Vigenere Cipher Encoding (Haskell)

vigenereCipher.hs

-- file: vigenereCipher.hs

import Data.Char

let2int                 ::  Char -> Int
let2int c               =   ord c - ord 'a'

int2let                 ::  Int -> Char
int2let n               =   chr (ord 'a' + n)

stringToWord            ::  String -> String
stringToWord s          =   [c | c <- cs, isLower c] where cs = concat (words (map toLower s))
-- Vigenere cipher takes input string and cycles to create a key.
-- Key/Message pair are then looked up in an alphabet table to find cipher.
-- Math: Ci = (Mi + Ki) mod 26
          -- Methodology:
          -- 1. concat ! replicate key n times where n is (size of message / size of key) + 1
          -- 2. let2int both message and key, add them together by index, and take mod; return cipher
keygen                  ::  String -> String -> String
keygen mes key          =   concat (replicate n key) where n = (length mes `div` length key) + 1

vEncodeWord             ::  String -> String -> String
vEncodeWord mes key     =   [int2let (((let2int m) + (let2int k)) `mod` 26) | (m,k)<-zip mes (keygen mes key)]

vEncodeString           ::  String -> String -> String
vEncodeString mes key   =   vEncodeWord m k
                               where m = stringToWord mes
                                     k = keygen m (stringToWord key)