Update and publish catamorphism article

code/catamorphisms/Cata.hs

@@ -99,3 +99,14 @@ invert :: BinaryTree a -> BinaryTree a
 invert = cata $ \case
     LeafF -> Leaf
     NodeF a l r -> Node a r l
+
+data MaybeF a b = NothingF | JustF a deriving Functor
+
+instance Cata (Maybe a) (MaybeF a) where
+    out Nothing = NothingF
+    out (Just x) = JustF x
+
+getOrDefault :: a -> Maybe a -> a
+getOrDefault d = cata $ \case
+    NothingF -> d
+    JustF a -> a