Do we need to have both Observable and Flowable ? #858

I wonder if the approach as taken in reactor (rx-lite) would just be simpler: Take one backpressurable type with optimizations for unbounded, non backpressured flows. That would also mean one for N items and one for 0 or 1 item, as we haven't really found a need for an explicit Completable type neither, like Optional in Java or a Promise in JS.

Beyond removing the overlap of concepts, I think this lite-rx approach makes it better positioned for providing simple adaptive layers for coroutines or enumerables, leaving the choice between one "functional" style and one "imperative-like".

We have a strong interest in helping reviewing API as well if necessary, don't hesitate to retake the Flux/Mono naming which are slightly more aligned in some situations (prefixes are aligned e.g. collectXxx, toXxx, fromXxx).