Haskell的Control.Concurrent.Async.mapConcurrent当前有限制吗？

我试图在Haskell中并行运行多个下载,我通常只使用Control.Concurrent.Async.mapConcurrently函数.但是,这样做会打开~3000个连接,这会导致Web服务器拒绝它们.是否可以完成与mapConcurrent相同的任务,但一次只打开有限数量的连接(即一次只打开2个或4个)？

一个快速的解决方案是使用信号量来限制并发操作的数量.它不是最优的(所有线程都是一次创建然后等待),但是有效:

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]

您也可以尝试使用pooled-io包来编写:

import qualified Control.Concurrent.PooledIO.Final as Pool
import Control.DeepSeq (NFData)
import Data.Traversable (Traversable, traverse)

mapPool ::
   (Traversable t, NFData b) =>
   Int -> (a -> IO b) -> t a -> IO (t b)
mapPool n f = Pool.runLimited n . traverse (Pool.fork . f)