open System
open System.Reactive
open System.Reactive.Subjects

type Async<'a> with
    member a.ToObservable() =        
        let subject = new AsyncSubject<'a>()
        Async.StartWithContinuations(a,
            (fun res -> subject.OnNext(res)
                        subject.OnCompleted()),
            (fun exn -> subject.OnError(exn)),
            (fun cnl -> ()))
        subject :> IObservable<'a>

// Test 
let c =  async { return 42}
c.ToObservable().Subscribe(fun x-> printfn "The answer to everything %d" x)