foobar.clj

user=> (defn foo [n]
         (with-local-vars [x 0]
           (dotimes [i n]
             (var-set x (inc @x)))))
#'user/foo
user=> (time (foo 5000000))
"Elapsed time: 514.042912 msecs"
user=> (time (foo 5000000))
"Elapsed time: 438.661454 msecs"

user=> (defn bar [n]
         (let [x (atom 0)]
           (dotimes [i n]
             (swap! x inc))))
#'user/bar
user=> (time (bar 5000000))
"Elapsed time: 113.794149 msecs"
nil
user=> (time (bar 5000000))
"Elapsed time: 100.323271 msecs"
nil

user=> (defn baz [n]
         (let [x (volatile! 0)]
           (dotimes [i n]
             (vswap! x inc))))
#'user/baz
user=> (time (baz 5000000))
"Elapsed time: 56.927072 msecs"
nil