Hi,
I have been working on a setup where I batch a number of updates in a
queue, which I store in an atom with multiple threads potentially adding
things to it. Periodically I want to flush that queue, leaving an empty
list in the atom and passing the current batch of values on for further
processing.
I can't use swap! for this, because my further processing is side effecting
and I wouldn't want to dispatch the same information twice, and I can't use
reset! because that just throws away the old contents of the atom, and
derefing it in the line before I am likely to lose data if another thread
adds something at the last minute.
I could change to an agent with append and flush actions which would get
queued up and run only once each, and that would work. But it has quite a
lot of thread overhead which I think is unnecessary.
What I ended up doing was writing a version of reset! which returns the old
value instead of the new value (why would I want the new value? I know
that, I put it in!)
(defn flush! [atom newval]
(let [val @atom]
(if (compare-and-set! atom val newval)
val
(recur atom newval))))
=> (def a (atom [1 2 3 4]))
#'user/a
=> (flush! a [])
[1 2 3 4]
=> @a
[]
Is there already a standard library fn I should be using for this? Is this
a bad idea for some reason I don't fathom?
Adam
https://github.com/AdamClements
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