< Concurrency demos
Revision as of 01:16, 29 November 2006 by Remi (zipWithM thread (repeat s) = mapM (thread s))
A simple example of parallelism in Haskell
This little piece of code computes an approximation of Riemann's zeta function, balancing the work to be done between N threads.
import Control.Concurrent import Control.Concurrent.MVar import Control.Monad import Data.Complex import System.Environment -- Return the list of the terms of the zeta function for the given range. -- We don't sum the terms here but let the main thread sum the lists returned -- by all the other threads so as to avoid accumulating rounding imprecisions. zetaRange :: (Floating a, Integral b) => a -> (b, b) -> [a] zetaRange s (x,y) = [ (fromIntegral n) ** (-s) | n <- [x..y] ] cut :: (Integral a) => (a, a) -> a -> [(a, a)] cut (x,y) n = (x, x + mine - 1) : cut' (x + mine) size (y - mine) where (size, modulo) = y `divMod` n mine = size + modulo cut' _ _ 0 =  cut' x' size' n' = (x', x' + size' - 1) : cut' (x' + size') size' (n' - size') getParams :: IO (Int, Int, Complex Double) getParams = do argv <- getArgs case argv of (t:n:s:) -> return (read t, read n, read s) _ -> error "usage: zeta <nthreads> <boundary> <s>" main :: IO () main = do (t, n, s) <- getParams childs <- mapM (thread s) (cut (1, n) t) results <- mapM takeMVar childs print (sum (concat results)) where thread s range = do putStrLn ("Starting thread for range " ++ show range) mvar <- newEmptyMVar forkIO (putMVar mvar (zetaRange s range)) return mvar
Insert benchmarks here! :-)