Is there a reason why the lift function in ReaderT's MonadTrans instance is implemented as:

instance MonadTrans (ReaderT r) where

    lift m = ReaderT $ \_ -> m



Instead of just using the monad's return function?     Could  "lift m" be implemented as "return m"?


instance (Monad m) => Monad (ReaderT r m) where

    return a = ReaderT $ \_ -> return a

    m >>= k  = ReaderT $ \r -> do

        a <- runReaderT m r
        runReaderT (k a) r

    fail msg = ReaderT $ \_ -> fail msg



Thanks,

Daryoush