add validation of parentheses balance.

src/parse.mjs

@@ -36,7 +36,15 @@ export function parse_line (line) {
 		};
 	}
 
-	const tokens = tokenize_line(line);
+	const tokenized_result = tokenize_line(line);
+
+	if (!tokenized_result.valid) {
+		return {
+			type: Line.Invalid,
+		};
+	}
+
+	const { tokens } = tokenized_result;
 
 	if (tokens.length === 0) {
 		return {

src/tokenize.mjs

@@ -28,6 +28,12 @@ export function tokenize_line (line) {
 		}
 
 		if (is_parens_close(c)) {
+			if (stack.length === 0) {
+				return {
+					valid: false,
+				};
+			}
+
 			push_token();
 			node = stack.pop();
 			continue;
@@ -41,8 +47,18 @@ export function tokenize_line (line) {
 		token += c;
 	}
 
+	if (stack.length > 0) {
+		return {
+			valid: false,
+		};
+	}
+
 	push_token();
-	return tree;
+
+	return {
+		valid: true,
+		tokens: tree,
+	};
 }
 
 function is_parens_open (c) {