lookup.clj

(defn get-apply
  [m & fields]
  (loop [this-m   m
         [f & fs] fields]
    (when-let [v (get this-m f)]
      (cond
        (map? v) (recur v fs)
        (ifn? v) (apply v fs)
        :else    v))))

(defn get-rule
  [rules & fields]
  (or
    (apply get-apply rules fields)
    (apply get-apply rules :default (drop 2 fields))
    (throw (ex-info "Lookup failure!" {:fields fields}))))