Haskell（嵌套）ReaderT
如何访问 ReaderT 的内部 monad。
就我而言，我的类型是：
newtype VCSSetupAction a = VCSSetupAction (ReaderT (Maybe VCSConf) IDEM a)
deriving (Monad, MonadIO, MonadReader (Maybe VCSConf))
我无法访问在这个 Monad 中运行的函数中的（也许是 VCSConf）
commitAction' :: Common.VCSAction ()
commitAction' = do
config <- ask
...
，但我也应该能够访问内部 IDEM，它的类型是：
type IDEM = ReaderT IDERef IO
所以我希望能够做一些像
commitAction' :: Common.VCSAction ()
commitAction' = do
config <- ask
ideRef <- lift $ ask -- this does not compile/work
我仍然不太了解 Monad 的事情。感谢您的任何帮助。
how can you access the inner monad of a ReaderT.
In my case I have the typ:
newtype VCSSetupAction a = VCSSetupAction (ReaderT (Maybe VCSConf) IDEM a)
deriving (Monad, MonadIO, MonadReader (Maybe VCSConf))
I cann access the (Maybe VCSConf) in a function running in this Monad like
commitAction' :: Common.VCSAction ()
commitAction' = do
config <- ask
...
but I should also be able to access the inner IDEM which turns out to be of type:
type IDEM = ReaderT IDERef IO
so I want to be able to do something like
commitAction' :: Common.VCSAction ()
commitAction' = do
config <- ask
ideRef <- lift $ ask -- this does not compile/work
I still do not understand Monads well enough for this. Thanks for any help.
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要直接使用
lift，您的newtype必须派生MonadTrans，但由于它不是转换器，因此在本例中这是不合适的。相反，您可以将操作包装在数据构造函数中。现在您直接与ReaderT打交道，因此您可以使用它的lift。您可能想为此定义一个助手以使事情变得更清晰。
To use
liftdirectly, yournewtypewould have to deriveMonadTrans, but since it's not a transformer, this is not appropriate in this case. Instead, you can just wrap the action in your data constructor. Now you're dealing directly with theReaderT, so you can use itslift.You might want to define a helper for this to make things cleaner.