Add |expected| and |found| properties to exceptions thrown by parsers

Based on a patch by Marcin Stefaniuk (marcin@stefaniuk.info).

README.md

@@ -70,7 +70,7 @@ To get parser’s source code, call the `toSource` method on the parser.
 Using the Parser
 ----------------
 
-Using the generated parser is simple — just call its `parse` method and pass an input string as a parameter. The method will return a parse result (the exact value depends on the grammar used to build the parser) or throw an exception if the input is invalid. The exception will contain `offset`, `line`, `column` and `message` properties with more details about the error.
+Using the generated parser is simple — just call its `parse` method and pass an input string as a parameter. The method will return a parse result (the exact value depends on the grammar used to build the parser) or throw an exception if the input is invalid. The exception will contain `offset`, `line`, `column`, `expected`, `found` and `message` properties with more details about the error.
 
     parser.parse("abba"); // returns ["a", "b", "b", "a"]
 

src/emitter.js

@@ -466,6 +466,8 @@ PEG.compiler.emitter = function(ast) {
             '    }',
             '    ',
             '    this.name = "SyntaxError";',
+            '    this.expected = expected;',
+            '    this.found = found;',
             '    this.message = buildMessage(expected, found);',
             '    this.offset = offset;',
             '    this.line = line;',

src/parser.js

@@ -3621,6 +3621,8 @@ PEG.parser = (function(){
     }
     
     this.name = "SyntaxError";
+    this.expected = expected;
+    this.found = found;
     this.message = buildMessage(expected, found);
     this.offset = offset;
     this.line = line;

test/compiler-test.js

@@ -331,55 +331,71 @@ test("indempotence", function() {
   strictEqual(parser1.toSource(), parser2.toSource());
 });
 
-test("error messages", function() {
+test("error details", function() {
   var literalParser = PEG.buildParser('start = "abcd"');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     literalParser,
     "",
+    ["\"abcd\""],
+    null,
     'Expected "abcd" but end of input found.'
   );
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     literalParser,
     "efgh",
+    ["\"abcd\""],
+    "e",
     'Expected "abcd" but "e" found.'
   );
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     literalParser,
     "abcde",
+    [],
+    "e",
     'Expected end of input but "e" found.'
   );
 
   var classParser = PEG.buildParser('start = [a-d]');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     classParser,
     "",
+    ["[a-d]"],
+    null,
     'Expected [a-d] but end of input found.'
   );
   var negativeClassParser = PEG.buildParser('start = [^a-d]');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     negativeClassParser,
     "",
+    ["[^a-d]"],
+    null,
     'Expected [^a-d] but end of input found.'
   );
 
   var anyParser = PEG.buildParser('start = .');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     anyParser,
     "",
+    ["any character"],
+    null,
     'Expected any character but end of input found.'
   );
 
   var namedRuleWithLiteralParser = PEG.buildParser('start "digit" = [0-9]');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     namedRuleWithLiteralParser,
     "a",
+    ["digit"],
+    "a",
     'Expected digit but "a" found.'
   );
 
   var namedRuleWithAnyParser = PEG.buildParser('start "whatever" = .');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     namedRuleWithAnyParser,
     "",
+    ["whatever"],
+    null,
     'Expected whatever but end of input found.'
   );
 
@@ -387,58 +403,74 @@ test("error messages", function() {
     'start "digits" = digit+',
     'digit "digit"  = [0-9]'
   ].join("\n"));
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     namedRuleWithNamedRuleParser,
     "",
+    ["digits"],
+    null,
     'Expected digits but end of input found.'
   );
 
   var choiceParser1 = PEG.buildParser('start = "a" / "b" / "c"');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     choiceParser1,
     "def",
+    ["\"a\"", "\"b\"", "\"c\""],
+    "d",
     'Expected "a", "b" or "c" but "d" found.'
   );
 
   var choiceParser2 = PEG.buildParser('start = "a" "b" "c" / "a"');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     choiceParser2,
     "abd",
+    ["\"c\""],
+    "d",
     'Expected "c" but "d" found.'
   );
 
   var simpleNotParser = PEG.buildParser('start = !"a" "b"');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     simpleNotParser,
     "c",
+    ["\"b\""],
+    "c",
     'Expected "b" but "c" found.'
   );
 
   var simpleAndParser = PEG.buildParser('start = &"a" [a-b]');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     simpleAndParser,
     "c",
+    [],
+    "c",
     'Expected end of input but "c" found.'
   );
 
   var emptyParser = PEG.buildParser('start = ');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     emptyParser,
     "something",
+    [],
+    "s",
     'Expected end of input but "s" found.'
   );
 
   var duplicateErrorParser = PEG.buildParser('start = "a" / "a"');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     duplicateErrorParser,
     "",
+    ["\"a\""],
+    null,
     'Expected "a" but end of input found.'
   );
 
   var unsortedErrorsParser = PEG.buildParser('start = "b" / "a"');
-  doesNotParseWithMessage(
+  doesNotParseWithDetails(
     unsortedErrorsParser,
     "",
+    ["\"a\"", "\"b\""],
+    null,
     'Expected "a" or "b" but end of input found.'
   );
 });

test/helpers.js

@@ -19,6 +19,25 @@ doesNotParseWithMessage = function(parser, input, message) {
   );
 };
 
+doesNotParseWithDetails = function(parser, input, expected, found, message) {
+  raises(
+    function() { parser.parse(input); },
+    function(e) {
+      var i;
+
+      if (!(e instanceof parser.SyntaxError))    { return false; }
+      if (e.expected.length !== expected.length) { return false; }
+      for (i = 0; i < e.expected.length; i++) {
+        if (e.expected[i] !== expected[i])       { return false; }
+      }
+      if (e.found !== found)                     { return false; }
+      if (e.message !== message)                 { return false; }
+
+      return true;
+    }
+  );
+};
+
 doesNotParseWithPos = function(parser, input, offset, line, column) {
   raises(
     function() { parser.parse(input); },