Passing data out of closure into the outer context

[Tom Krestle]

Greetings,

I've stumbled across the following problem, which I believe is common and should have some sort of generic solution. Imagine I have

... ;; code inside defn

;; I extract some information out of my atom in closure:

(swap! my-atom 

  (fn [val] 

    (let [extracted-info1 (extract-something-from val) 

      extracted-info2 (extract-something-else-from val)] 

        ;; update the atom

        (-> val

          (assoc ...) (assoc))))) ;; out of swap!

;; still code inside defn

;; now, here I need to use those extracted-info1, extracted-info2 that I got out of atom inside swap! operation

;; What would be the common way to pass that information here? Using vals doesn't sound right.

Have a beer!

Tom

[Cedric Greevey]

The ugly way, as you noted, is to change the atom from some-map to
[some-map extracted-info-from-last-swap].

A less ugly way might be

(let [ei (atom nil)]
(swap! my-atom
...
(reset! ei extracted-info1)
...)
(do-things-with ei))

If the swap! is retried, the ei atom will be reset! more than once,
but it will after the swap! contain the extracted-info1 from the
successful swap! of my-atom.

The most functional way would be to see if you couldn't do the work
with ei *inside* the closure. However, if it's expensive or
side-effecty that doesn't play nice with swap! retrying. In that
instance, you might want to think of replacing my-atom with my-ref and
doing something like

(dosync
(alter my-ref
...
(send-off ei-agent ei-do-things-func extracted-info1)
...))

Agent sends are held until a transaction commits, so the
ei-do-things-func will be called only once for each transaction on
my-ref.

[Tom Krestle]

Thanks, man! I ended up using atoms.