F#中的异步异常处理

Question

我试图在F#中编写非阻塞代码.我需要下载一个网页,但有时该网页不存在,AsyncDownloadString会抛出异常(404 Not Found).我尝试了下面的代码,但它没有编译.

我怎么能处理AsyncDownloadString的异常？

let downloadPage(url: System.Uri) = async {
    try
       use webClient = new System.Net.WebClient()
       return! webClient.AsyncDownloadString(url)
    with error -> "Error"
}

我怎么想在这里处理异常？如果抛出错误,我只想返回一个空字符串或带有消息的字符串.

Answer 1

只需return在返回错误字符串时添加关键字:

let downloadPage(url: System.Uri) = async {
    try
       use webClient = new System.Net.WebClient()
       return! webClient.AsyncDownloadString(url)
    with error -> return "Error"
}

IMO更好的方法是使用Async.Catch而不是返回错误字符串:

let downloadPageImpl (url: System.Uri) = async {
    use webClient = new System.Net.WebClient()
    return! webClient.AsyncDownloadString(url)
}

let downloadPage url =
    Async.Catch (downloadPageImpl url)