安装 async
$ cabal install async
async-2.2.1 installed
async / wait / concurrently
- async :: IO a -> IO (Async a)
启动新线程,执行异步操作。 - wait :: Async a -> IO a
等待异步操作完成,并返回值。 - concurrently :: IO a -> IO b -> IO (a, b)
并发(concurrently)运行两个 IO 操作,返回两个结果。
示例
安装 http-conduit
$ cabal install http-conduit
Installed http-conduit-2.3.2
Sample code to accompany the book "Parallel and Concurrent Programming in Haskell"
module GetURL (getURL) where
import Network.HTTP.Conduit
import Data.ByteString (ByteString)
import qualified Data.ByteString.Lazy as L
getURL :: String -> IO ByteString
getURL url = L.toStrict <$> simpleHttp url
一个下载模块 getURL
import GetURL
import Control.Concurrent.Async
import qualified Data.ByteString as B
main = do
a1 <- async (getURL "http://www.wikipedia.org/wiki/Shovel")
a2 <- async (getURL "http://www.wikipedia.org/wiki/Spade")
r1 <- wait a1
r2 <- wait a2
print (B.length r1, B.length r2) -- (87653,58155)
- 使用 async 开启两个线程同时下载两个网页。
- 使用 await 等待下载结束并得到结果。
import GetURL
import Control.Concurrent.Async
import qualified Data.ByteString as B
main = do
(r1, r2) <- concurrently
(getURL "http://www.wikipedia.org/wiki/Shovel")
(getURL "http://www.wikipedia.org/wiki/Spade")
print (B.length r1, B.length r2) -- (87653,58155)
- 使用 concurrently 开启两个线程同时下载两个网页。下载结束时返回结果。