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1

I am trying to write non-blocking code in F#. I need to download a webpage, but sometime that webpage doesn't exist and an exception is thrown (404 Not Found) by AsyncDownloadString. I tried the code below but it doesn't compile.

How could I handle exception from AsyncDownloadString?

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

How am I suppose to handle exception here? If an error is thrown, I simply want to return an empty string or a string with a message in it.

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1 Answer 1

up vote 6 down vote accepted

Just add the return keyword when you return your error string:

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

IMO a better approach would be to use Async.Catch instead of returning an error string:

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

let downloadPage url =
    Async.Catch (downloadPageImpl url)
share|improve this answer
    
Thank you Jack! It works! :) Why do you think Async.Catch is a better approach. I would think that exception handling should be done in downloadPage, no? –  Martin May 20 '13 at 15:08
2  
I think Async.Catch is better because: (1) it preserves information about the error...there are other reasons an exception might be thrown besides a 404, and having the exception instead of "Error" makes it easier to diagnose the problem; (2) using Choice<_,_> allows you to use the type system to enforce the processing paths for both the results and the errors. –  Jack P. May 20 '13 at 15:17

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