Async Exception Handling in F#

Question

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.

Answer 1

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)

• Martin: 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?
• Jack P.: 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.
• Jwosty: There are circumstances where Async.Catch can "leak" an exception (for example when an exception is thrown before the async is created - see: gist.github.com/jwosty/c6cb8ae1111ccc5caeb0f6db591d879e). Using a try-with block catches that exception.