http://wiki.haskell.jp/Workshop/StartHaskell2/exercise9#TOC

gistfile1.hs

import Data.Char
import System.IO
import System.Environment

main = do
  (command:argList) <- getArgs
  dispatch command argList

dispatch :: String -> [String] -> IO ()
dispatch "add"   = calc (+)
dispatch "sub"   = calc (-)
dispatch "times" = calc (*)
dispatch "div"   = calc (/)
dispatch command = doesntExist command

calc :: (Read a, Show b) => (a -> a -> b) -> [String] -> IO()
calc p [x,y] = putStrLn . show $ p (read x) (read y)
calc p _     = putStrLn "This command takes exactly two arguments"

doesntExist :: String -> [String] -> IO()
doesntExist command _ =
    putStrLn $ "The " ++ command ++ " command doesn't exist"