Futures and promises in Clojure

A future is an object that represents the result of running a function on a different thread. A promise is an object that represents a value that will be delivered at some point in time.

Futures

Futures are useful for long-running computations, or code that can block main threads. Let's create a function that runs for a relatively long time, as follows:

(defn get-user-accounts []
  (Thread/sleep 5000)
  [{:user "John"}{:user "Debbie"}])
(defn get-recent-purchases []
  (Thread/sleep 5000)
  ["vegetables" "fruits"])

The get-user-accounts and get-recent-purchases functions can potentially take a long time, depending on the amount of data. Here, we use Thread/sleep for five seconds, in order to simulate a long-running process. We can try to use these functions, as follows:

(defn long-process []
 (let [users (get-user-accounts)
       purchases (get-recent-purchases)]
   {:purchases purchases
    :users users}))

Our long-process function relies on data from functions...