caesar.hs

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

main = do
    args <- getArgs
    let action = args !! 0
        shift  = read (args !! 1) :: Int
        file   = args !! 2
    handle <- openFile file ReadMode
    contents <- hGetContents handle
    if action == "encrypt" then
        putStr $ encrypt shift contents
    else
        putStr $ decrypt shift contents
    hClose handle

encrypt :: Int -> String -> String
encrypt shift str = map (chr) $ map (+ shift) $ map (ord) str

decrypt :: Int -> String -> String
decrypt shift str = encrypt (negate shift) str