String rotation (rot13, etc.) in Haskell.

rot.hs

-- Usage:
-- rot 13 "Uryyb, jbeyq!"
-- --> "Hello, world!"

import Control.Applicative ((<|>))
import Data.Maybe (fromMaybe)
import System.Environment (getArgs)

caesarCipher :: Int -> [a] -> [(a, a)]
caesarCipher n xs = zip xs (drop n (cycle xs))

rot :: Int -> String -> String
rot n =
  let uppers = caesarCipher n ['A'..'Z']
      lowers = caesarCipher n ['a'..'z']
  in map (\c -> fromMaybe c (lookup c uppers <|> lookup c lowers))

main = do
  n <- (read . head) <$> getArgs
  contents <- getContents
  putStr $ rot n contents

mentioned and used your code with attribution in the @1HaskellADay exercise and solution for Y2017.M08.D04: https://github.com/geophf/1HaskellADay/blob/master/exercises/HAD/Y2017/M08/D04/Solution.hs