From: Jiansen He
Hi cafe,
I wounder if it is possible to tell a haskell system that two computations with side effects could be executed concurrently.
Here is an affected example:
Suppose two people want to compare their age, but do not want to leak their personal information. The following program reads one person's age after another then send back compared result.
age :: IO () age = do i <- readIntFrom a j <- readIntFrom b writeTo a (i-j) writeTo b (j-i)
How can I express the fact that two readings could be carried out in any order?
The best approach is probably to fork a thread for each concurrent evaluation. There are a few packages on Hackage that can help with this; I've used threads[1] and spawn[2] successfully. If the "readIntFrom" function is meant to be responding to a client, a threaded model may make sense organizationally, but for this to be a performance benefit there needs to be sufficient work for each thread to carry out. Haskell threads are lighter than Posix threads, but there's still overhead involved. There is an alternative I hesitate to mention because it involves some voodoo, but Roman Leshchinskiy came up with a clever technique that might be appropriate if you want the behavior of "par" but for IO[3]. [1] http://hackage.haskell.org/package/threads [2] http://hackage.haskell.org/package/spawn [3] http://unlines.wordpress.com/2010/04/21/sparking-imperatives/