throw exception in async

Question

I have the following code:

member public this.GetData(uri: string) = async {
 let! res = Async.AwaitTask(httpClient.GetAsync uri)
 return res
}

When the property res.IsSuccessStatusCode is false I would like to throw an exception, how can I achieve this. The following code won't compile:

member public this.GetData(uri: string) = async {
 let! res = Async.AwaitTask(httpClient.GetAsync uri)
 match res.IsSuccessStatusCode with
 | true -> return res
 | false -> raise new Exception("")
}

Answer 1

You certainly need to wrap new Exception(...) in brackets, but that is not sufficient in this case - both branches of the match statement need to return a value, so you also need to insert return:

async {
 let! res = Async.AwaitTask(httpClient.GetAsync uri)
 match res.IsSuccessStatusCode with
 | true -> return res
 | false -> return raise (new Exception(""))
}

This is actually easier to write using an if computation which can contain body that returns unit (and throws an exception if the operation did not succeed) - and so you do not need return in that case:

async {
 let! res = Async.AwaitTask(httpClient.GetAsync uri)
 if not res.IsSuccessStatusCode then
 raise (new Exception(""))
 return res 
}

• Knerd: Thanks, that compiles. Something else doesn't work, but that is another problem ;)

Answer 2

So the first part is that you need to wrap the new Exception() with brackets to make sure that F# interprets the code correctly.

raise (new Exception(""))

or you can use either of the pipe operators

raise <| new Exception("")
new Exception |> raise

or you can change the type and use failwith

failwith "some message"

Secondly, you need to return from both branches, so prefix raise with return

• Knerd: Thanks again :) The solution of @tomas-petricek works better :)