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.
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