`Deref clojure.core/promise` in fiber outputs stream of warnings

[puniverse]

`Deref clojure.core/promise` in fiber outputs stream of warnings

[pron]

Those warnings are true, though: you really are blocking a thread. This is why we have pulsar.core/promise. What behavior would you like to see? The only solution, AFAICT, is to have that other library (manifold?) integrate Pulsar, so that, if running in a fiber, would only block the fiber.

[andreasthoelke]

Ah thanks, it makes sense now. I guess I was somehow thinking that derefing even non-pulsar things in a fiber would still only block that fiber/Strand! So this warning is in fact super useful – I just overlooked that critical part “..is blocking a thread” in the long string.
In any case I think I can just link Aleph/Manifold to Pulsar on the callback level, e.g.

(defn websocket-handler [request]
  (-> (aleph.http/websocket-connection request)
      (manifold.deferred/on-realized (fn [user-socket]
                                       (,,,)))))

it will just not be as smooth. Will explore this a bit further and potentially propose an integration at the Manifold repo.

[pron]

I should note that it's trivial to transform an asynchronous API to a fiber-blocking one using await. Transforming a thread-blocking API to a fiber-blocking one requires a more work, and usually the cooperation of the library's author.

When integrating third-party libraries, you then have two options:

1. Use await to recreate a convenient synchronous API on top of the library's async API.
2. Use channels (that work on threads and fibers alike) to communicate between a thread and a fiber.