State Monad in Clojure

state-monad.clj

(ns foo)

(defn m-const [val]
  (fn [state]
    [state val]))

(defn m-do [& actions]
  (if (= (count actions) 1)
    (first actions)
    ; else
    (fn [state]
      (let [[state _] ((first actions) state)]
        ((apply m-do (rest actions)) state)))))

(defn m-set [key action]
  (fn [state]
    (let [[state val] (action state)]
      [(assoc state key val)])))

(defn m-get [key]
  (fn [state]
    [state (get state key)]))

(defn m-map [monads]
  (fn [state]
    (loop [res nil
           monads monads
           state state]
      (if (empty? monads)
        [state (reverse res)]
        ; else
        (let [[state val] ((first monads) state)]
          (recur (cons val res) (rest monads) state))))))

(defn monadic [func]
  (fn [& m-args]
    (fn [state]
      (let [[state args] ((m-map m-args) state)]
        [state (apply func args)]))))

(defn m-if [test then else]
  (fn [state]
    (let [[state cond] (test state)]
      (if cond
        (then state)
        (else state)))))

(let [test (m-do (m-set :foo (m-do (m-set :bar (m-const 3))
                                   (m-const 4)))
                 (m-set :foo ((monadic +) (m-get :foo) (m-get :bar)))
                 (m-if ((monadic =) (m-get :foo) (m-const 7))
                       (m-const "Success")
                                        ; else
                       (m-const "Failure")))]
  (test {}))