gistfile1.hs

import Control.Arrow ((&&&))
import Data.Bits (shiftR, (.&.), Bits(..))
import Data.Functor ((<$>))
import Data.List (intercalate)
import Data.Maybe (listToMaybe, fromMaybe)
import Network.BSD (getHostByName, HostEntry(..))
import System.Environment (getArgs)

main :: IO ()
main = do
    arg    <- fromMaybe (error "no arguments given") . listToMaybe <$> getArgs
    longIp <- hostAddresses `fmap` getHostByName arg
    mapM_ (putStrLn . showIPv4Addr) longIp

showIPv4Addr :: (Num a, Bits a, Show a) => a -> String
showIPv4Addr = toIPStr . ((a &&& b) &&& (c &&& d))
       where d = flip shiftR 24
             c = (.&. 0x00ff)   . flip shiftR 16
             b = (.&. 0x0000ff) . flip shiftR 8
             a = (.&. 0x000000ff)
             toIPStr ((a, b), (c, d)) = intercalate "." . map show $ [a, b, c, d]