[Haskell-cafe] Re: [Haskell] IVar

Simon Marlow simonmarhaskell at gmail.com
Thu Dec 6 04:04:01 EST 2007

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 

> 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.


More information about the Haskell-Cafe mailing list