module Main where
 
import qualified Data.Word as W
import qualified Data.Bits as Bts
import Data.Bits ((.|.))
import qualified Data.List as L
 
main :: IO ()
main = do
    print $ atDefault 0x00 (-1) myOctet
    print $ atDefault 0x00 0 myOctet
    print $ atDefault 0x00 1 myOctet
    print $ atDefault 0x00 2 myOctet
    print $ atDefault 0x00 3 myOctet
    print $ atDefault 0x00 4 myOctet
 
myOctet = toOctets (0xA4B3C2D1 :: W.Word32)
 
atDefault :: a -> Integer -> [a] -> a
atDefault aDef _ [] = aDef                        -- case: is empty anyway
atDefault _ 0  (a:_) = a                          -- case: index is 0 -> take it
atDefault aDef nIndex  (a:la) 
    | nIndex > 0 = atDefault aDef (nIndex - 1) la -- case: index is positive
    | otherwise  = aDef                           -- case: index is negative
 
class Octetable w where
    toOctets :: w -> [W.Word8]
 
instance Octetable W.Word32 where
    toOctets w32 = 
        [ fromIntegral (w32 `Bts.shiftR` 24)
        , fromIntegral (w32 `Bts.shiftR` 16)
        , fromIntegral (w32 `Bts.shiftR` 8)
        , fromIntegral w32
        ]