Shaden.hs

import Data.Char (ord)

main = print $ rot13 "unccl oveguqnl, funqra!"
  where
    rot13   = foldr replace []
    alpha   = ['a'..'z']
    index x = (ord x - ord 'a' + 13) `mod` 26
    replace x xs
        | x `elem` alpha = alpha !! index x:xs
        | otherwise      = x:xs