如何使用 monad-control 通过 ReaderT 进行简单的新型包装

Question

我定义了一个简单的 monad 转换器，EntityBuilderT，它只是 ReaderT 上的一个新类型。

data EntityBuilderState = ...

newtype EntityBuilderT m a = EntityBuilderT (ReaderT EntityBuilderState m a)

为了将函数包装在新的“环境”中，我编写了以下组合器：

withNewSource :: (Monad m) => String -> EntityBuilderT m a -> EntityBuilderT m a
withNewSource itemId builder = ...

在某些情况下，我还想构建更大的变压器堆栈。例如：

f :: MaybeT (EntityBuilderT m) a

显然，我无法将 withNewSource 应用于此函数 f，因为 monad 类型不再匹配。因此，我尝试使用 monad-control 编写此类组合器的新版本。

到目前为止我编写的代码如下所示。尽管实例定义似乎没问题，但编译器 (GHC 7.4.1) 拒绝了该代码并显示以下消息：

   Couldn't match type `IO' with `EntityBuilderT m0'
    When using functional dependencies to combine
      MonadBaseControl IO IO,
        arising from the dependency `m -> b'
        in the instance declaration in `Control.Monad.Trans.Control'
      MonadBaseControl (EntityBuilderT m0) IO,
        arising from a use of `control'
    In the expression: control
    In the expression: control $ \ run -> withNewSource itemId (run m)

我有点迷失了。有人知道问题到底是什么吗？

---

{-# LANGUAGE FlexibleInstances, GeneralizedNewtypeDeriving,
             MultiParamTypeClasses, TypeFamilies, UndecidableInstances #-}

import Control.Applicative (Applicative)
import Control.Monad (liftM)
import Control.Monad.Base
import Control.Monad.Trans (MonadTrans)
import Control.Monad.Trans.Control
import Control.Monad.Trans.Maybe (MaybeT)
import Control.Monad.Trans.Reader (ReaderT, withReaderT)


data EntityBuilderState

newtype EntityBuilderT m a = EntityBuilderT { unEB :: ReaderT EntityBuilderState m a }
  deriving (Applicative, Functor, Monad, MonadTrans)

instance MonadBase b m => MonadBase b (EntityBuilderT m) where
    liftBase = liftBaseDefault

instance MonadTransControl EntityBuilderT where
    newtype StT EntityBuilderT a = StEB { unStEB :: StT (ReaderT EntityBuilderState) a }
    liftWith f = EntityBuilderT $ liftWith $ \run ->
                   f $ liftM StEB . run . unEB
    restoreT = EntityBuilderT . restoreT . liftM unStEB

instance MonadBaseControl b m => MonadBaseControl b (EntityBuilderT m) where
    newtype StM (EntityBuilderT m) a = StMT { unStMT :: ComposeSt EntityBuilderT m a }
    liftBaseWith = defaultLiftBaseWith StMT
    restoreM     = defaultRestoreM   unStMT


withNewSource :: (Monad m) => String -> EntityBuilderT m a -> EntityBuilderT m a
withNewSource itemId (EntityBuilderT m) = EntityBuilderT (withReaderT undefined m)

withNewSource' :: String -> MaybeT (EntityBuilderT IO) a -> MaybeT (EntityBuilderT IO) a
withNewSource' itemId m = control $ \run -> withNewSource itemId (run m)

Answer 1

问题是，因为基本 monad 是 IO，所以 run 的类型是 MaybeT (EntityBuilderT IO) a -> IO (StM (MaybeT (EntityBuilderT IO) a))，但您将其返回值用作 EntityBuilderT IO 操作。此外，传递给 control 的函数的返回值必须位于 IO 中，而不是 EntityBuilderT IO 中。

这是因为您的 MonadBaseControl 实例表示您将事物提升到转换后的 monad m 的基础 monad 中；由于 MaybeT (EntityBuilderT IO) 的基础是 IO，因此 control 从 RunInBase (MaybeT (EntityBuilderT IO)) IO 获取函数 到 IO (StM (MaybeT (EntityBuilderT IO)) a)。

不幸的是，我在 monad-control 方面没有足够的经验来提出解决方案；也许您可以使用 MaybeT 的 MonadTransControl 实例来实现“下一级”功能？