Detect duplicate production errors

4 files changed, 87 insertions, 9 deletions

diff --git a/driver/parser_test.go b/driver/parser_test.go
index 272a14d..73d8fac 100644
--- a/driver/parser_test.go
+++ b/driver/parser_test.go
@@ -273,6 +273,48 @@ foo: "foo";
 `,
 			specErr: true,
 		},
+		// A production must not have a duplicate alternative.
+		{
+			specSrc: `
+s
+    : foo
+    | foo
+    ;
+foo: "foo";
+`,
+			specErr: true,
+		},
+		// A production must not have a duplicate alternative.
+		{
+			specSrc: `
+a
+    : foo
+    ;
+b
+    :
+    |
+    ;
+foo: "foo";
+`,
+			specErr: true,
+		},
+		// A production must not have a duplicate alternative.
+		{
+			specSrc: `
+a
+    : foo
+    ;
+b
+    : bar
+    ;
+a
+    : foo
+    ;
+foo: "foo";
+bar: "bar";
+`,
+			specErr: true,
+		},
 	}
 	for i, tt := range tests {
 		t.Run(fmt.Sprintf("#%v", i), func(t *testing.T) {
diff --git a/grammar/grammar.go b/grammar/grammar.go
index 7cd2409..6130e55 100644
--- a/grammar/grammar.go
+++ b/grammar/grammar.go
@@ -3,6 +3,7 @@ package grammar
 import (
 	"fmt"
 	"os"
+	"strings"
 
 	mlcompiler "github.com/nihei9/maleeni/compiler"
 	mlspec "github.com/nihei9/maleeni/spec"
@@ -343,6 +344,42 @@ func (b *GrammarBuilder) genProductionsAndActions(root *spec.RootNode, symTabAnd
 			if err != nil {
 				return nil, err
 			}
+			if _, exist := prods.findByID(p.id); exist {
+				// Report the line number of a duplicate alternative.
+				// When the alternative is empty, we report the position of its LHS.
+				var row int
+				if len(alt.Elements) > 0 {
+					row = alt.Elements[0].Pos.Row
+				} else {
+					row = prod.Pos.Row
+				}
+
+				var detail string
+				{
+					var b strings.Builder
+					fmt.Fprintf(&b, "%v →", prod.LHS)
+					for _, elem := range alt.Elements {
+						switch {
+						case elem.ID != "":
+							fmt.Fprintf(&b, " %v", elem.ID)
+						case elem.Pattern != "":
+							fmt.Fprintf(&b, ` "%v"`, elem.Pattern)
+						}
+					}
+					if len(alt.Elements) == 0 {
+						fmt.Fprintf(&b, " ε")
+					}
+
+					detail = b.String()
+				}
+
+				b.errs = append(b.errs, &verr.SpecError{
+					Cause:  semErrDuplicateProduction,
+					Detail: detail,
+					Row:    row,
+				})
+				continue LOOP_RHS
+			}
 			prods.append(p)
 
 			if alt.Directive != nil {
diff --git a/grammar/production.go b/grammar/production.go
index a935037..d4ea7a1 100644
--- a/grammar/production.go
+++ b/grammar/production.go
@@ -80,9 +80,9 @@ func newProductionSet() *productionSet {
 	}
 }
 
-func (ps *productionSet) append(prod *production) bool {
+func (ps *productionSet) append(prod *production) {
 	if _, ok := ps.id2Prod[prod.id]; ok {
-		return false
+		return
 	}
 
 	if prod.lhs.isStart() {
@@ -98,8 +98,6 @@ func (ps *productionSet) append(prod *production) bool {
 		ps.lhs2Prods[prod.lhs] = []*production{prod}
 	}
 	ps.id2Prod[prod.id] = prod
-
-	return true
 }
 
 func (ps *productionSet) findByID(id productionID) (*production, bool) {
diff --git a/grammar/semantic_error.go b/grammar/semantic_error.go
index 63f9325..ca84cf0 100644
--- a/grammar/semantic_error.go
+++ b/grammar/semantic_error.go
@@ -15,9 +15,10 @@ func (e *SemanticError) Error() string {
 }
 
 var (
-	semErrNoProduction    = newSemanticError("a grammar needs at least one production")
-	semErrUndefinedSym    = newSemanticError("undefined symbol")
-	semErrDuplicateSym    = newSemanticError("duplicate symbol")
-	semErrDirInvalidName  = newSemanticError("invalid directive name")
-	semErrDirInvalidParam = newSemanticError("invalid parameter")
+	semErrNoProduction        = newSemanticError("a grammar needs at least one production")
+	semErrUndefinedSym        = newSemanticError("undefined symbol")
+	semErrDuplicateProduction = newSemanticError("duplicate production")
+	semErrDuplicateSym        = newSemanticError("duplicate symbol")
+	semErrDirInvalidName      = newSemanticError("invalid directive name")
+	semErrDirInvalidParam     = newSemanticError("invalid parameter")
 )