为什么ContT不能成为MonadError的实例？

Question

我有一个monad变压器堆栈,包括一个ErrorT,我想ContT r围绕整个事情包装一个变压器.当我尝试这样做时,我调用throwError生成类型错误 - 显然ContT r不是自动的实例MonadError.好吧,我想 - 我会把它变成一个:

instance MonadError e m => MonadError e (ContT r m) where
  throwError = lift . throwError
  catchError = liftCatch . catchError

使用一些合适的定义liftCatch.但是现在我在编译时遇到错误:

src\Language\Types.hs:68:10:
    Illegal instance declaration for `MonadError e (ContT r m)'
      (the Coverage Condition fails for one of the functional dependencies;
       Use -XUndecidableInstances to permit this)
    In the instance declaration for `MonadError e (ContT r m)'

我很高兴使用UndecidableInstances pragma(我的印象并不是太令人担忧,例如看到这个问题)但是我想知道将延续变换器变成一个实例是否有困难MonadError- 我想如果它是很好,Control.Monad.Trans包的作者已经做到了......对吗？

Answer 1

ContT和ErrorT都允许非标准控制流程.有一种方法可以在mtl中围绕ContT包装ErrorT类型:

instance (Error e, MonadCont m) => MonadCont (ErrorT e m)

但这两个monad变压器不通勤.记住:

newtype Identity a = Identity {runIdentity :: a}
newtype ErrorT e m a = ErrorT {runErrorT :: m (Either e a)}
newtype ContT r m a = ContT {runContT :: (a -> m r) -> m r}

ErrorT String (ContT Bool Identity) () 在mtl包中没问题可能是:

ErrorT (ContT ( \ (k :: Either String () -> Identity Bool) -> k (Right ()) ) )

ContT r (ErrorT e Identity) a包mtl不好.但是你可以写出来.

你想在组合monad中使用(>> =)的语义是什么？您如何期望您的嵌套错误处理程序堆栈与非本地callCC交互？

我可以这样写它:

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, UndecidableInstances   #-}
import Control.Monad
import Control.Monad.Cont
import Control.Monad.Error
import Data.Function
import Data.IORef

handleError :: MonadError e m => (e -> m a) -> m a -> m a
handleError = flip catchError

test2 :: ErrorT String (ContT () IO) ()
test2 = handleError (\e -> throwError (e ++ ":top")) $ do
  x <- liftIO $ newIORef 1
  label <- callCC (return . fix)
  v <- liftIO (readIORef x)
  liftIO (print v)
  handleError (\e -> throwError (e ++ ":middle")) $ do
    when (v==4) $ do
      throwError "ouch"
  when (v < 10) $ do
         liftIO (writeIORef x (succ v))
         handleError (\e -> throwError (e ++ ":" ++ show v)) label
  liftIO $ print "done"

go2 = runContT (runErrorT test2) (either error return)

{-

*Main> go2
1
2
3
4
*** Exception: ouch:middle:top

-}

所以上面只使用mtl,这是新实例及其工作原理:

instance MonadError e m => MonadError e (ContT r m) where
  throwError = lift . throwError
  catchError op h = ContT $ \k -> catchError (runContT op k) (\e -> runContT (h e) k)

test3 :: ContT () (ErrorT String IO) ()
test3 = handleError (\e -> throwError (e ++ ":top")) $ do
  x <- liftIO $ newIORef 1
  label <- callCC (return . fix)
  v <- liftIO (readIORef x)
  liftIO (print v)
  handleError (\e -> throwError (e ++ ":middle")) $ do
    when (v==4) $ do
      throwError "ouch"
  when (v < 10) $ do
         liftIO (writeIORef x (succ v))
         handleError (\e -> throwError (e ++ ":" ++ show v)) label
  liftIO $ print "done"

go3 = runErrorT (runContT test3 return)

{-

*Main> go3
1
2
3
4
Left "ouch:middle:3:middle:2:middle:1:middle:top"

-}