tmp.clj

(ns tmp
  (:use clojure.contrib.java-utils))

(def *default* {:a 1})

(def *provided* {:c 2})

(def *mapping*
     {:c :b
      :a :c
      :b :a})

(defn apply-mapping
  "Applies mapping to given map."
  [map mapping]
  (merge map
         (loop [m (select-keys map (keys mapping))
                ks (seq (keys mapping))]
           (if ks
             (let [k (first ks)
                   v (k m)]
               (recur (assoc m (k mapping) v)
                      (next ks)))
             m))))

(with-system-properties
  (apply-mapping (merge *default* *provided*) *mapping*)
  (doseq [k [:a :b :c]] (prn k (get-system-property k))))