Poor's man state monad!

state_macros.clj

(defmacro << [k-or-ks f]
  `(fn [state#]
     (update-in state#
                ~(if (vector? k-or-ks)
                   k-or-ks
                   (vector k-or-ks))
                ~f)))

(defn >>* [state form]
  (condp = (first form)
    '>>> `(fn [~state] ~@(rest form) ~state)
    '<<< `(fn [~state] ~@(rest form))
    form))

(defmacro >> [state & forms]
  (reduce #(list (if (#{'<< 'fn 'fn*
                        'clojure.core/fn
                        'clojure.core/fn*} (first %2))
                   %2
                   `(fn [s#] ~%2 s#)) %)
          (conj (map (partial >>* state) forms)
                state)))

;; user> (let [state {}]
;;         (>> state
;;           (println "side-effect")
;;           (>>> (println state))
;;           (<< [:a :b] (constantly []))
;;           (>>> (println state))
;;           (<< [:a :b] #(conj % 1 2 3))
;;           (<<< (state :a))
;;           (>>> (println state))
;;           (<< [:b 1] #(inc %))
;;           (println state)
;;           (<<< (state :b))))
;; side-effect
;; {}
;; {:a {:b []}}
;; {:b [1 2 3]}
;; {}
;; => [1 3 3]