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。