[Haskell-cafe] Re: [Haskell] IVar

[Simon Marlow]

Jan-Willem Maessen wrote:
> 
> On Dec 5, 2007, at 3:58 AM, Simon Marlow wrote:
>
>> Ah, so I was thinking of the following readIVar:
>>
>> readIVar = unsafePerformIO . readIVarIO
>>
>> But clearly there's a better one.  Fair enough.
> 
> Hmm, so unsafePerformIO doesn't deal with any operation that blocks?  

Well, operations that block inside unsafePerformIO do block the whole 
thread, yes, and that may lead to a deadlock if the blocking operation is 
waiting to be unblocked by its own thread.  It sounds like you want to back 
off from any earlier-than-necessary evaluation at this point.  Fortunately 
this problem doesn't arise, because GHC won't commute evaluation past an IO 
operation.

> I'm wondering about related sorts of examples now, as well:
> 
> do
>   x <- newIVar
>   y <- unsafeInterleaveIO (readIVarIO x)
>   writeIVar x 3
>   print y
> 
> Or the equivalent things to the above with MVars.

Yes, this suffers from the same problem.  If the compiler were to eagerly 
evaluate y, then you'll get deadlock.

Cheers,
	Simon