Get and set record fields dynamically using DataKinds

EnvAccess.hs

{-# LANGUAGE AllowAmbiguousTypes #-}  -- GHC prompted me
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeApplications #-}  -- required for the examples
module EnvAccess where

data EnvType = LocalEnv | GlobalEnv

data Env = Env {localEnv :: Int, globalEnv :: Int} deriving Show

env :: Env
env = Env 1 2

class EnvGetSet (a :: EnvType) where
  getEnv :: Env -> Int
  setEnv :: Env -> Int -> Env

instance EnvGetSet LocalEnv where
  getEnv = localEnv
  setEnv e x = e {localEnv = x}

instance EnvGetSet GlobalEnv where
  getEnv = globalEnv
  setEnv e x = e {globalEnv = x}


-- getEnv @LocalEnv       ->  1
-- setEnv @LocalEnv 100   ->  Env {localEnv = 100, globalEnv = 2}
-- getEnv @GlobalEnv      ->  2
-- setEnv @GlobalEnv 200  ->  Env {localEnv = 1, globalEnv = 200}