Different platforms have different programming models. For instance, JavaScript applications are single-threaded and have an event loop. When making a network call, it is common to register a callback that will be invoked at a later stage, when that network call completes either successfully or with an error.

In contrast, when we're on the JVM, we can take full advantage of multithreading to achieve concurrency. It is simple to spawn new threads via one of the many concurrency primitives provided by Clojure, such as futures.

However, asynchronous programming becomes cumbersome. Clojure futures don't provide a native way for us to be notified of their completion at a later stage. In addition, retrieving values from a not-yet-completed future is a blocking operation. This can be seen clearly in the following snippet:

(defn do-something-important []
  (let [f (future (do (prn "Calculating...")
                      (Thread/sleep 10000)))]
    (prn "Perhaps...