lens.clj

(def users [{:name "Brian" :age 22} {:name "Ben" :age 19}])

;; Takes a "path", returns a function that takes an "object" and
;; returns a "costate"
(defn lens [p]
  (fn [o]
    {:get (get-in o p)
     :set #(assoc-in o p %1)}))

(def myAgeLens (lens [0 :age]))
(def usersCoState (myAgeLens users))

(println (:get usersCoState))
;; 22
(println ((:set usersCoState) 23))
;; [{:name Brian, :age 23} {:name Ben, :age 19}]

;; Laws
(println (= (:get (myAgeLens ((:set usersCoState) 23))) 23))
(println (= ((:set usersCoState) (:get usersCoState)) users))
(println (= ((:set (myAgeLens ((:set usersCoState) 23))) 24)  ((:set usersCoState) 24)))