Can Haskell's Control.Concurrent.Async.mapConcurrently have a limit?

Question

I\'m attempting to run multiple downloads in parallel in Haskell, which I would normally just use the Control.Concurrent.Async.mapConcurrently function for. However, doing so o

Answer 1

A quick solution would be to use a semaphore to restrict the number of concurrent actions. It's not optimal (all threads are created at once and then wait), but works:

import Control.Concurrent.MSem
import Control.Concurrent.Async
import Control.Concurrent (threadDelay)
import qualified Data.Traversable as T

mapPool :: T.Traversable t => Int -> (a -> IO b) -> t a -> IO (t b)
mapPool max f xs = do
    sem <- new max
    mapConcurrently (with sem . f) xs

-- A little test:
main = mapPool 10 (\x -> threadDelay 1000000 >> print x) [1..100]