问Monad变压器.显式提升

EN

{-# 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

Couldn't match type `t0 m0' with `MyApp'
Expected type: MyApp ()
Actual type: t0 m0 ()