i've just got all tests green in promisespromises using promesa-CSP (public CI is broken, because there is no new promesa release yet)- so that's a cross-platform manifold-like (plus error-propagation, plus chunking) streams API, with promesa-CSP underlying

On the end, my decision is going without the flag. Promesa has two API styles, one adapted for -> and other adaped to ->>. The first one, is base don then and catch and handle and finally and this will continue work as it working right now. The second one, is based on fmap, mcat, hmap, hcat and fnly, they were already stricter and performance oriented (in the same way as then' variant), so the posible breakage only will affect that API, not very common and not very well documented.

The API breakage is relative, because the docstrings are already clear on how it should work, If people is relying on the detail of implementation, it should to mean undefined behavior, so strictly it does not break anything.