Improved & simplified error handling code.

lib/runtime.js

@@ -87,7 +87,7 @@ PEG.Parser.prototype = {
       function buildExpectedFromMatchFailures(failures) {
         switch (failures.length) {
           case 0:
-            return "nothing";
+            return "end of input";
           case 1:
             return failures[0].toString();
           default:
@@ -100,19 +100,13 @@ PEG.Parser.prototype = {
         }
       }
 
-      if (that._pos === 0) {
-        var expected = buildExpectedFromMatchFailures(
-          that._rightmostMatchFailures
-        );
-        var actual = that._rightmostMatchFailuresPos < that._input.length
-          ? PEG.StringUtils.quote(
-              that._input.charAt(that._rightmostMatchFailuresPos)
-            )
-          : "end of input";
-      } else {
-        var expected = "end of input";
-        var actual = PEG.StringUtils.quote(that._input.charAt(that._pos));
-      }
+      var expected = buildExpectedFromMatchFailures(
+        that._rightmostMatchFailures
+      );
+      var pos = Math.max(that._pos, that._rightmostMatchFailuresPos);
+      var actual = pos < that._input.length
+        ? PEG.StringUtils.quote(that._input.charAt(pos))
+        : "end of input";
 
       return "Expected " + expected + " but " + actual + " found.";
     }
@@ -157,6 +151,33 @@ PEG.Parser.prototype = {
     };
 
     var result = this["_parse_" + this._startRule](initialContext);
+
+    /*
+     * The parser is now in one of the following three states:
+     *
+     * 1. The parser successfully parsed the whole input.
+     *
+     *    - |result !== null|
+     *    - |that._pos === input.length|
+     *    - |that._rightmostMatchFailures.length| may or may not contain
+     *      something
+     *
+     * 2. The parser successfully parsed only a part of the input.
+     *
+     *    - |result !== null|
+     *    - |that._pos < input.length|
+     *    - |that._rightmostMatchFailures.length| may or may not contain
+     *      something
+     *
+     * 3. The parser did not successfully parse any part of the input.
+     *
+     *   - |result === null|
+     *   - |that._pos === 0|
+     *   - |that._rightmostMatchFailures.length| contains at least one failure
+     *
+     * All code following this comment (including called functions) must
+     * handle these states.
+     */
     if (result === null || this._pos !== input.length) {
       var errorPosition = computeErrorPosition();
       throw new PEG.Parser.SyntaxError(

test/compiler-test.js

@@ -365,18 +365,25 @@ test("error messages", function() {
     'Expected digits but end of input found.'
   );
 
-  var choiceParser = PEG.buildParser('start: "a" / "b" / "c"');
+  var choiceParser1 = PEG.buildParser('start: "a" / "b" / "c"');
   doesNotParseWithMessage(
-    choiceParser,
+    choiceParser1,
     "def",
     'Expected "a", "b" or "c" but "d" found.'
   );
 
+  var choiceParser2 = PEG.buildParser('start: "a" "b" "c" / "a"');
+  doesNotParseWithMessage(
+    choiceParser2,
+    "abd",
+    'Expected "c" but "d" found.'
+  );
+
   var emptyParser = PEG.buildParser('start: ');
   doesNotParseWithMessage(
     emptyParser,
     "something",
-    'Expected nothing but "s" found.'
+    'Expected end of input but "s" found.'
   );
 });