On Fri, Jan 6, 2012 at 10:22 PM, Alan Malloy <a...@malloys.org> wrote:
> On Jan 6, 6:16 pm, Cedric Greevey <cgree...@gmail.com> wrote:
>> On Fri, Jan 6, 2012 at 4:34 PM, Alan Malloy <a...@malloys.org> wrote:
>> > On Jan 6, 12:56 pm, Jozef Wagner <jozef.wag...@gmail.com> wrote:
>> >> Thank you,
>>
>> >> But the things are more complicated. In my case, I need to update the atom
>> >> with the result of a (native) function which unfortunately also performs
>> >> some side effects and cannot be split in two.
>>
>> >> Updated example:
>>
>> >> (def state {:result nil :input [1 2 3]})
>>
>> >> (defn update-item!
>> >> [item]
>> >> (if (:result item) ; if already processed, do nothing
>> >> item
>> >> (let [result (side-effect-fn! (:input item))]
>> >> (assoc item :result result))))
>>
>> >> (swap! state update-item!) ;; same problem
>>
>> > Not doable with atoms if you have a side-effecty thing that you can't
>> > separate out. You'll have to use something lower-level like Java
>> > synchronization or locks.
>>
>> Or something higher level like transactions. Use a ref and have
>> update! alter the ref, then (send-off some-agent #(spit ...)), inside
>> a dosync. The agent send only goes if the transaction commits.
>
> This works exactly as well as with atoms: it works fine if you have a
> pure function and an I/O function, but doesn't work if you have a
> single function that does both pure computation and I/O side effects.
> Jozef has some Java magic that does I/O as a side effect of a
> computation, and he cannot untangle the two.
Wrong. Jozef has this:
(def aval (atom {:dumped false :contents "hello world"}))
(defn update!
[item]
(when-not (:dumped item)
(spit "a.out" (:contents item) :append true)
(assoc item :dumped true)))
(swap! aval update!)
Which can be transformed to this:
(def aval (ref {:dumped false :contents "hello world"}))
(def aagent (agent nil))
(defn update
[item]
(dosync
(if (:dumped @item)
(ref-set item nil)
(do
(alter item assoc :dumped true)
(send-off aagent (fn [_] (do (spit "a.out" (:contents @item)
:append true) nil)))))))
(update aval)
which should have the same semantics, except that the spit cannot be
done twice (or more) for one call to update.
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