caesar_cypher.hs

import Data.Char (chr, ord)
import System.Environment (getArgs)

cycleRange :: Char -> Int -> Char -> Char
cycleRange start n c | a <= x && x < a+26 = chr $ ((x - a + n) `mod` 26) + a
                     | otherwise = c
  where x = ord c
        a = ord start

caesarCypher :: Int -> String -> String
caesarCypher n = map caesarChar
  where caesarChar = cycleRange 'a' n . cycleRange 'A' n

main :: IO ()
main = do
  [file] <- getArgs
  putStrLn =<< caesarCypher 21 <$> readFile file