Haskell Language Parallelism rpar


Example

rpar :: Strategy a executes the given strategy (recall: type Strategy a = a -> Eval a) in parallel:

import Control.Concurrent
import Control.DeepSeq
import Control.Parallel.Strategies
import Data.List.Ordered

main = loop
  where 
    loop = do
      putStrLn "Enter a number"
      n <- getLine

      let lim = read n :: Int
          hf  = quot lim 2
          result = runEval $ do
            -- we split the computation in half, so we can concurrently calculate primes
            as <- rpar (force (primesBtwn 2 hf))
            bs <- rpar (force (primesBtwn (hf + 1) lim))
            return (as ++ bs)

      forkIO $ putStrLn ("\nPrimes are: " ++ (show result) ++ " for " ++ n ++ "\n")
      loop

-- Compute primes between two integers
-- Deliberately inefficient for demonstration purposes
primesBtwn n m = eratos [n..m]
  where
    eratos []     = []
    eratos (p:xs) = p : eratos (xs `minus` [p, p+p..])

Running this will demonstrate the concurrent behaviour:

Enter a number
12
Enter a number

Primes are: [2,3,5,7,8,9,10,11,12] for 12

100
Enter a number

Primes are: [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100] for 100

200000000
Enter a number
-- waiting for 200000000    
200
Enter a number

Primes are: [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200] for 200

-- still waiting for 200000000