The bare minimum to create a state monad in Clojure. No good reason, really.

minimal-state-monad.clj

(definline pair
  [x y]
  `(clojure.lang.MapEntry/create ~x ~y))

(defn create-state-monad
  [init]
  (fn [f in]
    (let [state' (f init in)]
      (pair (create-state-monad state') state'))))

(def s (create-state-monad 0))
(def s' (s + 1))
(def s'' ((s' 0) + 2))
(def s''' ((s'' 0) + 3))

;;; Other takes

(defn -state
  [i]
  (reify
    clojure.lang.IDeref
    (deref [this] i)
    clojure.lang.IFn
    (invoke [this f x]
      (-state (f i x)))))

(defn -accum
  [f i]
  (reify
    clojure.lang.IDeref
    (deref [this] i)
    clojure.lang.IFn
    (invoke [this x]
      (-accum f (f i x)))))