withenv.clj

(def loc
  "The location of lein's custom environment file."
  ".lein-env")

(defn with-env*
  "Executes the supplied function in an environment where conf's kv
  pairs override the default config."
  [conf f]
  {:pre [(map? conf)]}
  (let [old-env (try (slurp loc)
                     (catch java.io.FileNotFoundException _))]
    (spit loc (prn-str conf))
    (require '[environ.core :reload true])
    (try (f)
         (finally
           (when old-env (spit loc old-env))
           (require '[environ.core :reload true])))))

(defmacro with-env
  "Executes the supplied body in an environment where conf's bindings
  override the default env."
  [conf & body]
  `(with-env* ~conf (fn [] ~@body)))