So you're saying that it's expected behavior for the shim library you're providing to have drastically different behavior between different versions of GHC? I don't think that's a good idea at all. In any event, this approach is still predicated on the idea that you can identify an asynchronous event from its type. There are multiple problems with this: * As demonstrated with my previous example, not all asynchronous exceptions identify themselves as such. * There is no requirement that only asynchronous-type exceptions be thrown asynchronously. throwTo works with any instance of Exception. * If an asynchronous-type exception is caught and then rethrown as a synchronous exception, the type-based approach will still treat it as asynchronous, though it should be recognized as synchronous at that point. To demonstrate that last point, consider the example code below, which uses your asynchronous type machinery and the async package. The usage of `trySync` in `main` *should* catch that exception, since it is no longer asynchronous, but a type-only approach cannot handle that situation. ClassyPrelude's tryAny, on the other hand, gives the correct output. {-# LANGUAGE DeriveDataTypeable #-} import Control.Exception.Async import Control.Concurrent.Async import Control.Concurrent import Control.Exception import Data.Unique import Data.Typeable data Timeout = Timeout deriving (Typeable, Eq) instance Show Timeout where show _ = "Async Timeout" instance Exception Timeout where fromException = asyncExceptionFromException toException = asyncExceptionToException asyncTimeout n f = do pid <- myThreadId killer <- forkIO $ do threadDelay n throwTo pid Timeout res <- f killThread pid return res main :: IO () main = do res <- trySync f print res f :: IO String f = do x <- async $ asyncTimeout 1000000 $ do threadDelay 2000000 return "Finished" wait x On Wed, Feb 5, 2014 at 5:19 PM, Roman Cheplyaka <roma@ro-che.info> wrote:
It works as expected.
With GHC 7.8 it doesn't print the timeout exception.
With earlier GHC it does (again, as expected, because the timeout exception isn't marked as asynchronous). In practice one should rarely want to use System.Timeout anyway (because of the overflow issue), and I'm going to patch one of the better timeout packages (such as unbounded-delays) to support asynchronous-exceptions.
* Michael Snoyman <michael@snoyman.com> [2014-02-05 16:56:31+0200]
I don't think this package works as expected. Consider the following:
import Control.Concurrent import Control.Exception.Async import System.Timeout
main :: IO () main = do timeout 1000000 $ do threadDelay 10000000 `catchSync` \e -> do print e threadDelay 10000000 return ()
The expected behavior would be that the timeout- an async exception- would kill the thread delay, the catch would ignore the async exception, and the program would exit. In reality, catchSync treats the timeout as a synchronous exception, prints it, and delays once again. Compare this to classy-prelude's catchAny, which handles the situation correctly, via the technique I described in "Catching all exceptions."[1]
In this case, the issue is that the timeout exception type is not recognized as async, and a special case could be added to handle that exception type[2]. However, I think the overall approach of determining *how* an exception was thrown based on *what* was thrown is not tenable.
[1]
https://www.fpcomplete.com/user/snoyberg/general-haskell/exceptions/catching...
[2] It's a bit difficult to do so, since IIRC the type is never exported. But a hack using the Typeable instance- while ugly- is likely possible.
On Wed, Feb 5, 2014 at 1:28 PM, Roman Cheplyaka <roma@ro-che.info> wrote:
The links are:
http://hackage.haskell.org/package/asynchronous-exceptions https://github.com/feuerbach/asynchronous-exceptions
* Roman Cheplyaka <roma@ro-che.info> [2014-02-05 13:23:38+0200]
It is often useful to distinguish between synchronous and asynchronous exceptions. The common idiom is to run a user-supplied computation catching any synchronous exceptions but allowing asynchronous exceptions (such as user interrupt) pass through.
base 4.7 (shipped with GHC 7.8) will have SomeAsyncException type that solves this problem.
asynchronous-exceptions is a new package that serves two purposes: * provide compatibility with older `base` versions that lack the `SomeAsyncException` type * define convenient functions for catching only synchronous exceptions
Roman
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