从IO Maybe a转换为EitherT IO a

我正在使用API的servant库。它运行在：EitherT (Int, String) IO a monad中。我有一个类型IO Maybe a的功能，并且想要使用它。从IO Maybe a转换为EitherT IO a

这里有一个工作的例子：

sourcesGetAll :: EitherT (Int, String) IO [Source] 
sourcesGetAll = liftIO $ sourcesList h 

sourcesList :: IO [Source]

但现在我要使用这两个功能结合在一起

sourcesFind :: IO (Maybe Source) 
sourcesGetOne :: EitherT (Int, String) IO Source 
sourcesGetOne = ???

我想做到这一点：

maybeNotFound :: Maybe a -> Either (Int, String) a 
maybeNotFound Nothing = Left (404, "Not Found") 
maybeNotFound Just a = Right a

怎么办我是用所有花哨的单子做的？

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2015-04-15 Sean Clark Hess

您可以使用hoistEither :: Monad m => Either a b -> EitherT a m b来实现这一点：

maybeNotFoundT :: IO (Maybe a) -> EitherT (Int, String) IO a 
maybeNotFoundT maAct = do 
    ma <- liftIO maAct -- get the Maybe a. Now ma :: Maybe a 
    hoistEither $ maybeNotFound ma

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2015-04-15 17:03:06 bennofs

可以分解成两个独立的问题是：

变换IO (Maybe a)到MaybeT IO a
变换MaybeT IO a为EitherT (Int, String) a

第一种是通过使用MaybeT构造解决：

noteT :: Monad m => a -> MaybeT m b -> EitherT a m b

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2015-04-15 17:03:15

我想既然是切换到ExceptT而不是EitherT这不会以'错误2.0'工作？ – bennofs

正确。 'errors-2.0'将使用'ExceptT'而不是'EitherT' –

另一个答案感谢mpickering freenode上：

MaybeT :: IO (Maybe a) -> MaybeT IO a

二是通过使用noteT从errors库解决：

sourcesGetOne = EitherT $ maybeNotFound <$> sourcesFind

所以，问题是如何编写一个函数.. IO (Maybe a) -> EitherT (Int, String) IO a 给出一个函数f :: Maybe a -> Either (Int, String) a，再一个办法是..

myFunc action = EitherT (f <$> action)

如果你看一下文档的EitherT - https://hackage.haskell.org/package/either-4.3.3.2/docs/Control-Monad-Trans-Either.html。然后你会看到EitherT (Int, String) IO a，实际上只是一个包裹起来IO (Either (Int, String) a)

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2015-04-15 17:18:28

从IO Maybe a转换为EitherT IO a

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