Simple case: output depending on input

Scratch.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeFamilies #-}
module Scratch where

import Data.Kind

data Protection = None | Password

type family Secured (p :: Protection) (a :: Type) where
  Secured None a = a
  Secured Password a = Maybe a

data Box (p :: Protection) (a :: Type) where
  PlainBox :: a -> Box 'None a
  SecureBox :: String -> a -> Box 'Password a
deriving instance Functor (Box p)

openBox :: String -> Box p a -> Secured p a
openBox _ (PlainBox a) = a
openBox psswd (SecureBox secret a)
  | psswd == secret = Just a
  | otherwise = Nothing

main :: IO ()
main = do
  let plainBox = PlainBox "The cake is a lie."
      secureBox = SecureBox "1234" "The cake is a lie."
  print $ openBox "" plainBox
  print $ openBox "123" secureBox