{-# LANGUAGE LambdaCase #-}

data ArbitrarilyNested a = One a | Several [ArbitrarilyNested a]

flatten :: ArbitrarilyNested a -> [a]
flatten = \case
  One x -> pure x
  Several xs -> xs >>= flatten

x :: ArbitrarilyNested Integer
x = Several [One 1, One 2, Several [One 3, One 4, One 5, One 6], One 7]

main :: IO ()
main = print (flatten x)