words that rot13 to other words

rot13words.hs

import Data.Char (chr, ord)
import Data.List (sortOn)

rot13 :: String -> String
rot13 = map rot where
  rot x =
    let
      n       = ord x
      isUpper = n >= 65 && n <= 90
      isLower = n >= 97 && n <= 122
    in
      if      isUpper then chr (((n - 65 + 13) `mod` 26) + 65)
      else if isLower then chr (((n - 97 + 13) `mod` 26) + 97)
      else                 x

showPair :: (String, String) -> IO ()
showPair (x,y) = do
  putStr x
  putStr " / "
  putStrLn y

main :: IO ()
main = do
  dict <- (reverse . sortOn length . lines) <$> readFile "/usr/share/dict/web2"
  let rdict  = map rot13 dict
  let results = filter (flip elem dict . snd) (zip dict rdict)
  mapM_ showPair results