Hi,
an atom can be sufficient, but you need to drop to a lower level.
(defn !paws
"Like swap! but returns the old value of the atom."
[a f & args]
(loop []
(let [old-value @a
new-value (apply f old-value args)]
(if (compare-and-set! a old-value new-value)
old-value
(recur)))))
(peek (!paws atom-with-queue pop))
However, my suspicion is, that you don't clearly divide the state handling
from the program logic. The peek should happen in the update function you
pass to swap!. If this is not side-effect free, it means that you have to
coordinate with other identities and resources. Then a ref or an agent
might be indeed a better fit than the atom.
Sincerely
Meikel
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