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Background

IObservable<T> is .NET way of dealing with lazy event streams with publish/subscribe-pattern called Reactive Programming, "LINQ to Events and async operations".

Where a standard list (IEnumerable) is pull-based, IObservable is a push-based (infinite) list, like "a lazy list of mouse events": when an event happens, the corresponding list gets a new value. If Nullable is just "a list of 0 or 1", then async-await could be just an IObservable of 0 or 1.

There are many advantages of using reactive programming and Rx:

• Manual thread/lock -handling can be avoided
• No temporary class variables to capture the current or some previous state
• Testing is easy: generate lists like they would be event-lists.
• Testing the wrong async event order is easy.
• Also, testing long-duration workflows is easy as you can "fake" time passing

It's always good to have alternatives, and if R3 is your alternative to Rx, then FSharp.Control.R3 is your F# wrapper, like FSharp.Control.Reactive for F#.