Danny, thanks!
Eugen Martynov

not sure that can be compared. We don't really produce chains like in Rx as you have functions that have normal return types so you can check those. More comparable would be more Channels, which I haven’t used much yet.
For the Deferred type maybe what you would look for would be the CompletableDeferredas it has a method to check if it was actually completed: 
Completes this deferred value with a given [value]. The result is `true` if this deferred was completed as a result of this invocation and `false` otherwise (if it was already completed).