Finding the average of ages

In this section, we will explore a different use case for the Option Functor. We would like to, given a number of pirates, calculate the average of their ages. This is simple enough to do:

(defn avg [& xs]
  (float (/ (apply + xs) (count xs))))

(let [a (some-> (pirate-by-name "Jack Sparrow") age)
      b (some-> (pirate-by-name "Blackbeard") age)
      c (some-> (pirate-by-name "Hector Barbossa") age)]
  (avg a b c)) ;; 56.666668

Note how we are using some-> here to protect us from nil values. Now, what happens if there is a pirate for which we have no information?

(let [a (some-> (pirate-by-name "Jack Sparrow") age)
      b (some-> (pirate-by-name "Davy Jones") age)
      c (some-> (pirate-by-name "Hector Barbossa") age)]
  (avg a b c)) ;; NullPointerException   clojure.lang.Numbers.ops (Numbers.java:961)

It seems we're back at square one! It's worse now because using...