On 21 February 2010 09:24, ataggart <[email protected]> wrote:
> On Feb 20, 6:07 am, Jack Fang <[email protected]> wrote:
>> Suppose the transfer-money and print-info are run in different
>> thread. With the current Clojure STM, is it possible that print-info
>> prints the old source-account information and modified dest-account
>> information?
>
> Yes.
>
>> Do I need to add a dosync to the print-info function? I
>> read the Programming Clojure. It says do not have side-effect on a
>> transaction.
>
> If you want consistent results between refs, you'll need to be in a
> transaction, thus dosync. You might also need to use ensure, but I'm
> not certain. The reason side-effects are discouraged (not forbidden)
> is because a transaction may be run multiple times, thus you might get
> multiple printouts.
>
>> If I add dosync to the print-info, do I need to use agent
>
> Agents are desgned to be used for side-effects in a transaction.
> That's why sends to an agent are held and only actually happen if the
> tx commits. It mIght be overkill for your examle, but yes send
> ensured/altered values to an agent from inside a dosync block.
>> to print the info?
But surely in this case you could do:
(defn print-info []
(let [[src dst] (dosync [...@source-account @dest-account])]
(println src)
(println dst)))
or maybe:
(defn print-info []
(let [[src dst] (dosync
(ensure source-account)
(ensure dest-account)
[...@source-account @dest-account])]
(println src)
(println dst)))
I suspect the ensures are necessary, but I would appreciate it if
someone could explain why they are (or are not) necessary in this
case.
Thanks.
--
Michael Wood <[email protected]>
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