Added Optional AST node.

This does not speed up the benchmark suite execution statistically significantly
on V8.

Detailed results (benchmark suite totals):

---------------------------------
 Test #     Before       After
---------------------------------
      1   28.84 kB/s   28.75 kB/s
      2   28.76 kB/s   28.69 kB/s
      3   28.72 kB/s   28.69 kB/s
      4   28.84 kB/s   28.93 kB/s
      5   28.82 kB/s   28.70 kB/s
---------------------------------
Average   28.80 kB/s   28.75 kB/s
---------------------------------

Mozilla/5.0 (X11; U; Linux i686; en-US) AppleWebKit/533.2 (KHTML, like Gecko)
Chrome/5.0.342.9 Safari/533.2

lib/compiler.js

@@ -198,6 +198,8 @@ PEG.Grammar.Choice = function(alternatives) {
   this._alternatives = alternatives;
 };
 
+PEG.Grammar.Optional = function(element) { this._element = element; };
+
 PEG.Grammar.ZeroOrMore = function(element) { this._element = element; };
 
 PEG.Grammar.OneOrMore = function(element) { this._element = element; };
@@ -243,6 +245,9 @@ PEG.Grammar.extendNodes("checkReferencedRulesExist", {
       });
     },
 
+  Optional:
+    function(grammar) { this._element.checkReferencedRulesExist(grammar); },
+
   ZeroOrMore:
     function(grammar) { this._element.checkReferencedRulesExist(grammar); },
 
@@ -295,6 +300,11 @@ PEG.Grammar.extendNodes("checkNoLeftRecursion", {
       });
     },
 
+  Optional:
+    function(grammar, appliedRules) {
+      this._element.checkNoLeftRecursion(grammar, appliedRules);
+    },
+
   ZeroOrMore:
     function(grammar, appliedRules) {
       this._element.checkNoLeftRecursion(grammar, appliedRules);
@@ -862,6 +872,20 @@ PEG.Grammar.Choice.prototype.compile = function(resultVar) {
   return code;
 };
 
+PEG.Grammar.Optional.prototype.compile = function(resultVar) {
+  var elementResultVar = PEG.Compiler.generateUniqueIdentifier("result");
+
+  return PEG.Compiler.formatCode(
+    "${elementCode}",
+    "var ${resultVar} = ${elementResultVar} !== null ? ${elementResultVar} : '';",
+    {
+      elementCode:      this._element.compile(elementResultVar),
+      elementResultVar: elementResultVar,
+      resultVar:        resultVar
+    }
+  );
+};
+
 PEG.Grammar.ZeroOrMore.prototype.compile = function(resultVar) {
   var elementResultVar = PEG.Compiler.generateUniqueIdentifier("result");
 

lib/metagrammar.pegjs

@@ -34,9 +34,7 @@ prefixed
   / suffixed
 
 suffixed
-  : primary question {
-      return new PEG.Grammar.Choice([$1, new PEG.Grammar.Literal("")]);
-    }
+  : primary question { return new PEG.Grammar.Optional($1);   }
   / primary star     { return new PEG.Grammar.ZeroOrMore($1); }
   / primary plus     { return new PEG.Grammar.OneOrMore($1);  }
   / primary

test/compiler-test.js

@@ -191,6 +191,7 @@ test("buildParser reports missing referenced rules", function() {
     'start: "a" "b" missing',
     'start: missing / "a" / "b"',
     'start: "a" / "b" / missing',
+    'start: missing?',
     'start: missing*',
     'start: missing+',
     'start: &missing',
@@ -214,6 +215,7 @@ test("buildParser reports left recursion", function() {
     'start: start "a" "b"',
     'start: start / "a" / "b"',
     'start: "a" / "b" / start',
+    'start: start?',
     'start: start*',
     'start: start+',
     'start: &start',

test/metagrammar-test.js

@@ -17,12 +17,11 @@ global.grammarParserDoesNotParse = function(input) {
 module("Grammar Parser");
 
 with (PEG.Grammar) {
-  var literalEmpty = new Literal("");
   var literalAbcd  = new Literal("abcd");
   var literalEfgh  = new Literal("efgh");
   var literalIjkl  = new Literal("ijkl");
 
-  var choice = new Choice([literalAbcd, literalEmpty]);
+  var choice = new Optional(literalAbcd);
 
   var notAbcd = new NotPredicate(literalAbcd);
   var notEfgh = new NotPredicate(literalEfgh);