> Converting them to a cleaner ReaderT led to a 10% total run time regression, so I had to revert it. It makes me wonder about the speed penalty of code I desig

> Converting them to a cleaner ReaderT led to a 10% total run time regression, so I had to revert it. It makes me wonder about the speed penalty of code I designed better to begin with. I have heard from other Haskellers that you can sometimes get good performance by hand-rolling an application monad, and then writing out the MonadFoo instances so you get good ergonomics: -- Instead of this: newtype App a = App { runApp :: ReaderT AppEnv (ExceptT AppError IO) a } deriving (Functor, Applicative, Monad, MonadIO, MonadReader AppEnv, MonadError AppError) -- Try this: newtype App a = App { runApp :: AppEnv -> IO (Either AppError a) } deriving Functor instance Applicative App where ... instance Monad App where ... instance MonadIO App where ... instance MonadReader AppEnv App where ... instance MonadError AppError App where ...