Haskell's MVars in Clojure

mvar.clj

;; MCons Hack - https://gist.github.com/fredyr/6341286
;; Haskell MVars - http://chimera.labs.oreilly.com/books/1230000000929/ch07.html
;; http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurrent-MVar.html

defn empty-mvar []
  (let [take-sem (java.util.concurrent.Semaphore. 1)
        put-sem (java.util.concurrent.Semaphore. 1)]
    (.acquire take-sem)
    (mcons nil (mcons take-sem put-sem))))

(defn put-mvar [mvar val]
  (let [take-sem (mcar (mcdr mvar))
        put-sem  (mcdr (mcdr mvar))]
    (.acquire put-sem)
    (set-mcar! mvar val)
    (.release take-sem)))

(defn take-mvar [mvar]
  (let [take-sem (mcar (mcdr mvar))
        put-sem  (mcdr (mcdr mvar))]
    (.acquire take-sem)
    (let [val (mcar mvar)]
      (set-mcar! mvar nil)
      (.release put-sem)
      val)))

(defn new-mvar [val]
  (let [mvar (empty-mvar)]
    (put-mvar mvar val)
    mvar))

;; Examples
(def m (empty-mvar))
(put-mvar m [:a :b])
(take-mvar m)
(let [[a b] (take-mvar m)] a)
(take-mvar (new-mvar {:cool "man"}))

(let [mvar (empty-mvar)]
  (.start (Thread. #(do
                      (put-mvar mvar "x")
                      (put-mvar mvar "y"))))
  (str
   (take-mvar mvar)
   (take-mvar mvar)))