> (defn dec-or-dissoc! [key]
> (dosync
> (let [n (@*counts* key)]
> (if (< 1 n)
> (alter *counts* assoc key (dec n))
> (alter *counts* dissoc key)))))
> Is it true that in that case, one would have to use "ensure" to make the
> operation correct?
I don't think ensure is necessary here, coz the *counts* gets altered
in all cases.
Whereas consider this:
(defn dec-or-dissoc! [key]
(dosync
(let [n (@*counts* key)]
(when (< 1 n)
(alter *counts* assoc key (dec n))))))
In this case I think ensure is necessary.
Now consider this,
(defn dec-or-dissoc! [key]
(dosync
(alter *counts*
(fn [m]
(let [n (m key)]
(when (< 1 n)
(assoc m key (dec n))))))))
In this case ensure is not necessary.
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