NestedLists.hs

module NestedLists where

data NestedList a = Nest [NestedList a] | Item a
  deriving Show

nestedList = Nest [ Item 1
                  , Item 2
                  , Nest [ Item 3
                         , Nest [ Item 4
                                , Item 5
                                ]
                         , Item 6
                         ]
                  ]

instance Functor NestedList where
  fmap f (Item x) = Item (f x)
  fmap f (Nest nest) = Nest ((fmap . fmap) f nest)

main = do
  print $ fmap (*2) nestedList