state monad as a way to hold 'world' state

gistfile1.clj

(defmacro defworld [name bindings & body]
  `(defn ~name ~bindings
     (fn [~'world]
       (println ~'world)
       ~@body)))

(defmacro with-world [& body]
  `(domonad state-m
     ~@body))

(defworld inc-w [population]
  [(inc population) (update-in world [:peons] conj :a-peon)])

(defworld double-w [population]
  [(* 2 population) (update-in world [:peons] concat (:peons world))])

(defworld dec-w [population]
  [(dec population) (update-in world [:peons] rest)])

(defn alter-population [starting-world]
  (let [initial-population (count (:peons starting-world))
        state-fn (with-world [a (inc-w initial-population)
                              b (double-w a)
                              c (dec-w b)
                              d (dec-w c)]
                          d)]
    (state-fn starting-world)))

(fact
  (alter-population {:peons [:a-peon :a-peon :a-peon]})
  => [6 {:peons [:a-peon :a-peon :a-peon :a-peon :a-peon :a-peon]}])