EitherT如何运作？

Question

我花了一半时间试图弄清楚如何使用EitherT作为处理代码中错误的方法.

我已经定义了这样的变压器堆栈.

-- Stuff Monad

data StuffConfig = StuffConfig {
  appId     :: T.Text,
  appSecret :: T.Text
}

data StuffState = StuffState {
  stateToken :: Maybe Token,
  stateTime  :: POSIXTime
}

newtype Stuff a = Stuff {
  runStuff :: (ReaderT StuffConfig (StateT StuffState (EitherT T.Text IO))) a
} deriving (Monad, Functor, Applicative, 
            MonadIO, 
            MonadReader StuffConfig,
            MonadState StuffState
            )



askStuff :: StuffConfig -> Stuff a -> IO (Either T.Text a)
askStuff config a = do
  t <- getPOSIXTime 
  runEitherT (evalStateT (runReaderT (runStuff a) config) (StuffState Nothing t))

只要我只使用ReaderT和StateT函数,这个效果很好.我的印象是,现在我应该可以这样写:

faultyFunction :: String -> Stuff String
faultyFunction s = do
  when s == "left" $ left "breaking out"
  "right"

更重要的是捕获Either应该可以hoistEither从errors包中获得的返回值:

faultyLookup :: Map -> String -> Stuff String
faultyLookup m k = do
  hoistEither $ lookup k m

我阅读了关于monad变形金刚的现实世界haskell章节,并在其中摆弄lift.但我无法得到任何东西.

Answer 1

你不能只是使用的原因left和hoistEither功能是直接,不像StateT和ReaderT来自mtl包,either包不提供类似类型类MonadReader或MonadState.

前面提到的类型类可以透明地处理monad堆栈中的提升,但是EitherT,你必须自己完成提升(或编写MonadEither类似于MonadReaderet al 的类型类).

faultyFunction :: String -> Stuff String
faultyFunction s = do
  when (s == "left") $ Stuff $ lift $ lift $ left "breaking out"
  return "right"

首先,你需要应用Stuff的包装,然后lift通过ReaderT变压器,然后lift通过再次StateT变压器.

您可能想为自己编写实用程序函数,例如

stuffLeft :: T.Text -> Stuff a
stuffLeft = Stuff . lift . lift . left

然后你可以像这样简单地使用它:

faultyFunction :: String -> Stuff String
faultyFunction s = do
  when (s == "left") $ stuffLeft "breaking out"
  return "right"

或者,如果为其定义实例,则可以使用Control.Monad.Errorfrom .mtlErrorText

instance Error T.Text where
  strMsg = T.pack

现在你可以改变的定义,Stuff实现left和hoistEither这样的:

newtype Stuff a = Stuff {
  runStuff :: (ReaderT StuffConfig (StateT StuffState (ErrorT T.Text IO))) a
} deriving (Monad, Functor, Applicative,
            MonadIO,
            MonadReader StuffConfig,
            MonadState StuffState,
            MonadError T.Text
            )

left :: T.Text -> Stuff a
left = throwError

hoistEither :: Either T.Text a -> Stuff a
hoistEither = Stuff . lift . lift . ErrorT . return

使用此原始faultyFunction类型检查,无需任何手动提升.

您还可以为(使用from )的任何实例编写通用实现left并使用hoistEither它们:MonadErroreitherData.Either

left :: MonadError e m => e -> m a
left = throwError

hoistEither :: MonadError e m => Either e a -> m a
hoistEither = either throwError return