Caeser cipher written as part of a Chapter 9 challenge in the Haskell Book: http://haskellbook.com/

cipher.hs

module Cipher where

import           Data.Char

clampAlpha :: (Integral a) => a -> a
clampAlpha = (`mod` 26)

lowerBound :: Char -> Char
lowerBound c
  | isUpper c = 'A'
  | isLower c = 'a'
  | otherwise = undefined

caesarShift :: Int -> Char -> Char
caesarShift i c
  | not (isAlpha c) = c
  | otherwise = chr (clampped + baseN)
  where base = lowerBound c
        baseN = ord base
        n = ord c - baseN + i
        clampped = clampAlpha n

caesar :: Int -> String -> String
caesar shift = map (caesarShift shift)

unCaesar :: Int -> String -> String
unCaesar shift = map (caesarShift (-shift))

rot13 :: String -> String
rot13 = caesar 13

unRot13 :: String -> String
unRot13 = unCaesar 13