PEG.js grammar: Disallow empty sequences

Empty sequences are useless and they only confused users. Let's disallow
them.

lib/compiler/passes/generate-bytecode.js

@@ -351,8 +351,6 @@ module.exports = function(ast) {
     },
 
     sequence: function(node, context) {
-      var emptyArrayIndex;
-
       function buildElementsCode(elements, context) {
         var processedCount, functionIndex;
 
@@ -402,7 +400,6 @@ module.exports = function(ast) {
         }
       }
 
-      if (node.elements.length > 0) {
       failedIndex = addConst('peg$FAILED');
 
       return buildSequence(
@@ -413,11 +410,6 @@ module.exports = function(ast) {
           action: context.action
         })
       );
-      } else {
-        emptyArrayIndex = addConst('[]');
-
-        return [op.PUSH, emptyArrayIndex];
-      }
     },
 
     labeled: function(node, context) {

lib/parser.js

@@ -676,10 +676,14 @@ module.exports = (function() {
       s0 = peg$currPos;
       s1 = [];
       s2 = peg$parselabeled();
+      if (s2 !== peg$FAILED) {
         while (s2 !== peg$FAILED) {
           s1.push(s2);
           s2 = peg$parselabeled();
         }
+      } else {
+        s1 = peg$c0;
+      }
       if (s1 !== peg$FAILED) {
         s2 = peg$parseaction();
         if (s2 !== peg$FAILED) {
@@ -698,10 +702,14 @@ module.exports = (function() {
         s0 = peg$currPos;
         s1 = [];
         s2 = peg$parselabeled();
+        if (s2 !== peg$FAILED) {
           while (s2 !== peg$FAILED) {
             s1.push(s2);
             s2 = peg$parselabeled();
           }
+        } else {
+          s1 = peg$c0;
+        }
         if (s1 !== peg$FAILED) {
           peg$reportedPos = s0;
           s1 = peg$c8(s1);

spec/compiler/passes/generate-bytecode.spec.js

@@ -85,24 +85,25 @@ describe("compiler pass |generateBytecode|", function() {
 
   describe("for action", function() {
     describe("without labels", function() {
-      var grammar = 'start = { code }';
+      var grammar = 'start = "a" { code }';
 
       it("generates correct bytecode", function() {
         expect(pass).toChangeAST(grammar, bytecodeDetails([
           1,                           // PUSH_CURR_POS
-          0, 0,          // <expression>
+          14, 0, 2, 2, 18, 0, 19, 1,   // <expression>
           11, 6, 0,                    // IF_NOT_ERROR
           20, 1,                       //   * REPORT_SAVED_POS
-          22, 1, 1, 0,   //     CALL
+          22, 2, 1, 0,                 //     CALL
           5                            // NIP
         ]));
       });
 
       it("defines correct constants", function() {
-        expect(pass).toChangeAST(
+        expect(pass).toChangeAST(grammar, constsDetails([
-          grammar,
+          '"a"',
-          constsDetails(['[]', 'function() { code }'])
+          '{ type: "literal", value: "a", description: "\\"a\\"" }',
-        );
+          'function() { code }'
+        ]));
       });
     });
 
@@ -171,21 +172,6 @@ describe("compiler pass |generateBytecode|", function() {
   });
 
   describe("for sequence", function() {
-    describe("empty", function() {
-      var grammar = 'start = ';
-
-      it("generates correct bytecode", function() {
-        expect(pass).toChangeAST(grammar, bytecodeDetails([
-          0, 0   // PUSH
-        ]));
-      });
-
-      it("defines correct constants", function() {
-        expect(pass).toChangeAST(grammar, constsDetails(['[]']));
-      });
-    });
-
-    describe("non-empty", function() {
     var grammar = 'start = "a" "b" "c"';
 
     it("generates correct bytecode", function() {
@@ -223,7 +209,6 @@ describe("compiler pass |generateBytecode|", function() {
       ]));
     });
   });
-  });
 
   describe("for labeled", function() {
     it("generates correct bytecode", function() {

spec/generated-parser.spec.js

@@ -393,13 +393,7 @@ describe("generated parser", function() {
     });
 
     describe("sequence matching", function() {
-      it("matches empty sequence correctly", function() {
+      it("matches correctly", function() {
-        var parser = PEG.buildParser('start = ', options);
-
-        expect(parser).toParse("", []);
-      });
-
-      it("matches non-empty sequence correctly", function() {
         var parser = PEG.buildParser('start = "a" "b" "c"', options);
 
         expect(parser).toParse("abc", ["a", "b", "c"]);
@@ -845,9 +839,9 @@ describe("generated parser", function() {
 
       describe("expectations reporting", function() {
         it("reports expectations correctly with no alternative", function() {
-          var parser = PEG.buildParser('start = ', options);
+          var parser = PEG.buildParser('start = "a"', options);
 
-          expect(parser).toFailToParse("a", {
+          expect(parser).toFailToParse("ab", {
             expected: [{ type: "end", description: "end of input" }]
           });
         });
@@ -916,10 +910,10 @@ describe("generated parser", function() {
 
       describe("message building", function() {
         it("builds message correctly with no alternative", function() {
-          var parser = PEG.buildParser('start = ', options);
+          var parser = PEG.buildParser('start = "a"', options);
 
-          expect(parser).toFailToParse("a", {
+          expect(parser).toFailToParse("ab", {
-            message: 'Expected end of input but "a" found.'
+            message: 'Expected end of input but "b" found.'
           });
         });
 

spec/parser.spec.js

@@ -8,7 +8,6 @@ describe("PEG.js grammar parser", function() {
       labeledAbcd        = { type: "labeled", label: "a", expression: literalAbcd },
       labeledEfgh        = { type: "labeled", label: "b", expression: literalEfgh },
       labeledIjkl        = { type: "labeled", label: "c", expression: literalIjkl },
-      sequenceEmpty      = { type: "sequence", elements: [] },
       sequenceOfLiterals = {
         type:     "sequence",
         elements: [literalAbcd, literalEfgh, literalIjkl]
@@ -235,13 +234,6 @@ describe("PEG.js grammar parser", function() {
 
   /* Canonical sequence is "\"abcd\" \"efgh\" \"ijkl\"". */
   it("parses sequence", function() {
-    expect('start = { code }').toParseAs(
-      oneRuleGrammar({
-        type:       "action",
-        expression: sequenceEmpty,
-        code:       " code "
-      })
-    );
     expect('start = a:"abcd" { code }').toParseAs(
       oneRuleGrammar({ type: "action", expression: labeledAbcd, code: " code " })
     );
@@ -253,9 +245,6 @@ describe("PEG.js grammar parser", function() {
       })
     );
 
-    expect('start = ').toParseAs(
-      oneRuleGrammar(sequenceEmpty)
-    );
     expect('start = a:"abcd"').toParseAs(
       oneRuleGrammar(labeledAbcd)
     );

src/parser.pegjs

@@ -54,7 +54,7 @@ choice
     }
 
 sequence
-  = elements:labeled* code:action {
+  = elements:labeled+ code:action {
       var expression = elements.length !== 1
         ? {
             type:     "sequence",
@@ -67,7 +67,7 @@ sequence
         code:       code
       };
     }
-  / elements:labeled* {
+  / elements:labeled+ {
       return elements.length !== 1
         ? {
             type:     "sequence",