Check out the parallel combinators in parallel-io:
http://hackage.haskell.org/packages/archive/parallel-io/0.3.2/doc/html/Contr...
On Fri, Sep 28, 2012 at 1:01 PM, Greg Fitzgerald
I'm new to concurrent programming in Haskell. I'm looking for a drop-in replacement for 'mapM' to parallelize a set of independent IO operations. I hoped 'mapConcurrently' might be it, but I need something that will only spawn as many threads as I have CPUs available [1].
I also tried Control.Parallel.Strategies [2]. While that route works, I had to use unsafePerformIO. Considering that IO is for sequencing effects and my IO operation doesn't cause any side-effects (besides hogging a file handle), is this a proper use of unsafePerformIO?
Attempt 1 --------------
import System.Process(readProcess) import Control.Concurrent.Async(mapConcurrently)
main :: IO [String] main = mapConcurrently (\n -> readProcess "echo" ["test: " ++ show n] "") [0..1000]
$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.6.1
$ runghc test.hs test.hs: runInteractiveProcess: pipe: Too many open files test.hs: runInteractiveProcess: pipe: Too many open files test.hs: runInteractiveProcess: pipe: Too many open files test.hs: runInteractiveProcess: pipe: Too many open files test.hs: runInteractiveProcess: pipe: Too many open files test.hs: runInteractiveProcess: pipe: Too many open files test.hs: runInteractiveProcess: pipe: Too many open files test.hs: runInteractiveProcess: pipe: Too many open files test.hs: runInteractiveProcess: pipe: Too many open files test.hs: echo: createProcess: resource exhausted (Too many open files)
Attempt 2 --------------
import System.Process(readProcess) import Control.Parallel.Strategies(parMap, rpar) import System.IO.Unsafe(unsafePerformIO)
main :: IO [String] main = myMapConcurrently (\n -> readProcess "echo" ["test: " ++ show n] "") [0..1000] where myMapConcurrently f = return . parMap rpar (unsafePerformIO . f)
$ runghc test.hs > /dev/null && echo Success Success
Thanks, Greg
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