If you have two streams of time/value pairs - using MVars as write-once sampling variables - and both streams are fed from another thread (e.g. timers firing), and you want to merge these two streams into a single stream with monotonic time stamps, then you want to be able to check if at time t an occurrence exists in a stream. In the case an old time was read but not yet written to an mvar, this could lead to the merged stream not being monotonic. At least in my C# prototype that was the case, I used many very high frequency timers to stress test the merger, and this bug popped up. I found similar code in the Reactive library, but Reactive is much more clever operationally (and semantically) than my little prototype so it might not be a problem. But if it is, I guess it can be solved by introducing another MVar to indicate "I'm reading time", at least I solved it in the C# prototype in that way. 

On Sat, Feb 14, 2009 at 8:01 PM, Paul Johnson <paul@cogito.org.uk> wrote:

I'm not entirely sure what you are trying to achieve here.  Presumably you want v to contain the (value, time) pair as soon as possible after time "t".  Of course it won't be instantaneous.  So another thread could observe that at time (t+delta) the variable "v" does not yet contain (x,t).  Is this a problem?

Atomic transactions won't work because "getCurrentTime" is of type "IO Time", whereas anything inside "atomic" has to be of type "STM a".

In theory you could lock out context switches by messing around with the GHC runtime, but if you are running on a multicore machine then that won't work either.

Paul.