Question

I have an atom,

(def a (atom {:a <some-value>}))

and it needs to be updated continuously, what would be the most memory efficient call in the long run…?

(swap! a assoc :a <next-value>)

or

(swap! a (fn [_] {:a <next-value>}))

Intuitively, based on the talks I have heard on persistent structures, I’m thinking that the second way would be a little slower but better in the long-run… but would like a second opinion.

Answer 1

1. The first form doesn’t work.
2. Memory efficiency is irrelevant: how you get to the new value has no impact on the long-term amount of memory usage once the old value is thrown away.
3. You appear to want reset!, not swap!.
4. Consider why you’re updating an atom so often, especially if you don’t care about its previous value at all. You can usually accomplish something similar more easily with a purely functional approach, or at least with a swap! function that takes the old value into account.