Hi Rich, all,
Ever since the new implementation of watchers I've been itching to try
out Cells again. It worked great for agents. I ran into a snag,
though, when I tried to make cells out of refs. Here's an example.
I make two refs:
(def r1 (ref 1))
(def r2 (ref (+ 10 @r1)))
Add a watcher so that r2 gets updated whenever r1 changes:
(add-watcher r1 :send (agent nil) (fn [a r] (dosync (ref-set r2 (+
10 @r)))))
Seems to work:
(println @r1 @r2)
;;=> 1 11
(dosync (ref-set r1 2))
(println @r1 @r2)
;;=> 2 12
Unless I go too fast:
(do (dosync (ref-set r1 3)) (println @r1 @r2))
;;=> 3 12
And I have to wait for the watcher to catch up:
(println @r1 @r2)
;;=> 3 13
I guess I was hoping that the watcher on a ref would be called
synchronously in the same transaction that modified the ref. I'm not
sure if that's possible, or even desirable. I found a way around, but
I had to abandon watchers in the process.
So I suppose my question is, do watchers have to be agents?
-Stuart Sierra
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