Solving the expression problem in Clojure using maps.

commands.clj

(defn move-formatter [command]
  (format "<MoveCommand %s,%s>" (:x command) (:y command)))

(defn stop-formatter [command]
  "<StopCommand>")

(defn default-formatter [command]
  (format "<Unknown command %s>" command))

(def formatters {
    "move" move-formatter
    "stop" stop-formatter})



(defn format-command [command]
  (let [formatter (or 
                   (formatters (:type command)) 
                   default-formatter)]
    (formatter command)))

(defn print-command [command]
  (println (format-command command)))


(def mc {:type "move" :x 10 :y 20})
(def sc {:type "stop"})
(def d (java.util.Date. 1348040520000))


(print-command mc)
(print-command sc)
(print-command d)