After I have spawned a thread with 'forkIO', how can I check if that thread work has finished already? Or wait for it?
The best way to do this is using Control.Exception.finally: (...)
Changing ugly code for bad performance is not that usual in Haskell code :(
I think you misunderstand Chris' remark. He's saying that MVars and forkIO give you bot clean control, and high performance.
Sorry if I sound rude. I just saw a place for a small joke, and used it. Chris code is pretty elegant to what it is supposed to do. However, knowing if a thread has finished is just 1 bit of information. There's probably a reason why that would hurt performance, but I don't understand it. For most situations, I believe you want to know when a thread has finished, and have that in the implementation is probably more efficient than creating a MVar for each one. Please understand that I'm not criticising anyone's work, I just want to understand it better. Threads are a deep problem with many issues involved, and I have no proper knowledge of any of them.
This code seems quite elegant, for the job you were asking:
import Control.Concurrent import Control.Exception
main = do done <- run (print (last [1..100000000])) print "Waiting...." takeMVar done print "OK." where (...)
Sorry, I don't agree. I try to write things in a way that when you read it you can get an intuition on why it's doing what it's doing; even when the code is for my reading only (which, in Haskell, is almost always the case). For instance: in the code I'm writing now, I need to know if threads have finished since only them I can use the files they generate. So, instead of checking if threads have finished, I decided to check if files exist and are available for writing. When I read 'takeMVar done', it's difficult to think why you want to read a value you are never going to use. But, of course, maybe this is just my prejudice, and if I understand anything about threads I would have a different feeling about it.
And the lovely thread-ring benchmark, is also very nice:
http://shootout.alioth.debian.org/gp4/benchmark.php?test=threadring&lang=all
(...)
Sorry, I didn't think that's nice either. I read the description of the task, and it's one where in the real world you would never use threads to do it, except for benchmarking threads. Of course, that is important for many people, like those who study threads and their implementation. But do you know of a benchmark where the task is some kind of situation where you actually get a result faster by using threads than by using a single thread? Thanks, Maurício