Peter Verswyvelen wrote:
Consider the following code
stamp v x = do t <- getCurrentTime putMVar v (x,t)
Is it possible - with GHC - that a thread switch happens after the t <- getCurrentTime and the putMVar v (x,t)?
If so, how would it be possible to make sure that the operation of reading the current time and writing the pair to the MVar is an "atomic" operation, in the sense that no thread switch can happen between the two? Would this require STM?
Thanks again for sharing your wisdom with me :)
Peter
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.