FLOLAC 2018 0-4: Caesar Cipher & Decipher

caesar.hs

import Data.Char (ord, chr)
import Data.List
import Data.Function (on)

encode :: Int -> String -> String
encode n = map (shift n)

decode :: String -> (String, Int)
decode = aarrr mkPair (flip minimumBy [0..25] . compareOnDiff . count)
         where aarrr f g x = f x $ g x
               mkPair xs n = (encode n xs, mod (26-n) 26)
               compareOnDiff c = compare `on` diff c

diff :: (Char -> Int) -> Int -> Int
diff c n = sum . change . sort' $ rank
           where change = zipWith (\x y -> abs $ x - snd y) [0..25]
                 sort' = sortBy (flip compare `on` (count' . fst))
                 count' = c . shift (mod (26-n) 26)

count :: Eq a => [a] -> a -> Int
count = foldl count' (const 0)
        where count' :: Eq a => (a -> Int) -> a -> (a -> Int)
              count' f x y | x == y    = f y + 1
                           | otherwise = f y

shift :: Int -> Char -> Char
shift n = chr . (\x -> mod (x-oA + n) 26 + oA) . ord
          where oA = ord 'A'

-- https://en.wikipedia.org/wiki/Letter_frequency#Relative_frequencies_of_letters_in_the_English_language
-- ETAOINSHRDLCUMWFGYPBVKJXQZ
rank :: [(Char, Int)]
rank = [('A',  2), ('B', 19), ('C', 11), ('D',  9),
        ('E',  0), ('F', 15), ('G', 16), ('H',  7),
        ('I',  4), ('J', 22), ('K', 21), ('L', 10),
        ('M', 13), ('N',  5), ('O',  3), ('P', 18),
        ('Q', 24), ('R',  8), ('S',  6), ('T',  1),
        ('U', 12), ('V', 20), ('W', 14), ('X', 23),
        ('Y', 17), ('Z', 25)]