从IO转换为EitherT IO a

Question

我正在使用API的奴仆库。它运行在：EitherT (Int, String) IO a monad中。我有一个带有IO Maybe a类型的函数，并且希望使用它。

下面是一个有用的例子：

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

sourcesList :: IO [Source]

但是现在我想把这两个函数结合起来

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

我想这样做：

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

我怎么处理那些花哨的单簧管呢？

Answer 1

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

Answer 2

您可以将其分解为两个独立的问题：

• 将IO (Maybe a)转换为MaybeT IO a
• 将MaybeT IO a转换为EitherT (Int, String) a

第一个问题通过使用MaybeT构造函数解决：

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

第二个问题通过使用来自noteT库的errors来解决：

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

Answer 3

另一个答案是，由于在freenode上争吵不休：

sourcesGetOne = EitherT $ maybeNotFound <$> sourcesFind

所以问题是如何写一个函数。IO (Maybe a) -> EitherT (Int, String) IO a给出了一个函数f :: Maybe a -> Either (Int, String) a，那么一种方法是..。 myFunc action = EitherT (f <$> action) 如果你看一下EitherT的文档-- 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，实际上只是一个包装好的IO (Either (Int, String) a)