Encode and decode text using a Vigenere cipher

cipher.hs

module Scratch where

-- This impl. is incomplete. I'm sketching out a basic working proof-of-concept to help
-- me better understand the Vigenere cipher.
--
-- Conceptually, a faster, more complete version exists; this isn't it.

import Data.Function ((&))
import Data.Map (Map)
import qualified Data.Map as Map
import qualified Data.Char as Char
import qualified Data.Maybe as Maybe
import qualified Data.List as List

-- | Create a Vigenere matrix
vSquare :: Map Char [Char]
vSquare =
  alpha & fmap (\c -> (c, rotate (Char.ord c - Char.ord 'A') alpha))
        & Map.fromList
  where alpha = ['A'..'Z']

-- | Attempt to return the ciphertext character at a given `row` and `col` pair.
vSquareGet :: Char -> Char -> Maybe Char
vSquareGet row col =
  case Map.lookup row vSquare of
    Nothing -> Nothing
    Just alpha -> Just (alpha !! (Char.ord col - Char.ord 'A'))

-- | Attempt to return the plaintext character for a ciphertext char, `char` at `row`.
vSquareReverseGet :: Char -> Char -> Maybe Char
vSquareReverseGet row char =
  case Map.lookup row vSquare of
    Nothing -> Nothing
    Just alpha ->
      case List.elemIndex char alpha of
        Nothing -> Nothing
        Just i -> Just $ Char.chr (i + Char.ord 'A')

-- | Return the ciphertext for `x` using `keyword`.
encode :: String -> String -> String
encode x keyword =
  zip x (cycle keyword)
    & fmap (\(c, c') -> vSquareGet c' c)
    & Maybe.catMaybes

-- | Return the plaintext for `x` using `keyword`.
decode :: String -> String -> String
decode x keyword =
  zip x (cycle keyword)
    & fmap (\(char, row) -> vSquareReverseGet row char)
    & Maybe.catMaybes