Rewrite handling of optional parameters

Instead of testing arguments.length to see whether an optional parameter
was passed to a function, compare its value to "undefined". This
approach has two advantages:

  * It is in line with handling of default parameters in ES6.

  * Optional parameters are actually spelled out in the parameter
    list.

There is also one important disadvantage, namely that it's impossible to
pass "undefined" as an optional parameter value. This required a small
change in two tests.

Additional notes:

  * Default parameter values are set in assignments immediately
    after the function header. This reflects the fact that these
    assignments really belong to the parameter list (which is where they
    are in ES6).

  * Parameter values are checked against "void 0" in places where
    "undefined" can potentially be redefiend.

lib/compiler.js

@@ -38,10 +38,12 @@ var compiler = {
    * during the generation and some may protrude to the generated parser and
    * cause its malfunction.
    */
-  compile: function(ast, passes) {
-    var options = arguments.length > 2 ? objects.clone(arguments[2]) : {},
-        stage;
+  compile: function(ast, passes, options) {
+    options = options !== void 0 ? options : {};
 
+    var stage;
+
+    options = objects.clone(options);
     objects.defaults(options, {
       allowedStartRules:  [ast.rules[0].name],
       cache:              false,

lib/compiler/passes/generate-js.js

@@ -437,13 +437,12 @@ function generateJS(ast, options) {
             return code;
           },
 
-          pop: function() {
-            var n, values;
+          pop: function(n) {
+            var values;
 
-            if (arguments.length === 0) {
+            if (n === void 0) {
               return s(this.sp--);
             } else {
-              n = arguments[0];
               values = arrays.map(arrays.range(this.sp - n + 1, this.sp + 1), s);
               this.sp -= n;
 
@@ -851,9 +850,10 @@ function generateJS(ast, options) {
   }
 
   parts.push([
-    '  function peg$parse(input) {',
-    '    var options = arguments.length > 1 ? arguments[1] : {},',
-    '        parser  = this,',
+    '  function peg$parse(input, options) {',
+    '    options = options !== void 0 ? options : {};',
+    '',
+    '    var parser = this,',
     '',
     '        peg$FAILED = {},',
     ''

lib/parser.js

@@ -28,9 +28,10 @@ module.exports = (function() {
 
   peg$subclass(peg$SyntaxError, Error);
 
-  function peg$parse(input) {
-    var options = arguments.length > 1 ? arguments[1] : {},
-        parser  = this,
+  function peg$parse(input, options) {
+    options = options !== void 0 ? options : {};
+
+    var parser = this,
 
         peg$FAILED = {},
 

lib/peg.js

@@ -22,7 +22,9 @@ var PEG = {
    * errors are detected during the generation and some may protrude to the
    * generated parser and cause its malfunction.
    */
-  buildParser: function(grammar) {
+  buildParser: function(grammar, options) {
+    options = options !== void 0 ? options : {};
+
     function convertPasses(passes) {
       var converted = {}, stage;
 
@@ -35,8 +37,9 @@ var PEG = {
       return converted;
     }
 
-    var options = arguments.length > 1 ? objects.clone(arguments[1]) : {},
-        plugins = "plugins" in options ? options.plugins : [],
+    options = objects.clone(options);
+
+    var plugins = "plugins" in options ? options.plugins : [],
         config  = {
           parser: this.parser,
           passes: convertPasses(this.compiler.passes)

spec/behavior/generated-parser-behavior.spec.js

@@ -34,14 +34,15 @@ describe("generated parser behavior", function() {
 
   beforeEach(function() {
     this.addMatchers({
-      toParse: function(input, expected) {
-        var options = arguments.length > 2 ? arguments[2] : {},
-            result;
+      toParse: function(input, expected, options) {
+        options = options !== undefined ? options : {};
+
+        var result;
 
         try {
           result = this.actual.parse(input, options);
 
-          if (arguments.length > 1) {
+          if (expected !== undefined) {
             this.message = function() {
               return "Expected " + jasmine.pp(input) + " "
                    + "with options " + jasmine.pp(options) + " "
@@ -58,7 +59,7 @@ describe("generated parser behavior", function() {
           this.message = function() {
             return "Expected " + jasmine.pp(input) + " "
                  + "with options " + jasmine.pp(options) + " "
-                 + "to parse" + (arguments.length > 1 ? " as " + jasmine.pp(expected) : "") + ", "
+                 + "to parse" + (expected !== undefined ? " as " + jasmine.pp(expected) : "") + ", "
                  + "but it failed to parse with message "
                  + jasmine.pp(e.message) + ".";
           };
@@ -67,9 +68,10 @@ describe("generated parser behavior", function() {
         }
       },
 
-      toFailToParse: function(input, details) {
-        var options = arguments.length > 2 ? arguments[2] : {},
-            result;
+      toFailToParse: function(input, details, options) {
+        options = options !== undefined ? options : {};
+
+        var result;
 
         try {
           result = this.actual.parse(input, options);
@@ -417,9 +419,14 @@ describe("generated parser behavior", function() {
     describe("positive semantic predicate", function() {
       describe("when the code returns a truthy value", function() {
         it("returns |undefined|", function() {
-          var parser = PEG.buildParser('start = &{ return true; }', options);
+          /*
+           * The |""| is needed so that the parser doesn't return just
+           * |undefined| which we can't compare against in |toParse| due to the
+           * way optional parameters work.
+           */
+          var parser = PEG.buildParser('start = &{ return true; } ""', options);
 
-          expect(parser).toParse("", undefined);
+          expect(parser).toParse("", [undefined, ""]);
         });
       });
 
@@ -617,9 +624,14 @@ describe("generated parser behavior", function() {
     describe("negative semantic predicate", function() {
       describe("when the code returns a falsey value", function() {
         it("returns |undefined|", function() {
-          var parser = PEG.buildParser('start = !{ return false; }', options);
+          /*
+           * The |""| is needed so that the parser doesn't return just
+           * |undefined| which we can't compare against in |toParse| due to the
+           * way optional parameters work.
+           */
+          var parser = PEG.buildParser('start = !{ return false; } ""', options);
 
-          expect(parser).toParse("", undefined);
+          expect(parser).toParse("", [undefined, ""]);
         });
       });
 

spec/unit/compiler/passes/helpers.js

@@ -4,7 +4,9 @@
 
 beforeEach(function() {
   this.addMatchers({
-    toChangeAST: function(grammar, details) {
+    toChangeAST: function(grammar, details, options) {
+      options = options !== undefined ? options : {};
+
       function matchDetails(value, details) {
         function isArray(value) {
           return Object.prototype.toString.apply(value) === "[object Array]";
@@ -42,8 +44,7 @@ beforeEach(function() {
         }
       }
 
-      var options = arguments.length > 2 ? arguments[2] : {},
-          ast     = PEG.parser.parse(grammar);
+      var ast = PEG.parser.parse(grammar);
 
       this.actual(ast, options);