gistfile1.txt

#!/usr/bin/runhaskell
import Data.Char

translate_char tmap c = helper tmap where
  helper [] = c
  helper ((c',ct):ps)
    | c == c' = ct
    | otherwise = helper ps

translate_string tmap = map $ translate_char tmap

cipher_helper s = do
  putStrLn s
  c1 <- getChar
  c2 <- getChar
  cipher_helper $ translate_string [(c1, c2) 
                                   ,(c2, c1)
                                   ,(toUpper c1, toUpper c2)
                                   ,(toUpper c2, toUpper c1)] s

main = getLine >>= cipher_helper