STM in Clojure vs Haskell, why no retry or orElse?

A few months ago I reread Simon Peyton Joneses article on STM in the Beautiful Code book and decided to try and translate it into clojures STM

See the paper here

He says ‘Atomic blocks as we have introduced them so far are utterly inadequate to coordinate concurrent programs. They lack two key facilities: blocking and choice’ so I guess the implication is Clojures STM is inferior, any thoughts?

I had to use a constraint on a ref and try/catch to get the same effect (though I hate using exceptions for control flow it does seem to work)

I think a better solution might be had using watchers, how would you do it? Any good links explaining differences in the STMs?

(ns santa.core)

(declare join-group)

(defn start-thread
   (Thread. fn)))

(defn random-delay []
  (Thread/sleep (rand-int 1000)))

(def printer (agent 0))

(defn threadsafe-print
  (send printer println somestring))

(defn make-gate
  (ref {:max-size max-size
        :remaining 0}
       :validator #(>= (:remaining %) 0)))

(defn pass-gate
    (dosync (alter gate #(update-in % [:remaining] dec)))
    (catch IllegalStateException e
      (do ;(Thread/sleep 20)
          (pass-gate gate)))))

(defn helper
  [group task]
  (let [[in-gate out-gate] (join-group group)]
    (do (pass-gate in-gate)
        (pass-gate out-gate)

(defn thread [group id work-fn]
   (fn []
     (helper group #(work-fn id))

(defn deliver-toys [id]
   (str "Reindeer " id " delivering toys")))

(defn meet-in-study [id]
   (str "Elf " id " meeting in the study")))

(defn reindeer-thread [group id]
  (thread group id deliver-toys))

(defn elf-thread [group id]
  (thread group id meet-in-study))

(defn operate-gate
  (dosync (alter gate assoc :remaining (:max-size @gate)))
  (while (> (:remaining @gate) 0)
    ;(Thread/sleep 20)

(defn new-group
  (ref {:max-size max-size
        :remaining max-size
        :in-gate (make-gate max-size)
        :out-gate (make-gate max-size)
       :validator #(>= (:remaining %) 0)))

(defn join-group
    (dosync (let [{:keys [remaining max-size in-gate out-gate]} @group]
              (alter group #(update-in % [:remaining] dec))
              [in-gate out-gate]))
    (catch IllegalStateException e
      (do ;(Thread/sleep 20)
          (join-group group)))))

(def elf-group (new-group 3))
(def reindeer-group (new-group 9))

(defn handle-group [group group-name]
  (let [current-group @group
        {:keys [max-size remaining in-gate out-gate]} current-group]
    (if (= 0 remaining)
      (do (dosync
           (ref-set group {:max-size max-size
                           :remaining max-size
                           :in-gate (make-gate max-size)
                           :out-gate (make-gate max-size)}))
          (threadsafe-print (str "***** With" group-name "*****"))
          (operate-gate in-gate)
          (operate-gate out-gate)

(defn santa []
  (while (handle-group elf-group "Elves"))
  (handle-group reindeer-group "Reindeer")

(defn -main
  [& args]
  (dotimes [i 9]
    (do (println "Launching reindeer thread:" i)
        (reindeer-thread reindeer-group i)))
  (dotimes [i 10]
    (do (println "Launching elf thread" i)
        (elf-thread elf-group i)))