Hi…
in my little project I do some concurrent stuff using atoms.
Now, during the last days using swap! I encountered the same problem
several times:
Some result value is produced inside the swap! function besides the updated
data that I would like to return.
I first played with two atoms and considered modeling the entire result as
the atom value and even using refs.
The "fat atom" approach is ugly in this case as the enhanced result is just
a local artifact and not of interest outside my calls. Refs/STM looks
somewhat big for this.
But then I saw compare-and-set! and came up with this:
(defn complex-swap! [atoom get-ref-val f & args]
(loop [last-val @atoom]
(let [result (apply f last-val args)]
(if (compare-and-set! atoom last-val (get-ref-val result))
result
(recur @atoom)))))
It allows to produce an augmented result value, only part of which is the
new atom content passing an additional getter function to extract the new
value part.
For example:
(def my-atom (atom '(1 2 3 4)))
(complex-swap! my-atom second (partial split-with #(<= % 2)))
=> [(1 2) (3 4)]
@my-atom
=> (3 4).
The original swap! would be like this:
(swap! my-atom (comp second (partial split-with #(<= % 2))))
=> (3 4)
@my-atom
=> (3 4)
Now, is there some even easier way to achieve this or is this the best
approach?
Ciao
…Jochen
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