MailboxProcessor 和异常

问题描述

我想知道，为什么 MailboxProcessor 处理异常的默认策略只是默默地忽略它们.例如:

I wonder, why MailboxProcessor's default strategy of handling exceptions is just silently ignore them. For example:

let counter =
    MailboxProcessor.Start(fun inbox ->
        let rec loop() =
            async { printfn "waiting for data..."
                    let! data = inbox.Receive()
                    failwith "fail" // simulate throwing of an exception
                    printfn "Got: %d" data
                    return! loop()
            }
        loop ())
()
counter.Post(42)
counter.Post(43)
counter.Post(44)
Async.Sleep 1000 |> Async.RunSynchronously

然后什么也没有发生.没有程序执行的致命停止，或出现未处理的异常"的消息框.没什么.

and nothing happens. There is no fatal stop of the program execution, or message box with "An unhandled exception" arises. Nothing.

如果有人使用 PostAndReply 方法，这种情况会变得更糟:结果是有保证的死锁.

This situation becomes worse if someone uses PostAndReply method: a guaranteed deadlock as the result.

这种行为有什么原因吗?

Any reasons for such behavior?

推荐答案

我认为 F# 中的 MailboxProcessor 不包含任何处理异常的机制的原因是不清楚是什么这样做的最佳方式.例如，您可能希望在发生未处理的异常时触发一个全局事件，但您可能希望在下一次调用 Post 或 PostAndReply 时重新抛出异常.

I think the reason why the MailboxProcessor in F# does not contain any mechanism for handling exceptions is that it is not clear what is the best way for doing that. For example, you may want to have a global event that is triggered when an unhandled exception happens, but you may want to rethrow the exception on the next call to Post or PostAndReply.

这两个选项都可以基于标准的MailboxProcessor 实现，因此可以添加您想要的行为.例如，以下代码段显示了添加全局异常处理程序的 HandlingMailbox.它与普通的MailboxProcessor 接口相同(我省略了一些方法)，但它添加了发生异常时触发的OnError 事件:

Both of the options can be implemented based on the standard MailboxProcessor, so it is possible to add the behaviour you want. For example, the following snippet shows HandlingMailbox that adds a global exception handler. It has the same interface as normal MailboxProcessor (I omitted some methods), but it adds OnError event that is triggered when an exception happens:

type HandlingMailbox<'T> private(f:HandlingMailbox<'T> -> Async<unit>) as self =
  let event = Event<_>()
  let inbox = new MailboxProcessor<_>(fun inbox -> async {
    try 
      return! f self
    with e ->
      event.Trigger(e) })
  member x.OnError = event.Publish
  member x.Start() = inbox.Start()
  member x.Receive() = inbox.Receive()
  member x.Post(v:'T) = inbox.Post(v)
  static member Start(f) =
    let mbox = new HandlingMailbox<_>(f)
    mbox.Start()
    mbox

要使用它，您需要编写与之前相同的代码，但现在您可以异步处理异常:

To use it, you would write the same code as what you wrote before, but you can now handle exceptions asynchronously:

let counter = HandlingMailbox<_>.Start(fun inbox -> async {
  while true do 
    printfn "waiting for data..." 
    let! data = inbox.Receive() 
    failwith "fail" })

counter.OnError.Add(printfn "Exception: %A")
counter.Post(42) 

这篇关于MailboxProcessor 和异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！