Optional

src/combinators.rs

@@ -16,3 +16,13 @@ where
         Ok((results, acc))
     }
 }
+
+pub fn optional<I, O, E>(parser: impl Parser<I, O, E>) -> impl Parser<I, Option<O>, E>
+where
+    I: Clone,
+{
+    move |input: I| match parser.parse(input.clone()) {
+        Ok((output, rest)) => Ok((Some(output), rest)),
+        Err(_e) => Ok((None, input)),
+    }
+}

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"));
+    }
 }