression condition;
@Override
public Kind getKind() {
return Kind.IF;
}
public IfClause(Expression condition) {
this.condition = condition;
}
@Override
public void eval(Environment env, OutputCollector collector, int step)
throws EvalException, InterruptedException {
if (EvalUtils.toBoolean(condition.eval(env))) {
evalStep(env, collector, step);
}
}
@Override
public void validate(ValidationEnvironment env, Location loc) throws EvalException {
condition.validate(env);
}
@Override
public LValue getLValue() {
return null;
}
@Override
public Expression getExpression() {
return condition;
}
@Override
public String toString() {
return String.format("if %s", condition);
}
}
/**
* The output expressions, e.g. "a+d" in the example above. This list has either one (list) or two
* (dict) items.
*/
private final ImmutableList<Expression> outputExpressions;
private final ImmutableList<Clause> clauses;
public AbstractComprehension(List<Clause> clauses, Expression... outputExpressions) {
this.clauses = ImmutableList.copyOf(clauses);
this.outputExpressions = ImmutableList.copyOf(outputExpressions);
}
protected abstract char openingBracket();
protected abstract char closingBracket();
public ImmutableList<Expression> getOutputExpressions() {
return outputExpressions;
}
@Override
public String toString() {
StringBuilder sb = new StringBuilder();
sb.append(openingBracket()).append(printExpressions());
for (Clause clause : clauses) {
sb.append(' ').append(clause);
}
sb.append(closingBracket());
return sb.toString();
}
/** Base class for comprehension builders. */
public abstract static class AbstractBuilder {
protected List<Clause> clauses = new ArrayList<>();
public void addFor(Expression loopVar, Expression listExpression) {
Clause forClause = new ForClause(new LValue(loopVar), listExpression);
clauses.add(forClause);
}
public void addIf(Expression condition) {
clauses.add(new IfClause(condition));
}
public abstract AbstractComprehension build();
}
public ImmutableList<Clause> getClauses() {
return clauses;
}
@Override
public void accept(SyntaxTreeVisitor visitor) {
visitor.visit(this);
}
@Override
Object doEval(Environment env) throws EvalException, InterruptedException {
OutputCollector collector = createCollector(env);
evalStep(env, collector, 0);
return collector.getResult(env);
}
@Override
void validate(ValidationEnvironment env) throws EvalException {
for (Clause clause : clauses) {
clause.validate(env, getLocation());
}
// Clauses have to be validated before expressions in order to introduce the variable names.
for (Expression expr : outputExpressions) {
expr.validate(env);
}
}
/**
* Evaluate the clause indexed by step, or elementExpression. When we evaluate the
* comprehension, step is 0 and we evaluate the first clause. Each clause may
* recursively call evalStep any number of times. After the last clause,
* the output expression(s) is/are evaluated and added to the results.
*
* <p> In the expanded example above, you can consider that evalStep is equivalent to
* evaluating the line number step.
*/
private static void evalStep(Environment env, OutputCollector collector, int step)
throws EvalException, InterruptedException {
List<Clause> clauses = collector.getClauses();
if (step >= clauses.size()) {
collector.evaluateAndCollect(env);
} else {
clauses.get(step).eval(env, collector, step + 1);
}
}
/**
* Returns a {@link String} representation of the output expression(s).
*/
abstract String printExpressions();
abstract OutputCollector createCollector(Environment env);
/**
* Interface for collecting the intermediate output of an {@code AbstractComprehension} and for
* providing access to the final results.
*/
interface OutputCollector {
/** Returns the location for the comprehension we are evaluating. */
Location getLocation();
/** Returns the list of clauses for the comprehension we are evaluating. */
List<Clause> getClauses();
/**
* Evaluates the output expression(s) of the comprehension and collects the result.
*/
void evaluateAndCollect(Environment env) throws EvalException, InterruptedException;
/**
* Returns the final result of the comprehension.
*/
Object getResult(Environment env) throws EvalException;
}
}