import Prelude 

{- Prelude has 
     null :: [a] -> Bool    that retunrs True for empty lists,
     length :: [a] -> Int    to get the length of litss, and
     sum :: [Int] -> Int to get the sum of elements in the list
     
  so these implementations could be simpler still -}

{-| An example list of integers 1 - 4 -}
ex01 :: [Int]
ex01 = [1,2,3,4]

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

main :: IO ()
main = do
  putStrLn $ "ex1         : " ++ (show ex01) 
  putStrLn $ "ex1 length  : " ++ (show (foldl (\acc _ -> acc + 1) 0 ex01))
  putStrLn $ "doubled ex1 : " ++ (show (map (2 *) ex01))
  putStrLn $ "sum ex1     : " ++ (show (foldl (+) 0 ex01))
  putStrLn $ "histogram   : " ++ (show (map (makeStringOf "*") ex01))
  
  -- bonus - mapping using foldr for a right fold...
  putStrLn $ "doubled ex1 : " ++
    (show (foldr (\x acc -> (2 * x):acc) [] ex01))