すごいHaskell読書会 in 大阪 #4 第6章「モジュール」持ち寄り練習問題 解答例

Rot13.hs

module Toy.Rot13 (encrypt) where

import Data.Char

lowerA = ord 'a'
lowerZ = ord 'z'

upperA = ord 'A'
upperZ = ord 'Z'

encrypt :: String -> String
encrypt = rot 13

rot :: Int -> String -> String
rot r = map (rotChar r)

rotChar :: Int -> Char -> Char
rotChar r c = chr $ go r $ ord c
  where
    go r x
      | lowerA <=  x && x <= lowerZ = rotate r lowerA lowerZ x
      | upperA <=  x && x <= upperZ = rotate r upperA upperZ x
      | otherwise                   = x
    rotate r a z x =
      let width = (z - a + 1)
      in (x - a + r) `rem` width + a