open number char monad io string

chars = "ABCDEFGHIJKLMOPQRSTUVWXYZ"

caesar _ _ [] = ""
caesar op key (x::xs) = check shifted ++ caesar op key xs
  where orig = indexOf (string.upper $ toString x) chars
        shifted = orig `op` key
        check val | orig == -1 = x
                  | val > 24 = trans $ val - 25
                  | val < 0 = trans $ 25 + val
                  | else = trans shifted
        trans idx = chars:idx

cypher = caesar (+)
decypher = caesar (-)

key = 2

do
  putStrLn "A string to encode:"
  str <- readStr
  putStr "Encoded string: "
  cstr <- return <| cypher key str
  put cstr
  putStrLn ""
  putStr "Decoded string: "
  put $ decypher key cstr