ebnf: follow EBNF for EBNF faithfully

Apply analogous changes in godoc/spec.go.

Fixes #1829.

R=nmessenger, r
CC=golang-dev
https://golang.org/cl/4528127

src/cmd/godoc/spec.go

@@ -129,6 +129,9 @@ func (p *ebnfParser) parseTerm() bool {
 
 
 func (p *ebnfParser) parseSequence() {
+	if !p.parseTerm() {
+		p.errorExpected(p.pos, "term")
+	}
 	for p.parseTerm() {
 	}
 }
@@ -148,7 +151,9 @@ func (p *ebnfParser) parseExpression() {
 func (p *ebnfParser) parseProduction() {
 	p.parseIdentifier(true)
 	p.expect(token.ASSIGN)
+	if p.tok != token.PERIOD {
 		p.parseExpression()
+	}
 	p.expect(token.PERIOD)
 }
 

src/pkg/ebnf/ebnf.go

@@ -82,6 +82,12 @@ type (
 		Body   Expression // {body}
 	}
 
+	// A Bad node stands for pieces of source code that lead to a parse error.
+	Bad struct {
+		TokPos token.Pos
+		Error  string // parser error message
+	}
+
 	// A Production node represents an EBNF production.
 	Production struct {
 		Name *Name
@@ -103,6 +109,7 @@ func (x *Range) Pos() token.Pos      { return x.Begin.Pos() }
 func (x *Group) Pos() token.Pos      { return x.Lparen }
 func (x *Option) Pos() token.Pos     { return x.Lbrack }
 func (x *Repetition) Pos() token.Pos { return x.Lbrace }
+func (x *Bad) Pos() token.Pos        { return x.TokPos }
 func (x *Production) Pos() token.Pos { return x.Name.Pos() }
 
 

src/pkg/ebnf/ebnf_test.go

@@ -14,7 +14,7 @@ import (
 var fset = token.NewFileSet()
 
 
-var grammars = []string{
+var goodGrammars = []string{
 	`Program = .`,
 
 	`Program = foo .
@@ -38,7 +38,19 @@ var grammars = []string{
 }
 
 
-func check(t *testing.T, filename string, src []byte) {
+var badGrammars = []string{
+	`Program = | .`,
+	`Program = | b .`,
+	`Program = a … b .`,
+	`Program = "a" … .`,
+	`Program = … "b" .`,
+	`Program = () .`,
+	`Program = [] .`,
+	`Program = {} .`,
+}
+
+
+func checkGood(t *testing.T, filename string, src []byte) {
 	grammar, err := Parse(fset, filename, src)
 	if err != nil {
 		t.Errorf("Parse(%s) failed: %v", src, err)
@@ -49,9 +61,20 @@ func check(t *testing.T, filename string, src []byte) {
 }
 
 
+func checkBad(t *testing.T, filename string, src []byte) {
+	_, err := Parse(fset, filename, src)
+	if err == nil {
+		t.Errorf("Parse(%s) should have failed", src)
+	}
+}
+
+
 func TestGrammars(t *testing.T) {
-	for _, src := range grammars {
-		check(t, "", []byte(src))
+	for _, src := range goodGrammars {
+		checkGood(t, "", []byte(src))
+	}
+	for _, src := range badGrammars {
+		checkBad(t, "", []byte(src))
 	}
 }
 
@@ -67,6 +90,6 @@ func TestFiles(t *testing.T) {
 		if err != nil {
 			t.Fatal(err)
 		}
-		check(t, filename, src)
+		checkGood(t, filename, src)
 	}
 }

src/pkg/ebnf/parser.go

@@ -85,6 +85,7 @@ func (p *parser) parseToken() *Token {
 }
 
 
+// ParseTerm returns nil if no term was found.
 func (p *parser) parseTerm() (x Expression) {
 	pos := p.pos
 
@@ -131,7 +132,8 @@ func (p *parser) parseSequence() Expression {
 	// no need for a sequence if list.Len() < 2
 	switch len(list) {
 	case 0:
-		return nil
+		p.errorExpected(p.pos, "term")
+		return &Bad{p.pos, "term expected"}
 	case 1:
 		return list[0]
 	}
@@ -144,20 +146,16 @@ func (p *parser) parseExpression() Expression {
 	var list Alternative
 
 	for {
-		if x := p.parseSequence(); x != nil {
-			list = append(list, x)
-		}
+		list = append(list, p.parseSequence())
 		if p.tok != token.OR {
 			break
 		}
 		p.next()
 	}
+	// len(list) > 0
 
 	// no need for an Alternative node if list.Len() < 2
-	switch len(list) {
-	case 0:
-		return nil
-	case 1:
+	if len(list) == 1 {
 		return list[0]
 	}
 
@@ -168,7 +166,10 @@ func (p *parser) parseExpression() Expression {
 func (p *parser) parseProduction() *Production {
 	name := p.parseIdentifier()
 	p.expect(token.ASSIGN)
-	expr := p.parseExpression()
+	var expr Expression
+	if p.tok != token.PERIOD {
+		expr = p.parseExpression()
+	}
 	p.expect(token.PERIOD)
 	return &Production{name, expr}
 }