{-# LANGUAGE DeepSubsumption #-}

module Foo where

import Control.Monad.Reader
import Control.Monad.State

data Bar

db :: forall m. Monad m => ReaderT Bar m Int
db = undefined

newtype DBR a = DBR {unDBR :: forall m. Monad m => ReaderT Bar m a}

instance Functor DBR where
  fmap :: (a -> b) -> DBR a -> DBR b
  fmap f (DBR a) = DBR $ fmap f a

instance Applicative DBR where
  pure :: a -> DBR a
  pure a = DBR $ pure a

  (<*>) :: DBR (a -> b) -> DBR a -> DBR b
  (<*>) (DBR f) (DBR a) = DBR $ f <*> a

instance Monad DBR where
  (>>=) :: DBR a -> (a -> DBR b) -> DBR b
  (>>=) (DBR a) f = DBR $ a >>= unDBR . f

data Baz
  
works :: StateT Baz DBR Int
works = lift $ DBR $ db

doesnotwork :: StateT Baz DBR Int
doesnotwork = lift . DBR $ db