{-# LANGUAGE PatternSynonyms #-}

data F = F [Int] [Int]

-- a bidirectional pattern synonym
-- which
-- * when used to match: returns concatenation of constructor parameter
-- * when used to construct: sets the list to the first constructor parameter
pattern F' = undefined


foo :: IO ()
foo = do
  let f = F [1,2,3] [4,5,6]
      f' = F' [7,8,9]
      F' x1 = f
      F' x2 = f'
  -- prints [1,2,3,4,5,6
  print x1
  -- prints [7,8,9]
  print x2