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