import Control.Monad.Reader

data Env = Env
  { counter :: !Int,
    flag :: !Bool
  }
  deriving (Show, Read)

class Environment r where
  getCounter :: r -> Int
  getFlag :: r -> Bool

instance Environment Env where
  getCounter = counter
  getFlag = flag

newtype App m a = App {run :: ReaderT Env m a}
  deriving (Functor, Applicative, Monad, MonadIO, MonadReader Env)

runApp :: Env -> App IO a -> IO a
runApp e (App r) = runReaderT r e

f1 :: (Environment e) => e -> IO ()
f1 e = putStrLn $ (("Counter: " ++) . show . getCounter) e

f2 :: (Environment e) => e -> IO ()
f2 e = putStrLn $ (("Counter: " ++) . show . (+ 4) . getCounter) e


f4 :: App IO ()
f4 = do
  env <- ask
  liftIO $ do
    f1 env
    f2 env

main :: IO ()
main = runReaderT (run f4) (Env {counter = 0, flag = False})