-- | 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, String, 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 eeach element in a list 
    to produce a new list -}
lstMap :: (a -> b) -> Lst a -> Lst b
lstMap _ NIL         = NIL
lstMap f (Cons x xs) = Cons (f x) (lstMap f xs)

{-| Makes a string of repeated strings -}
makeStringOf :: String -> Int -> String
makeStringOf _ 0 = ""
makeStringOf s n = s ++ (makeStringOf s (n - 1))

{-| Fold for Lst -}
lstFold :: (b -> a -> b) -> b -> Lst a -> b
lstFold _ acc NIL = acc
lstFold f acc (Cons x xs) = lstFold f (f acc x) xs

{-| Get the length of a list by folding over the list with
    an accumulator function -}
lstLen :: Lst a -> Int
lstLen xs = lstFold (\acc _ -> 1 + acc) 0 xs

main :: IO ()
main = do
  putStrLn $ "ex1         : " ++ (show ex01) 
  putStrLn $ "ex1 length  : " ++ (show (lstLen ex01))
  putStrLn $ "doubled ex1 : " ++ (show (lstMap dub ex01))
  putStrLn $ "histogram   : " ++ (show (lstMap (makeStringOf "*") ex01))