flatten.hs

-- To run, you will need the Haskell Platform
-- $> runhaskell flatten.hs
data Node a = Single a 
            | Many [Node a]
            deriving Show

flatten :: [Node a] -> [a]
flatten [] = []
flatten ((Single v):xs) = v : flatten xs
flatten ((Many xs):xs') = flatten xs ++ flatten xs'

list = [Single 1, Single 2, Many [Single 3, Many [Single 4, Single 5, Many [Single 6], Single 7], Single 8, Single 9], Single 10]

main = do
  putStrLn $ "We are going to flatten the following list: " ++ show list
  let flattened = flatten list
  print flattened 
  putStrLn "END"