i is a haskell programmer

binary_tree_invert.hs

{-# LANGUAGE DeriveFunctor #-}

data MaybePair a = Pair (Maybe a) (Maybe a)
  deriving (Eq, Show, Functor)

data BinaryTree a =
  Node a (MaybePair (BinaryTree a))
  deriving (Eq, Show)

invert :: BinaryTree a -> BinaryTree a
invert (Node x children) = Node x $ invertPair $ invert <$> children
  where invertPair (Pair left right) = Pair right left