PEG.buildParser reports left recursion (both direct and indirect).

lib/compiler.js

@@ -44,6 +44,9 @@ PEG.buildParser = function(grammar, startRule) {
   if (ast[startRule] === undefined) {
     throw new PEG.Grammar.GrammarError("Missing \"" + startRule + "\" rule.");
   }
+  for (var key in ast) {
+    ast[key].checkNoLeftRecursion(ast, []);
+  }
 
   return PEG.Compiler.compileParser(ast, startRule);
 };
@@ -140,6 +143,47 @@ PEG.Grammar.Action.prototype.checkReferencedRulesExist = function(grammar) {
   this._expression.checkReferencedRulesExist(grammar);
 };
 
+/* ===== Left Recursion Checks ===== */
+
+PEG.Grammar.Rule.prototype.checkNoLeftRecursion = function(grammar, appliedRules) {
+  this._expression.checkNoLeftRecursion(grammar, appliedRules.concat(this._name));
+};
+
+PEG.Grammar.Literal.prototype.checkNoLeftRecursion = function(grammar, appliedRules) {};
+
+PEG.Grammar.Any.prototype.checkNoLeftRecursion = function(grammar, appliedRules) {};
+
+PEG.Grammar.Sequence.prototype.checkNoLeftRecursion = function(grammar, appliedRules) {
+  if (this._elements.length > 0) {
+    this._elements[0].checkNoLeftRecursion(grammar, appliedRules);
+  }
+};
+
+PEG.Grammar.Choice.prototype.checkNoLeftRecursion = function(grammar, appliedRules) {
+  PEG.ArrayUtils.each(this._alternatives, function(alternative) {
+    alternative.checkNoLeftRecursion(grammar, appliedRules);
+  });
+};
+
+PEG.Grammar.ZeroOrMore.prototype.checkNoLeftRecursion = function(grammar, appliedRules) {
+  this._element.checkNoLeftRecursion(grammar, appliedRules);
+};
+
+PEG.Grammar.NotPredicate.prototype.checkNoLeftRecursion = function(grammar, appliedRules) {
+  this._expression.checkNoLeftRecursion(grammar, appliedRules);
+};
+
+PEG.Grammar.RuleRef.prototype.checkNoLeftRecursion = function(grammar, appliedRules) {
+  if (appliedRules.indexOf(this._name) !== -1) {
+    throw new PEG.Grammar.GrammarError("Left recursion detected for rule \"" + this._name + "\".");
+  }
+  grammar[this._name].checkNoLeftRecursion(grammar, appliedRules);
+};
+
+PEG.Grammar.Action.prototype.checkNoLeftRecursion = function(grammar, appliedRules) {
+  this._expression.checkNoLeftRecursion(grammar, appliedRules);
+};
+
 /* ===== PEG.Compiler ===== */
 
 PEG.Compiler = {

test/compiler-test.js

@@ -131,6 +131,27 @@ test("buildParser reports missing referenced rules", function() {
   });
 });
 
+test("buildParser reports left recursion", function() {
+  var grammars = [
+    /* Direct */
+    'start: start',
+    'start: start "a" "b"',
+    'start: start / "a" / "b"',
+    'start: "a" / "b" / start',
+    'start: start*',
+    'start: !start',
+    'start: &start',
+    'start: start { }',
+
+    /* Indirect */
+    'start: stop\nstop: start'
+  ];
+
+  PEG.ArrayUtils.each(grammars, function(grammar) {
+    throws(function() { PEG.buildParser(grammar); }, PEG.Grammar.GrammarError);
+  });
+});
+
 test("buildParser allows custom start rule", function() {
   var parser = PEG.buildParser('s: "abcd"', "s");
   parses(parser, "abcd", "abcd");