Future and promise in Clojure

future-promise.clj

(defn start-thread [f]
  (.start (Thread. f)))

(let
    [sum    (future (apply + (range 1e7)))
     answer (promise)]
     
    (println "0. Started...")

    (start-thread
      (fn [] 
        (println "1. I'm in thread 1")
        (println "1. Done: " @sum)
        (println "1. Delivering answer")
        (deliver answer 42)
        (println "1. The thread is over")))
    (start-thread
      (fn []
        (println "2. Waiting in thread 2")
        (println (str "2. " @answer))
        (println "2. unlocked")))
    
    (println "0. And I'm just here..."))

Output:

"0. Started..."
"1. I'm in thread 1"
"2. Waiting in thread 2"
"0. And I'm just here..."
nil
user=> "1. Done: 49999995000000"
"1. Delivering answer"
"1. The thread is over"
"2. 42"
"2. unlocked"