Checks refactoring

Changed two things:

  1. Checks are run in |PEG.Compiler.compileParser|, not in
     |PEG.buildParser|.

  2. Checks are extracted into separate functions.

lib/compiler.js

@@ -23,19 +23,10 @@ function nop() {}
 PEG.buildParser = function(grammar, startRule) {
   startRule = startRule || "start";
 
-  var ast = PEG.grammarParser.parse(grammar);
-
-  for (var rule in ast) {
-    ast[rule].checkReferencedRulesExist(ast);
-  }
-  if (ast[startRule] === undefined) {
-    throw new PEG.Grammar.GrammarError("Missing \"" + startRule + "\" rule.");
-  }
-  for (var rule in ast) {
-    ast[rule].checkNoLeftRecursion(ast, []);
-  }
-
-  return PEG.Compiler.compileParser(ast, startRule);
+  return PEG.Compiler.compileParser(
+    PEG.grammarParser.parse(grammar),
+    startRule
+  );
 };
 
 /* ===== PEG.ArrayUtils ===== */
@@ -462,18 +453,57 @@ PEG.Compiler = {
   },
 
   /*
-   * Generates a parser from a specified grammar and start rule.
+   * Checks made on the grammar AST before compilation. Each check is a function
+   * that is passed the AST and start rule and does not return anything. If the
+   * check passes, the function does not do anything special, otherwise it
+   * throws |PEG.Grammar.GrammarError|. The checks are run in sequence in order
+   * of their definition.
    */
-  compileParser: function(grammar, startRule) {
+  _checks: [
+    /* Checks that all referenced rules exist. */
+    function(ast, startRule) {
+      for (var rule in ast) {
+        ast[rule].checkReferencedRulesExist(ast);
+      }
+    },
+
+    /* Checks that the start rule is defined. */
+    function(ast, startRule) {
+      if (ast[startRule] === undefined) {
+        throw new PEG.Grammar.GrammarError(
+          "Missing \"" + startRule + "\" rule."
+        );
+      }
+    },
+
+    /* Checks that no left recursion is present. */
+    function(ast, startRule) {
+      for (var rule in ast) {
+        ast[rule].checkNoLeftRecursion(ast, []);
+      }
+    }
+  ],
+
+  /*
+   * Generates a parser from a specified grammar AST and start rule. Throws
+   * |PEG.Grammar.GrammarError| if the AST contains a semantic error. Note that
+   * not all errors are detected during the generation and some may protrude to
+   * the generated parser and cause its malfunction.
+   */
+  compileParser: function(ast, startRule) {
     /*
      * This ensures that the same grammar and start rule always generate exactly
      * the same parser.
      */
     this._resetUniqueIdentifierCounters();
 
+    for (var i = 0; i < this._checks.length; i++) {
+      this._checks[i](ast, startRule);
+    }
+
     var parseFunctionDefinitions = [];
-    for (var rule in grammar) {
-      parseFunctionDefinitions.push(grammar[rule].compile());
+    for (var rule in ast) {
+      parseFunctionDefinitions.push(ast[rule].compile());
     }
 
     var source = this.formatCode(