Clojure STM simple program - clojure

Question

I am writing on a simple program to transform the values of two integers using Clojure's STM.
I am following the approch of Lewandowski (http://lewandowski.io/2016/01/clojure-summary/), using his function a06.
My code:
(defn trans [p1 p2]
(println "a")
(dosync
(let [newval1 (#p1 + 50)
newval2 (#p2 - 30)]
(do
(println "b")
(ref-set p1 newval1)
(ref-set p2 newval2)))))
(defn main []
(let [p1 (ref 20) p2 (ref 100)]
(do
(future (trans p1 p2))
(future (trans p1 p2))
(Thread/sleep 500))
(println #p1)
(println #p2)))
For some reason, my function main is not getting into the "do part" of the function trans. My output is hence only:
a
a
20
100
=> nil
I added "a" and "b" to show my problem.
I am sorry if this is a very simple mistake I made but I simply don't see what is missing in my code. Every answer is much apreciated! Thank you!

Answer 1

Compare to the following, which works correctly:
(defn trans [p1 p2]
(println "a")
(dosync
(let [newval1 (+ #p1 50)
newval2 (- #p2 30)]
(do
(println "b")
(ref-set p1 newval1)
(ref-set p2 newval2)))))
(defn main []
(let [p1 (ref 20) p2 (ref 100)]
(let [f1 (future (trans p1 p2))
f2 (future (trans p1 p2))]
(Thread/sleep 500))
(println (pr-str #p1))
(println (pr-str #p2))))
That said, futures aren't really the ideal tool for the job when your interest is in a side effect, rather than in later retrieval of the result of the calculation itself.