如何捕获在C函数调用的Haskell回调函数中引发的Haskell异常？ [英] How to catch a Haskell exception that is thrown in a Haskell callback function called by a C function?

问题描述

是否有什么好方法可以捕获由ac函数调用的haskell回调函数引发的haskell异常？

Is there any good way to catch a haskell exception, which is thrown in a haskell callback function called by a c function?

例如，让我有一个简单的例子仅调用给定回调的c函数

For example, let me have a simple c function which just calls a given callback,

void callmeback ( void (*callback) () ) {
  callback ();
}

以及通过ffi使用此功能的haskell代码。

and a haskell code which uses this function via ffi.

{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE ForeignFunctionInterface #-}
module Main (main) where

import Foreign
import Control.Exception
import Data.Typeable

foreign import ccall safe "wrapper"
        mkCallback :: IO () -> IO (FunPtr (IO ()))

foreign import ccall safe "callmeback"
        callmeback :: FunPtr (IO ()) -> IO ()

data AnError = AnError String deriving (Show, Eq, Typeable)
instance Exception AnError             

callback :: IO ()
callback = throwIO $ AnError "Catch me."

callMeBack :: IO () -> IO ()
callMeBack f = do fp <- mkCallback f
                  callmeback fp
main = do callMeBack callback `catch`
             (\ e -> do putStrLn $ show (e :: AnError)
                        putStrLn "I caught you." )
          putStrLn "-- Happy end."

执行结果（使用GHC 7.8.2编译）如下：

The exection result (compiling with GHC 7.8.2) is as follows:

% ./Catch
Catch: AnError "Catch me."

因此，似乎在回调中抛出了异常无法在 main 中捕获。
如何使这种代码正常工作？

So it seems like the exception thrown in callback can not be caught in main. How can I make this kind code work well?

推荐答案

您必须手动执行此操作，这看起来像

You have to do this manually, which would look like this:

• 在Haskell代码中包装您的回调函数，该函数调用 try ，然后将生成的 Esomeer SomeException（）序列化为可以从C处理的格式（可以使用 StablePtr SomeException ，但关键是您需要以某种方式处理 Either ）。

• Wrap your callback function in Haskell code that calls try, and then serializes the resulting Either SomeException () to a format you can process from C (you can use a StablePtr for the SomeException, but the point is that you need to handle the Either somehow).

在您的C代码调用回调的地方，检查结果是否为 Left exn ，如果是，则将错误传播到C代码的顶层，并在此过程中适当地释放资源。此步骤是非机械的，因为C没有异常。

Where your C code calls the callback, check whether the result was a Left exn and if so, propagate the error to the top level of your C code, freeing resources as appropriate along the way. This step is non-mechanical, since C does not have exceptions.

在C代码的顶层，重新序列化异常或结果并读取将其保存在Haskell函数中，该函数包装对C代码的调用，引发异常或适当地返回结果。

At the top level of your C code, re-serialize the exception or result and read it in a Haskell function that wraps your call to the C code, raising an exception or returning the result as appropriate.

我不知道执行此操作的程序的任何示例。

I'm not aware of any example of a program that does this.

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