rotN :: forall a. (Bounded a, Enum a) => a -> a
rotN c = toEnum rotation
 where alphabetSize = 1 + fromEnum (maxBound :: a)
       halfAlphabet = alphabetSize `div` 2
       offset = fromEnum c + halfAlphabet
       rotation =  offset `mod` alphabetSize

rotNdecoder :: forall a. (Bounded a, Enum a) => a -> a
rotNdecoder c = toEnum rotation
 where n = 1 + fromEnum (maxBound :: a)
       halfN = n `div` 2
       offset = if even n
                then fromEnum c + halfN
                else 1 + fromEnum c + halfN
       rotation =  offset `mod` n

rotEncoder :: (Bounded a, Enum a) => [a] -> [a]
rotEncoder text = map rotN text

rotDecoder :: (Bounded a, Enum a) => [a] -> [a]
rotDecoder text = map rotNdecoder text