Prohibit error node having children

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]

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,
 		},