An alternative implementation of map that won't silently return nils for missing keys.

map.clj

(ns strict-map.map)

(deftype StrictMap [inner]
  clojure.lang.IPersistentMap
  (assoc [this k v]
    (StrictMap. (.assoc inner k v)))
  (assocEx [this k v]
    (StrictMap. (.assocEx inner k v)))
  (without [this k]
    (StrictMap. (.without inner k)))

  java.lang.Iterable
  (iterator [this]
    (.iterator inner))

  clojure.lang.Associative
  (containsKey  [this k]
    (.containsKey inner k))
  (entryAt [this k]
    (.entryAt inner k))

  clojure.lang.IPersistentCollection
  (count [this]
    (.count inner))
  (cons [this x]
    (StrictMap. (.cons inner x)))
  (empty [this]
    (.empty inner))
  (equiv [this other]
    (and (map? other) (= (seq other) (seq inner))))

  clojure.lang.Seqable
  (seq [this]
    (.seq inner))

  clojure.lang.IFn
  (invoke [this k]
    (.invoke inner k))
  (invoke [this k default]
    (.invoke inner k default))
  (applyTo [this args]
    (.applyTo inner args))

  clojure.lang.ILookup
  (valAt [this k]
    (let [v (get inner k)]
      (cond (not (contains? inner k))
              (throw (new Exception  (str "Key '" k "' not found in strict map '" inner "'.")))  
            (map? v)
              (StrictMap. v)
            :otherwise
              v)))
  (valAt [this k default]
    (let [v (get inner k)]
      (cond (not (contains? inner k))
              default 
            (map? v)
              (StrictMap. v)
            :otherwise
              v))))
  
(defn strict
  "Convert an ordinary map into one that throws an Exception if an unknown key is requested."
  [m] (StrictMap. m))

You mean invoke?