Implement Alternative Parser

app/Parser.hs

@@ -37,3 +37,9 @@ instance Applicative Parser where
         Just (rest, f) -> case a rest of
             Nothing -> Nothing
             Just (rest', x) -> Just (rest', f x)
+
+instance Alternative Parser where
+    empty = Parser $ const Nothing
+    (<|>) (Parser p1) (Parser p2) = Parser $ \input -> case p1 input of
+        Nothing -> p2 input
+        result -> result