Evaluating a procedure in parallel

In this recipe, we will conduct two time-consuming tasks in parallel. We will use the rpar function provided by the parallel package from hackage. The rpar function annotates its argument to be evaluated in parallel. Then, we call runEval to actually perform the computation.

Getting ready

Install the parallel package using cabal as follows:

$ cabal install parallel

How to do it…

1. Import the parallel package as follows:

   import Control.Parallel.Strategies (runEval, rpar)

2. Evaluate two tasks in parallel, and wait for both tasks to finish before returning as seen in the following code snippet:

   main = do
     print $ runEval $ do
       a <- rpar task1
       b <- rpar task2
       return (a, b)

3. A time-consuming task can be created as follows:

   task1 = 8^8^9 :: Integer

4. Another time-consuming task can be created as follows:

   task2 = 8^8^8 :: Integer

5. Compile the code with the threaded and rtsopts flags enabled, as follows:

   $ ghc -O2 --make Main.hs -threaded –rtsopts...