BusyBuilder.fs

type BusyBuilder(blockUi, unblockUi) =
    member this.Bind(x, f) = async.Bind(x, f)
    member this.Combine(e1, e2) = async.Combine(e1, e2)
    member this.Delay(f) =
        async {
            blockUi ()
            let! result = async.Delay f
            unblockUi ()
            return result
        }
    member this.For(coll, f) = async.For(coll, f)
    member this.Return(x) = async.Return(x)
    member this.ReturnFrom(m) = async.ReturnFrom(m)
    member this.TryWith(expr, handler) = async.TryWith(expr, handler)
    member this.TryFinally(expr, compensation) = async.TryFinally(expr, compensation)
    member this.Using(resource, binder) = async.Using(resource, binder)
    member this.While(guard, computation) = async.While(guard, computation)
    member this.Zero() = async.Zero()

let busy = new BusyBuilder((fun () -> printfn "blocking UI"), (fun () -> printfn "unblocking UI"))

busy {
    let uri = new System.Uri("https://www.google.com")
    let webClient = new System.Net.WebClient()
    let! html = webClient.AsyncDownloadString(uri)
    printfn "%s" html
    return html
} |> Async.RunSynchronously