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.
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