UMD parsers: Allow generating parsers in UMD format from the API

Introduce new "format" and "exportVar" options to PEG.buildParser which
together allow generating parsers in UMD format.

Part of work on #362.

README.md

@@ -120,6 +120,11 @@ object to `PEG.buildParser`. The following options are supported:
     `false`)
   * `allowedStartRules` — rules the parser will be allowed to start parsing from
     (default: the first rule in the grammar)
+  * `exportVar` — name of a global variable into which the parser object is
+    assigned to when no module loader is detected; valid only when `format` is
+    set to `"umd"` (default: `null`)
+  * `format` — format of the genreated parser (`"bare"` or `"umd"`); valid only
+    when `output` is set to `"source"` (default: `"bare"`)
   * `output` — if set to `"parser"`, the method will return generated parser
     object; if set to `"source"`, it will return parser source code as a string
     (default: `"parser"`)

lib/compiler.js

@@ -49,7 +49,9 @@ var compiler = {
       cache:             false,
       trace:             false,
       optimize:          "speed",
-      output:            "parser"
+      output:            "parser",
+      format:            "bare",
+      exportVar:         null
     });
 
     for (stage in passes) {

lib/compiler/passes/generate-js.js

@@ -1168,6 +1168,8 @@ function generateJS(ast, options) {
           ].join('\n');
     }
 
+    var generators = {
+      bare: function() {
         var parts = [];
 
         parts.push('(function() {');
@@ -1179,6 +1181,47 @@ function generateJS(ast, options) {
         parts.push('})()');
 
         return parts.join('\n');
+      },
+
+      umd: function() {
+        var parts = [];
+
+        parts.push([
+          '(function(root, factory) {',
+          '  if (typeof define === "function" && define.amd) {',
+          '    define([], factory);',
+          '  } else if (typeof module === "object" && module.exports) {',
+          '    module.exports = factory();'
+        ].join('\n'));
+
+        if (options.exportVar !== null) {
+          parts.push([
+            '  } else {',
+            '    root.' + options.exportVar + ' = factory();'
+          ].join('\n'));
+        }
+
+        parts.push([
+          '  }',
+          '})(this, function() {'
+        ].join('\n'));
+
+        parts.push(indent2(generateIntro()));
+        parts.push('');
+        parts.push(indent2(toplevelCode));
+        parts.push('');
+        parts.push(indent2('return ' + generateParserObject() + ';'));
+
+        parts.push([
+          '});',
+          ''
+        ].join('\n'));
+
+        return parts.join('\n');
+      }
+    };
+
+    return generators[options.format]();
   }
 
   ast.code = generateWrapper(generateToplevel());

spec/api/pegjs-api.spec.js

@@ -199,6 +199,12 @@ describe("PEG.js API", function() {
       });
     });
 
+    /*
+     * The |format| and |exportVars| options are not tested becasue there is no
+     * meaningful way to thest their effects without turning this into an
+     * integration test.
+     */
+
     /* The |plugins| option is tested in plugin API specs. */
 
     it("accepts custom options", function() {