Asynchronous exceptions in threadWait

Andrew Martin andrew.thaddeus at
Thu Jan 17 22:43:05 UTC 2019

Thanks. That makes sense. So, in the code I’m looking at, since takeMVar is the last effectful thing that happens, there is no semantic difference between having mask_ extend all the way down to the bottom of the block (as it currently does) and cutting it off right before the line involving takeMVar?

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On Jan 17, 2019, at 5:23 PM, Michael Walker <mike at> wrote:

>> Although the entire function has asynchronous exceptions masked, the call to takeMVar uses onException to deal with the possibility of an exception. According to the docs in Control.Concurrent, takeMVar can throw exceptions. But my understand (which may be wrong) is that the only exception this could throw would be something like BlockedIndefinitelyOnMVar, which I don't believe can happen here.
> That's not quite right.  The mask_ function blocks asynchronous
> exceptions from being delivered to the thread while it's not
> "interruptible".  Most blocking functions, such as takeMVar, make the
> thread interruptible while they're blocked.  So any asynchronous
> exception (such as ThreadKilled) could be delivered to the thread if
> it blocks in the takeMVar.
> -- 
> Michael Walker (

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