From 2fcab705ddf787fa0f4edab2f2dc7e579ca982d9 Mon Sep 17 00:00:00 2001
From: Ryo Nihei <nihei.dev@gmail.com>
Date: Wed, 3 Aug 2022 23:07:06 +0900
Subject: [PATCH] Prohibit error node having children

---
 spec/test/parser.go      |  8 +++++
 spec/test/parser_test.go | 77 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 85 insertions(+)

diff --git a/spec/test/parser.go b/spec/test/parser.go
index 33e035b..e9c557d 100644
--- a/spec/test/parser.go
+++ b/spec/test/parser.go
@@ -289,6 +289,14 @@ func formatSyntaxError(synErr *SyntaxError, gram Grammar, lineOffset int) []byte
 }
 
 func (tp *treeParser) genTree(node *Node) (*Tree, error) {
+	// A node labeled 'error' cannot have children. It always must be (error).
+	if sym := node.Children[0]; sym.Text == "error" {
+		if len(node.Children) > 1 {
+			return nil, fmt.Errorf("%v:%v: error node cannot take children", tp.lineOffset+sym.Row+1, sym.Col+1)
+		}
+		return NewTerminalNode(sym.Text, ""), nil
+	}
+
 	if len(node.Children) == 2 && node.Children[1].KindName == "string" {
 		var text string
 		str := node.Children[1].Children[0]
diff --git a/spec/test/parser_test.go b/spec/test/parser_test.go
index 7a8984e..cbf96c0 100644
--- a/spec/test/parser_test.go
+++ b/spec/test/parser_test.go
@@ -327,6 +327,83 @@ foo
 foo
 --
 (foo)
+`,
+			parseErr: true,
+		},
+		// A node may have just one string node.
+		{
+			src: `test
+----
+foo bar
+----
+(foo (bar 'bar'))
+`,
+			tc: &TestCase{
+				Description: "test",
+				Source:      []byte("foo bar"),
+				Output: NewNonTerminalTree("foo",
+					NewTerminalNode("bar", "bar"),
+				).Fill(),
+			},
+		},
+		// A node may have just one pattern node.
+		{
+			src: `test
+----
+foo bar
+----
+(foo (bar "bar"))
+`,
+			tc: &TestCase{
+				Description: "test",
+				Source:      []byte("foo bar"),
+				Output: NewNonTerminalTree("foo",
+					NewTerminalNode("bar", "bar"),
+				).Fill(),
+			},
+		},
+		// A node may be the error node.
+		{
+			src: `test
+----
+foo x
+----
+(foo (error))
+`,
+			tc: &TestCase{
+				Description: "test",
+				Source:      []byte("foo x"),
+				Output: NewNonTerminalTree("foo",
+					NewTerminalNode("error", ""),
+				).Fill(),
+			},
+		},
+		// The error node cannot have a string node.
+		{
+			src: `test
+----
+foo x
+----
+(foo (error 'x'))
+`,
+			parseErr: true,
+		},
+		// The error node cannot have a pattern node.
+		{
+			src: `test
+----
+foo x
+----
+(foo (error "x"))
+`,
+			parseErr: true,
+		},
+		{
+			src: `test
+----
+foo x
+----
+(foo (error (_ (x 'x'))))
 `,
 			parseErr: true,
 		},