I'm trying to use Reactive Extensions from Boo and am running into type problems. Here's the basic example:

def OnSubscribe(observer as IObservable[of string]) as callable:
    print "subscribing"

    def Dispose():
        print "disposing"

    return Dispose

observable = System.Linq.Observable.Create[of string](OnSubscribe)
observer = System.Linq.Observer.Create[of string]({x as string | print x})
observable.Subscribe(observer)

The Subscribe here gives a System.InvalidCastException: Cannot cast from source type to destination type. The issue appears to be with how I'm creating the observable, but I've struggled to see where the type problem arises from.

Ideas?