Haskeline:为` `ReaderT r (StateT S m)`堆栈实现`MonadException`实例

Question

假设我有以下monad转换器堆栈(为简单起见，将r和s保留为() )，

newtype MyMonad m a = MM (ReaderT () (StateT () m a)

如果我想使用它作为haskeline的InputT的基单体，我需要一个System.Console.Haskeline.MonadException实例。考虑到这些实例的apparent complexity，我更愿意让编译器用GeneralizedNewtypeDeriving为我派生它。具体地说，我希望下面的代码进行类型检查，

{-# LANGUAGE GeneralizedNewtypeDeriving, StandaloneDeriving, FlexibleContexts #-}

import Control.Monad.State
import Control.Monad.Reader
import Control.Monad.IO.Class
import Control.Applicative
import System.Console.Haskeline.MonadException

newtype MyMonad m a = MM (ReaderT () (StateT () m) a)
                    deriving (Functor, Applicative, Monad, MonadIO)
deriving instance (MonadException m) => MonadException (MyMonad m)        

但不幸的是，这给了我

/home/bgamari/hi.hs:11:1:
    Could not deduce (MonadException (StateT () m))
      arising from the superclasses of an instance declaration
    from the context (MonadIO (MyMonad m), MonadException m)
      bound by the instance declaration at /home/bgamari/hi.hs:11:1-66
    Possible fix:
      add an instance declaration for (MonadException (StateT () m))
    In the instance declaration for `MonadException (MyMonad m)'

查看为StateT和ReaderT提供的实例，

instance MonadException m => MonadException (ReaderT r m)
instance MonadException m => MonadException (StateT s m)

期望编译器推导出StateT实例似乎是完全合理的。我对巧妙的GeneralizedNewtypeDeriving期望太高了吗？如果不对其进行开放编码，如何实现此实例？

Answer 1

有两个版本的State monad：strict和lazy。import Control.Monad.State brings in the lazy version，但MonadException的实例似乎是针对严格版本的。

尝试使用import Control.Monad.State.Strict而不是import Control.Monad.State。