[prev in list] [next in list] [prev in thread] [next in thread] 

List:       haskell-cafe
Subject:    Re: [Haskell-cafe] Shared/Exclusive Locks
From:       Robert Dockins <robdockins () fastmail ! fm>
Date:       2005-12-28 19:08:04
Message-ID: 913953D1-BBD0-429E-A38F-BC3EDC3D48EB () fastmail ! fm
[Download RAW message or body]


On Dec 28, 2005, at 1:38 PM, Tomasz Zielonka wrote:

> On Wed, Dec 28, 2005 at 05:28:28PM +0000, Chris Kuklewicz wrote:
>> But STM, wrapped in small pieces, makes for interesting IO commands
>> (untested):
>
>> waitForZero :: (Num a, Ord a) => (TVar a) -> IO ()
>> waitForZero tv = atomically $ do
>>   v <- readTVar tv
>>   when (v>0) retry
>
> This function is rather useless in IO - you will get race conditions.
> That's what you get for leaving the STM wonderland ;-)

Actually, I think it works in the particular instance given of  
constructing read-write locks because the call to waitForZero is  
guarded by a mutex.  But it would certainly be easy to introduce a  
race condition using constructs like this.  Given the alternatives  
{use STM fully, use STM some, don't use STM}, I would have a hard  
time justifying the "use STM some" alternative (at least for new  
programs).  If you are OK with introducing a dependency on STM, why  
not go whole hog?


Rob Dockins

Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
           -- TMBG




[prev in list] [next in list] [prev in thread] [next in thread]