Monad transformer – Explicit lifting - monads

Question

I'm reading about monad transformers in Real World Haskell. In the following example, the stack is Writer on top State on top of Reader on top of IO.
{-# Language GeneralizedNewtypeDeriving #-}
import Control.Monad
import Control.Monad.State
import Control.Monad.Reader
import Control.Monad.Writer
import System.Directory
import System.FilePath
data AppConfig = AppConfig {
cfgMaxDepth :: Int
} deriving Show
data AppState = AppState {
stDeepestReached :: Int
} deriving Show
newtype MyApp a = MyA {
runA :: WriterT [(FilePath,Int)] (StateT AppState (ReaderT AppConfig IO)) a
} deriving (Monad, MonadIO, Functor, MonadReader AppConfig,
MonadWriter [(FilePath,Int)], MonadState AppState)
runApp :: MyApp a -> Int -> IO ([(FilePath,Int)], AppState)
runApp k maxDepth = let config = AppConfig maxDepth
state' = AppState 0
in runReaderT (runStateT (execWriterT $ runA k) state') config
constrainedCount :: Int -> FilePath -> MyApp ()
constrainedCount curDepth path = do
contents <- liftIO . getDirectoryContents $ path
cfg <- ask
let maxDepth = cfgMaxDepth cfg
tell [(path,curDepth)]
forM_ (filter (\d' -> d' /= ".." && d' /= ".") contents) $ \d -> do
let newPath = path </> d
isDir <- liftIO $ doesDirectoryExist newPath
when (isDir && curDepth < maxDepth) $ do
let newDepth = curDepth+1
st <- get
when (stDeepestReached st < newDepth) $
put st { stDeepestReached = newDepth }
constrainedCount newDepth newPath
main = runApp (constrainedCount 0 "/tmp") 2 >>= print
I (think I) understand how I can simply call ask, get and put since these are defined in the MonadReader, MonadWriter and MonadState typeclasses and there are instances such as MonadWriter (StateT s m) and so on.
What I don't understand is why I cannot explicit lift an action from the layer below up to the current monad transformer. In constrainedCount I'm in the Reader monad, if I understand correctly, and I thought both st <- get and st <- lift get should work. (And that tell and lift . lift . tellshould be the same). If I changest <- gettost <- lift get` I get the error
Couldn't match type `t0 m0' with `MyApp'
Expected type: MyApp ()
Actual type: t0 m0 ()
which tells me very little... Is my understanding of this completely wrong?

Answer 1

Let's have a look at the type of lift get:
lift get :: (MonadTrans t, MonadState a m) => t m a
But your MyApp isn't a monad transformer, it's only a monad. But what's inside is, of course, so if you use
st <- MyA $ lift get
it works.