On 23 June 2004 17:46, S. Alexander Jacobson wrote:

Basically, what I want is a tryReadSampleVar function.

Or, if we stick with MVars, I don't want another thread to operate between the tryTakeMVar and the putMVar. In particular, I don't want another thread to believe (mistakenly) that the mvar is empty when it is simply being updated.

I suppose I could define a new abstraction where I have a second mvar that acts as an exclusive lock on the the first MVar. e.g.

data XMVar a = XMVar (MVar ()) MVar a

overWriteXMVar (XMVar lock mvar) val = withMVar lock (\_->tryTakeMVar mvar >> putMVar mvar val) tryTakeXMVar (XMVar lock mvar) val = withMVar lock (\_->tryTakeMVar mvar)

But, I think it would be more sane simply to have a tryReadSampleVar somewhere.

If you send me code for tryReadSampleVar, I'll incorporate it. Unfortunately SampleVar is abstract, so you'll need to take the source for that library (libraries/base/Control/Concurrent/SampleVar.hs in a GHC distribution) and modify it to create your own version. Cheers, Simon