On Jan 6, 7:32 pm, Cedric Greevey <cgree...@gmail.com> wrote:
> 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.
That's the simplification he made in his first post, but if you look
at the post I was actually replying to he's confessed he actually has
a different situation.
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to clojure@googlegroups.com
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
