Let and apply - oliyh/learning-clojure GitHub Wiki

Apply

Calls a function with each element of the sequence as a distinct argument

(apply + [1 2 3]) ;; 6, equivalent of (+ 1 2 3)

Let

Let allows you to assign an intermediate result to a var so that you can refer to it without having to recompute it.

(defn stats [s]
  (let [n (count s)]
    (println "The mean is" (/ (apply + s) n) "and the median is" (s (long (/ n 2))))))
    ;; The mean is 5/2 and the median is 3