Haskell vigenereCipher

VigenereCipher.hs

import Data.Char (chr, ord, toLower, isLetter)

-- | Vigenere Cipher
vigenereCipher2 :: String -> String -> String
vigenereCipher2 message key = zipWith (curry shiftCharByKey) message (prepareKey message key) where
  -- | Prepares the key to skip foreign chars by setting the key to 'a' at these positions and repeats the key so it matches the messages length
  prepareKey :: String -> String -> String
  prepareKey [] _ = []
  prepareKey (m:message) (k:key) = if isLetter m then k : prepareKey message (key ++ [k]) else 'a' : prepareKey message (k : key)
  -- | Shifts a character depending on the position of the key's character in the alphabet
  shiftCharByKey :: (Char, Char) -> Char
  shiftCharByKey (m, k) = chr $ ord m + shiftAmount where
    mIndex = ord (toLower m) - ord 'a'
    kIndex = ord (toLower k) - ord 'a'
    shiftAmount = if mIndex + kIndex < 26 then kIndex else kIndex - 26