-- | The Prelude defines the most common data types and functions for
-- | us, but the LANGUAGE construct at the top of the page excludes the
-- | normally automatically imported Prelude. Here we add back in what
-- | we might need in this module.
import Prelude (Show, IO, Int, putStrLn, ($), show, (++), (+), (*))

-- | our own version of a list datatype
data Lst a 
  = NIL
  | Cons a (Lst a)
  deriving (Show)

-- | our own version of a boolean datatype
data Booly
  = Yep
  | Nope
  deriving (Show)

-- | Returns true if an instance of @Lst@ is empty, false otherwise
isEmpty :: Lst a -> Booly
isEmpty NIL = Yep
isEmpty _   = Nope

{-| An example list of integers 1 - 4 -}
ex01 :: Lst Int
ex01 = Cons 1 (Cons 2 (Cons 3 (Cons 4 NIL)))

{-| Doubles the input parameter -}
dub :: Int -> Int
dub n = n * 2

{-| Apply a function to every element in the list to generate
    a new element -}
lstModder :: (Int -> Int) -> Lst Int -> Lst Int
lstModder _ NIL = NIL
lstModder modFn (Cons x xs) = Cons (modFn x) (lstModder modFn xs)

main :: IO ()
main = do
  putStrLn $ "ex1         : " ++ (show ex01) 
  putStrLn $ "doubled ex1 : " ++ (show (lstModder dub ex01))