spy-example.clj

(def beatle->email
  {:john   "john.lennon@beatles.com"
   :paul   "paul.mccartney@beatles.com"
   :george "george.harrison@beatles.com"
   :ringo  "ringo.starr@beatles.com"})

(defn lookup-email [beatle-id]
  (get beatle->email beatle-id))

(defn send-message [email message]
  (println (str "Sending " message " to " email))
  nil)

(defn email-beatle [beatle-id message]
  (when-let [email (lookup-email beatle-id)]
    (send-message email message)))