What does interruptibility mean exactly?

[Bas van Dijk]

On Mon, Jun 14, 2010 at 11:20 PM, Don Stewart <dons at galois.com> wrote:
>
> v.dijk.bas:
>> Hello,
>>
>> I've a short question about interruptible operations. In the following
>> program is it possible for 'putMVar' to re-throw asynchronous
>> exceptions even when asynchronous exception are blocked/masked?
>>
>>   newEmptyMVar >>= \mv -> block $ putMVar mv x
>>
>> The documentation in Control.Exception about interruptible
>> operations[1] confused me:
>>
>> "Some operations are interruptible, which means that they can receive
>> asynchronous exceptions even in the scope of a block. Any function
>> which may itself block is defined as interruptible..."
>>
>
> I think the best definition of interruptible is in this paper:
>
>    www.haskell.org/~simonmar/papers/async.pdf
>
> Section 5.3
>

Thanks for the link Don! Next time I will re-read the paper before asking ;-)

The definition makes it clear indeed:

"Any operation which may need to wait indefinitely for a resource
(e.g., takeMVar) may receive asynchronous exceptions even within an
enclosing block, BUT ONLY WHILE THE RESOURCE IS UNAVAILABLE"

So I guess I can update my threads package to use MVars again. Nice!
because they were a bit faster in an informal benchmark I performed
some time ago.

A later quote from 5.3 emphasizes the definition even more:

"...an interruptible operation cannot be interrupted if the resource
it is attempting to acquire is always available..."

The following darcs patch makes the definition of
interruptibility in the documentation in Control.Exception a bit
clearer in this regard:

http://bifunctor.homelinux.net/~bas/doc-interruptibility.dpatch

Regards,

Bas