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 <p...@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. > >
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