一个常用的示例,用于说明在F#中使用异步工作流程的是并行检索多个网页。此类示例可以在以下链接中找到:http://en.wikibooks.org/wiki/F_Sharp_Programming/Async_Workflows 如果链接在未来更改,这里也提供了代码:
open System.Text.RegularExpressions
open System.Net
let download url =
let webclient = new System.Net.WebClient()
webclient.DownloadString(url : string)
let extractLinks html = Regex.Matches(html, @"http://\S+")
let downloadAndExtractLinks url =
let links = (url |> download |> extractLinks)
url, links.Count
let urls =
[@"http://www.craigslist.com/";
@"http://www.msn.com/";
@"http://en.wikibooks.org/wiki/Main_Page";
@"http://www.wordpress.com/";
@"http://news.google.com/";]
let pmap f l =
seq { for a in l -> async { return f a } }
|> Async.Parallel
|> Async.Run
let testSynchronous() = List.map downloadAndExtractLinks urls
let testAsynchronous() = pmap downloadAndExtractLinks urls
let time msg f =
let stopwatch = System.Diagnostics.Stopwatch.StartNew()
let temp = f()
stopwatch.Stop()
printfn "(%f ms) %s: %A" stopwatch.Elapsed.TotalMilliseconds msg temp
let main() =
printfn "Start..."
time "Synchronous" testSynchronous
time "Asynchronous" testAsynchronous
printfn "Done."
main()
我想知道如何处理全局状态的变化,例如网络连接丢失?有没有一种优雅的方法来解决这个问题?
可以在进行Async.Parallel调用之前检查网络状态,但是状态可能会在执行期间发生变化。假设我们想要暂停执行直到网络再次可用,那么有没有一种函数式的方法来实现呢?