Allow labeled expressions in the metagrammar (without any meaning yet)

lib/compiler.js

@@ -275,6 +275,7 @@ PEG.Compiler = {
         rule:          checkExpression,
         choice:        checkSubnodes("alternatives"),
         sequence:      checkSubnodes("elements"),
+        labeled:       checkExpression,
         and_predicate: checkExpression,
         not_predicate: checkExpression,
         optional:      checkExpression,
@@ -340,6 +341,7 @@ PEG.Compiler = {
             }
           },
 
+        labeled:       checkExpression,
         and_predicate: checkExpression,
         not_predicate: checkExpression,
         optional:      checkExpression,
@@ -406,6 +408,7 @@ PEG.Compiler = {
           rule:          replaceInExpression,
           choice:        replaceInSubnodes("alternatives"),
           sequence:      replaceInSubnodes("elements"),
+          labeled:       replaceInExpression,
           and_predicate: replaceInExpression,
           not_predicate: replaceInExpression,
           optional:      replaceInExpression,
@@ -596,6 +599,10 @@ PEG.Compiler = {
       );
     },
 
+    labeled: function(node, resultVar) {
+      return PEG.Compiler.compileNode(node.expression, resultVar);
+    },
+
     and_predicate: function(node, resultVar) {
       var savedPosVar                 = PEG.Compiler.generateUniqueIdentifier("savedPos");
       var savedReportMatchFailuresVar = PEG.Compiler.generateUniqueIdentifier("savedReportMatchFailuresVar");

lib/metagrammar.pegjs

@@ -35,7 +35,7 @@ choice
     }
 
 sequence
-  = prefixed* action {
+  = labeled* action {
       var expression = $1.length != 1
         ? {
             type:     "sequence",
@@ -48,7 +48,7 @@ sequence
         action:     $2
       };
     }
-  / prefixed* {
+  / labeled* {
       return $1.length != 1
         ? {
             type:     "sequence",
@@ -57,6 +57,16 @@ sequence
         : $1[0];
     }
 
+labeled
+  = identifier colon prefixed {
+      return {
+        type:       "labeled",
+        label:      $1,
+        expression: $3
+      };
+    }
+  / prefixed
+
 prefixed
   = and suffixed { return { type: "and_predicate", expression: $2 }; }
   / not suffixed { return { type: "not_predicate", expression: $2 }; }
@@ -90,6 +100,7 @@ nonBraceCharacter
   = [^{}]
 
 equals   = "=" __ { return $1; }
+colon    = ":" __ { return $1; }
 slash    = "/" __ { return $1; }
 and      = "&" __ { return $1; }
 not      = "!" __ { return $1; }

test/compiler-test.js

@@ -206,6 +206,7 @@ test("buildParser reports missing referenced rules", function() {
     'start = "a" / "b" / missing',
     'start = missing "a" "b"',
     'start = "a" "b" missing',
+    'start = label:missing',
     'start = &missing',
     'start = !missing',
     'start = missing?',
@@ -230,6 +231,7 @@ test("buildParser reports left recursion", function() {
     'start = start / "a" / "b"',
     'start = "a" / "b" / start',
     'start = start "a" "b"',
+    'start = label:start',
     'start = &start',
     'start = !start',
     'start = start?',
@@ -289,6 +291,12 @@ test("sequences", function() {
   parses(posTestParser, "a", "a");
 });
 
+test("labels", function() {
+  var parser = PEG.buildParser('start = label:"a"');
+  parses(parser, "a", "a");
+  doesNotParse(parser, "b");
+});
+
 test("and predicate", function() {
   var parser = PEG.buildParser('start = "a" &"b" "b"');
   parses(parser, "ab", ["a", "", "b"]);

test/metagrammar-test.js

@@ -43,6 +43,14 @@ function sequence(elements) {
   };
 }
 
+function labeled(label, expression) {
+  return {
+    type:       "labeled",
+    label:      label,
+    expression: expression
+  };
+}
+
 function nodeWithExpressionConstructor(type) {
   return function(expression) {
     return {
@@ -100,12 +108,12 @@ var literalIjkl  = literal("ijkl");
 
 var optionalLiteral = optional(literalAbcd);
 
-var notAbcd = notPredicate(literalAbcd);
-var notEfgh = notPredicate(literalEfgh);
-var notIjkl = notPredicate(literalIjkl);
+var labeledAbcd = labeled("a", literalAbcd);
+var labeledEfgh = labeled("e", literalEfgh);
+var labeledIjkl = labeled("i", literalIjkl);
 
 var sequenceEmpty    = sequence([]);
-var sequenceNots     = sequence([notAbcd, notEfgh, notIjkl]);
+var sequenceLabeleds = sequence([labeledAbcd, labeledEfgh, labeledIjkl]);
 var sequenceLiterals = sequence([literalAbcd, literalEfgh, literalIjkl]);
 
 var choiceLiterals = choice([literalAbcd, literalEfgh, literalIjkl]);
@@ -194,19 +202,31 @@ test("parses sequence", function() {
     oneRuleGrammar(action(sequenceEmpty, " code "))
   );
   grammarParserParses(
-    'start = !"abcd" { code }',
-    oneRuleGrammar(action(notAbcd, " code "))
+    'start = a:"abcd" { code }',
+    oneRuleGrammar(action(labeledAbcd, " code "))
   );
   grammarParserParses(
-    'start = !"abcd" !"efgh" !"ijkl" { code }',
-    oneRuleGrammar(action(sequenceNots, " code "))
+    'start = a:"abcd" e:"efgh" i:"ijkl" { code }',
+    oneRuleGrammar(action(sequenceLabeleds, " code "))
   );
 
   grammarParserParses('start = ',         oneRuleGrammar(sequenceEmpty));
-  grammarParserParses('start = !"abcd"', oneRuleGrammar(notAbcd));
+  grammarParserParses('start = a:"abcd"', oneRuleGrammar(labeledAbcd));
   grammarParserParses(
-    'start = !"abcd" !"efgh" !"ijkl"',
-    oneRuleGrammar(sequenceNots)
+    'start = a:"abcd" e:"efgh" i:"ijkl"',
+    oneRuleGrammar(sequenceLabeleds)
+  );
+});
+
+/* Canonical labeled is "label:\"abcd\"". */
+test("parses labeled", function() {
+  grammarParserParses(
+    'start = label:!"abcd"',
+    oneRuleGrammar(labeled("label", notPredicate(literalAbcd)))
+  );
+  grammarParserParses(
+    'start = !"abcd"',
+    oneRuleGrammar(notPredicate(literalAbcd))
   );
 });