Re: [Haskell-cafe] Printing of asynchronous exceptions to stderr

On 13/11/2010 19:08, Bit Connor wrote:

On Wed, Nov 10, 2010 at 5:48 PM, Simon Marlow wrote:

...

[...] So we should say there are a few things that you can do that guarantee not to call any interruptible operations:

- IORef operations - STM transactions that do not use retry - everything from the Foreign.* modules

and probably some other things. Maybe we should put this list in the documentation.

A list would be very helpful. I am specifically interested in knowing about:

- The try family of MVar functions - the forkIO function itself

I added the following text to the Control.Exception documentation, in the section on "Interruptible operations". Suggestions for improvements are welcome: It is useful to think of 'mask' not as a way to completely prevent asynchronous exceptions, but as a filter that allows them to be raised only at certain places. The main difficulty with asynchronous exceptions is that they normally can occur anywhere, but within a 'mask' an asynchronous exception is only raised by operations that are interruptible (or call other interruptible operations). In many cases these operations may themselves raise exceptions, such as I\/O errors, so the caller should be prepared to handle exceptions arising from the operation anyway. Sometimes it is too onerous to handle exceptions in the middle of a critical piece of stateful code. There are three ways to handle this kind of situation: * Use STM. Since a transaction is always either completely executed or not at all, transactions are a good way to maintain invariants over state in the presence of asynchronous (and indeed synchronous) exceptions. * Use 'mask', and avoid interruptible operations. In order to do this, we have to know which operations are interruptible. It is impossible to know for any given library function whether it might invoke an interruptible operation internally; so instead we give a list of guaranteed-not-to-be-interruptible operations below. * Use 'uninterruptibleMask'. This is generally not recommended, unless you can guarantee that any interruptible operations invoked during the scope of 'uninterruptibleMask' can only ever block for a short time. Otherwise, 'uninterruptibleMask' is a good way to make your program deadlock and be unresponsive to user interrupts. The following operations are guaranteed not to be interruptible: * operations on 'IORef' from "Data.IORef" * STM transactions that do not use 'retry' * everything from the @Foreign@ modules * everything from @Control.Exception@ * @tryTakeMVar@, @tryPutMVar@, @isEmptyMVar@ * @takeMVar@ if the @MVar@ is definitely full, and conversely @putMVar@ if the @MVar@ is definitely empty * @newEmptyMVar@, @newMVar@ * @forkIO@, @forkIOUnmasked@, @myThreadId@ Cheers, Simon