Concurrency demos

A large range of small demonstration programs for using concurrent and parallel Haskell are in the Haskell concurrency regression tests. In particular, they show the use of MVars and forkIO.

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

zetaRange :: (Floating (Complex a), RealFloat a, Integral b) => Complex a -> (b, b) -> Complex a
zetaRange s (x,y) = sum [ (fromIntegral n :+ 0) ** (-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 <- zipWithM thread (repeat s) (cut (1, n) t)
  results <- mapM takeMVar childs
  print $ sum results
  where
    thread s range = do
      putStrLn $ "Starting thread for range " ++ show range
      mvar <- newEmptyMVar
      forkIO (putMVar mvar (zetaRange s range))
      return mvar