[Haskell-cafe] Parallel executing of actions

[Bertram Felgenhauer]

Joel Reymont wrote:
> 
> On Apr 15, 2007, at 8:23 PM, Spencer Janssen wrote:
> 
> >parSequence_ xs = do
> >    m <- newEmptyMVar
> >    mapM_ (\x -> forkIO x >> putMVar m ()) xs

should be

       mapM_ (\x -> forkIO (x >> putMVar m ())) xs

> >    replicateM_ (length xs) (takeMVar m)
> 
> mapM_ above spawns (length xs) threads blocking on a single "lock",  
> right?

yes.

> replicateM_ then makes sure that the lock is "unlocked" as many times  
> as threads spawned, right?

right.

> Since all the threads block on a single MVar how do they run in  
> parallel?

The idea is that before the threads block on the MVar, they run their
action x to completion.

Bertram