Optional

src/lib.rs

@@ -74,4 +74,17 @@ mod tests {
         let output = parser.parse("aaaaaaaabcd").unwrap();
         assert_eq! {((10, 'b'), "cd"), output};
     }
+
+    #[test]
+    fn test_optional() {
+        let parser = sequence(
+            optional(literal("alpha")),
+            sequence(repeated(literal(" ")), literal("beta")),
+        );
+
+        let output1 = parser.parse("   beta").unwrap();
+        assert_eq!(output1.0 .0, None);
+        let output2 = parser.parse("alpha   beta").unwrap();
+        assert_eq!(output2.0 .0, Some("alpha"));
+    }
 }