From 68a644a559db0d2e117066a413e9279988576164 Mon Sep 17 00:00:00 2001
From: Ryo Nihei <nihei.dev@gmail.com>
Date: Mon, 19 Jul 2021 00:58:25 +0900
Subject: Detect duplicate definitions of terminal symbols and fragments in
 advance

---
 grammar/grammar.go        | 20 ++++++++++++++++++++
 grammar/semantic_error.go |  1 +
 2 files changed, 21 insertions(+)

diff --git a/grammar/grammar.go b/grammar/grammar.go
index 0cd055d..00c894c 100644
--- a/grammar/grammar.go
+++ b/grammar/grammar.go
@@ -66,6 +66,15 @@ func (b *GrammarBuilder) genSymbolTableAndLexSpec(root *spec.RootNode) (*symbolT
 	skipKinds := []mlspec.LexKind{}
 	entries := []*mlspec.LexEntry{}
 	for _, prod := range root.LexProductions {
+		if _, exist := symTab.toSymbol(prod.LHS); exist {
+			b.errs = append(b.errs, &verr.SpecError{
+				Cause:  semErrDuplicateSym,
+				Detail: prod.LHS,
+				Row:    prod.Pos.Row,
+			})
+			continue
+		}
+
 		_, err := symTab.registerTerminalSymbol(prod.LHS)
 		if err != nil {
 			return nil, err
@@ -129,7 +138,18 @@ func (b *GrammarBuilder) genSymbolTableAndLexSpec(root *spec.RootNode) (*symbolT
 	// Anonymous patterns take precedence over explicitly defined lexical specifications.
 	entries = append(anonEntries, entries...)
 
+	checkedFragments := map[string]struct{}{}
 	for _, fragment := range root.Fragments {
+		if _, exist := checkedFragments[fragment.LHS]; exist {
+			b.errs = append(b.errs, &verr.SpecError{
+				Cause:  semErrDuplicateSym,
+				Detail: fragment.LHS,
+				Row:    fragment.Pos.Row,
+			})
+			continue
+		}
+		checkedFragments[fragment.LHS] = struct{}{}
+
 		entries = append(entries, &mlspec.LexEntry{
 			Fragment: true,
 			Kind:     mlspec.LexKind(fragment.LHS),
diff --git a/grammar/semantic_error.go b/grammar/semantic_error.go
index 01446ed..63f9325 100644
--- a/grammar/semantic_error.go
+++ b/grammar/semantic_error.go
@@ -17,6 +17,7 @@ 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")
 )
-- 
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