Async exception handling in F#

asyncWorkflow.fs

open System
open System.Net

// exception handling in async using Async.Catch
let fetchAsync (name, url:string) =
    async {
        let uri = new System.Uri(url)
        let webClient = new WebClient()
        let! html = Async.Catch (webClient.AsyncDownloadString(uri))
        match html with
        | Choice1Of2 html -> printfn "Read %d characters for %s" html.Length name
        | Choice2Of2 error -> printfn "Error! %s" error.Message
    } |> Async.Start

// exception handling in async using regular try/with
let fetchAsync2 (name, url:string) =
    async {
        let uri = new System.Uri(url)
        let webClient = new WebClient()
        try
            let! html = webClient.AsyncDownloadString(uri)
            printfn "Read %d characters for %s" html.Length name
        with error -> printfn "Error! %s" error.Message
    } |> Async.Start

fetchAsync2 ("blah", "http://asdlkajsdlj.com")


//another eg
let work a = async {
    return 
        match a with 
        | 1 -> "Success!"
        | _ -> failwith "Darnit"
}

let printResult (res:Choice<'a,System.Exception>) = 
    match res with
    | Choice1Of2 a -> printfn "%A" a
    | Choice2Of2 e -> printfn "Exception: %s" e.Message


let callingWorkflow = 
    async {
        let result = work 1 |> Async.Catch
        let result2 = work 0 |> Async.Catch

        [ result; result2 ]
        |> Async.Parallel
        |> Async.RunSynchronously
        |> Array.iter printResult
    }

callingWorkflow |> Async.RunSynchronously
Async.Catch returns a Choice<'T1,'T2>. Choice1Of2 for a successful execution, and the exception thrown for the Choice2Of2.