Example usage of Scope Type Variables

Cipher.hs

{-# LANGUAGE ScopedTypeVariables #-}

module Cipher where

data FourLetterAlphabet = L1 | L2 | L3 | L4 deriving (Show, Enum, Bounded)

-- Because of ScopedTypeVariables, the type variable "a" in the where clause
-- refers to the same type variable "a" as in the function's signature.
--
-- When we apply rotN to a value, all instances of "a" will be instantiated to
-- the type of that value. If we had not used ScopedTypeVariables, then the "a"
-- type variable in the where clause would not be instantiated, and we could not
-- resolve minBound and maxBound.
rotN :: forall a. (Bounded a, Enum a) => a -> a
rotN char = toEnum rotated
    where rotated = offset `mod` alphabetSize
          offset = fromEnum char + halfAlphabetSize
          halfAlphabetSize = alphabetSize `div` 2
          alphabetSize = mL [minBound .. maxBound]
          mL :: [a] -> Int
          mL = length