send

user=> (def my-agent (agent 100))
#'user/my-agent
user=> @my-agent
100

;; Note the following happens asynchronously in a thread
;; pool
user=> (send my-agent + 100)
#<Agent@5afc0f5: 200>

;; Assuming the addition has completed the value will
;; now be updated when we look at it.
user=> @my-agent
200

;; update agent value
user => (def foo (agent 100)) 
#'user/my-agent
user => @foo
100

;; function get "old value"
user => (send foo (fn [old-foo] 
                      (println  old-foo "foo will change")
                      (+ old-foo 100)))
100 foo will change
#agent[{:status :ready, :val 200} 0x000000]

user => @foo
200