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Question

Starting with Clojure I discovered a talk by Rich Hickey where he demonstrates some of Clojure's strengths on a basic Ant-Simulator.

Can this code still be considered as a good reference for Clojure? Especially the parts when he recursively sends off functions to agents to simulate a game loop. Example:

(defn animation [x] (when b/running (send-off *agent* #'animation)) (. panel (repaint)) (. Thread (sleep defs/animation-sleep-ms)) nil)

Edit:

I am not interested in the #' reader macro but more wether it is idiomatic/good Clojure to recursively call a function on a agent or not.

What specific code snipped can you post so a problem can be stated, so we can help you? Otherwise, I'm thinking this question belongs over on Programmers. — octopusgrabbus
– octopusgrabbus, Commented Sep 3, 2012 at 15:45
Why is the #' reader macro needed when recursively sending animation to *agent*? — noahlz
– noahlz, Commented Sep 3, 2012 at 16:14
This evaluates animation every time it is used. This way animationcan be changed on the fly — Ole Krüger
– Ole Krüger, Commented Sep 3, 2012 at 18:14

noahlz · Accepted Answer · 2012-09-05 22:19:53Z

This snippet is current in Clojure 1.4. Is it idiomatic for a function to submit a task back to the agent that called it? Yes.

Here is an example that uses a similar approach to recursively calculate a factorial:

(defn fac [n limit total] (if (< n limit) (let [next-n (inc n)] (send-off *agent* fac limit (* total next-n)) next-n) total)) (def a (agent 1)) (await (send-off a fac 5 1)) ; => nil @a ;=> 120

Update

The above is a contrived example and actually not a good one, as there is a race condition between the various recursive send-off calls and the later await. There may be some send-off calls yet to be added to the agent's task queue.

I re-wrote the above as follows:

(defn factorial-using-agent-recursive [x] (let [a (agent 1)] (letfn [(calc [n limit total] (if (< n limit) (let [next-n (inc n)] (send-off *agent* calc limit (* total next-n)) next-n) total))] (await (send-off a calc x 1))) @a))

and observed the following behavior:

user=> (for [x (range 10)] (factorial-using-agent-recursive 5)) (2 4 3 120 2 120 120 120 120 2) user=> (for [x (range 10)] (factorial-using-agent-recursive 5)) (2 2 2 3 2 2 3 2 120 2) user=> (for [x (range 10)] (factorial-using-agent-recursive 5)) (120 120 120 120 120 120 120 120 120 120)

Moral of the story is: don't use agents for synchronous calculations. Use them for asynchronous independent tasks - like updating animations displayed to a user :)

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