name_that_nil.clj

(defn lookup-contract [m allowed]
  (reify
    clojure.lang.ILookup
    (valAt [coll k]
      (.valAt coll k nil))
    (valAt [coll k not-found]
      (if (contains? allowed m)
        (.valAt coll k not-found)
        (throw (IllegalArgumentException. 
                 (str k " violates lookup contract, allowed " allowed)))))))

(defmacro cmap [& ks]
  `(fn [m#]
     (lookup-contract m# ~(set ks))))

(def scene (cmap :subject :action :object))

(def scenes [(scene
               {:subject  "Frankie"
                :action   "say"
                :object   "relax"})

             (scene
               {:subject  "Lucy"
                :action   "?s"
                :object   "Clojure"})

             (scene
               {:subject  "Rich"
                :action   "tries"
                :object   "a new conditioner"})])


(defn people-in-scenes [scenes]
  (->> scenes
    (map :bad-key)
    (interpose ", ")
    (reduce str)))

(comment
  (println "People:" (people-in-scenes scenes))
  )