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 some-> doesn't help if we need to use all values at once, as opposed to threading them through a chain of function calls.

Of course, not all is lost. All we need to do is check if all values are present before calculating the average:

(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)]
  (when (and a b c)
    (avg a b c))) ;; nil

While this works perfectly fine, our implementation suddenly had to become aware that any or all of the values a, b, and c could be nil. The next abstraction we will look at, Applicative Functors, fixes this.