haskell - 使用透镜对 StateT 中的内部单子(monad)进行操作

Question

请参阅下面的increment函数中liftIO的使用。我有预感，有更好的方法可以使用镜头来处理它。有什么建议吗？

{-# LANGUAGE TemplateHaskell #-}

import Control.Lens
import Control.Monad.IO.Class
import Control.Monad.State

data PersistentCounter = PersistentCounter {
    _cValue :: Int,
    _cFilename :: FilePath
  } deriving Show
makeLenses ''PersistentCounter

increment :: StateT PersistentCounter IO ()
increment = do
  t <- cValue <+= 1
  f <- use cFilename
  liftIO $ writeFile f $ show t

---

编辑:我认为可能有一种类似于下面的“zoom2”的机制。

increment :: StateT PersistentCounter IO ()
increment = do
  t <- cValue <+= 1
  zoom2 store 

store :: PersistentCounter -> IO ()
store counter =
  writeFile (counter^.cFilename) $ show (counter^.cValue)

-- | Invoke a function in the inner monad, and pass the state as
--   a parameter.
zoom2 :: Monad m => (s -> m ()) -> StateT s m ()
zoom2 f = get >>= (\s -> lift (f s)) >> return ()

Answer 1

我之前(几乎)写过很多次zoom2——我通常称之为getsM。通用版本看起来像

-- from Control.Monad.State
gets  :: MonadState s m => (s ->   a) -> m a
getsM :: MonadState s m => (s -> m a) -> m a
getsM f = state  $ \s -> (,s) <$> f s

或者，如果您不介意修复特定堆栈

-- from Control.Monad.Trans.State
gets  :: Monad m => (s ->   a) -> StateT s m a
getsM :: Monad m => (s -> m a) -> StateT s m a 
getsM f = StateT $ \s -> (,s) <$> f s