gistfile1.hs

module Main where


-- A MoreList is either a More value which contains a list of MoreList's, or
-- an Elem value, which contains a single value of type a
data MoreList a = More [MoreList a]
                | Elem a

flatten :: MoreList a -> [a]
flatten (Elem a) = [a]
flatten (More l) = concatMap flatten l

-- printE takes a MoreList and prints all its Elem values,
-- recursing on More values
-- (it needs the (Show a) context to print values of type a)
printE :: Show a => MoreList a -> IO ()
printE = print . flatten


main :: IO ()
main = do
  -- Construct list using Elem and More value constructors
  -- list is a MoreList and can contain More and Elem values
  let list = More
             [Elem 1, Elem 2,
              More [Elem 3,
                     More [Elem 4, Elem 5,
                            More [Elem 6, Elem 7]
                           ],
                     Elem 8,
                     More [ Elem 9, Elem 10]
                    ],
              Elem 11]
          :: MoreList Int -- type of list
  -- Apply printE on list
  printE list