CaesarCipher

caesarcipher.hs

import Data.Char

data CaesarCipherShift = CipherShift Int

caesarEncode :: CaesarCipherShift -> String -> String
caesarEncode _ [] = []
caesarEncode (CipherShift shiftCount) (x:xs)
  | shiftedOrd > printableMax =
    (chr $ ((shiftedOrd - 1 - printableMin) `mod` printableRange) + printableMin) :
      (caesarEncode (CipherShift shiftCount) xs)
  | shiftedOrd < 0 =
    (chr $ printableMax + ((shiftedOrd + 1) `rem` printableMax)) :
      (caesarEncode (CipherShift shiftCount) xs)
  | shiftedOrd < printableMin =
    (chr $ ((shiftedOrd + 1) `mod` printableRange) + printableRange) :
      (caesarEncode (CipherShift shiftCount) xs)
  | otherwise = chr shiftedOrd : (caesarEncode (CipherShift shiftCount) xs)
    where
      printableMin = 0x20
      printableMax = 0x7e
      printableRange = printableMax - printableMin
      shiftedOrd = ord x + shiftCount

caesarDecode :: CaesarCipherShift -> String -> String
caesarDecode (CipherShift shiftCount) xs = 
  caesarEncode (CipherShift (negate shiftCount)) xs

Usage:

-- use negative CipherShift to shift left, and positive to shift right
caesarDecode (CipherShift 13) . caesarEncode (CipherShift 13) $ "Hello World"