Would some of this difficulty be ameliorated by calling memoize on the
function that you use? That way, if it's an expensive function, and the
value hasn't changed, it's just looked up rather than recalculated.

- DAemon

On Fri, May 18, 2012 at 6:28 AM, Stuart Sierra
<the.stuart.sie...@gmail.com>wrote:

> I think the point with `commute` is to allow for more concurrency at the
> expense of more computation.
>
> If you want assurance that your function is only called once, you can use
> `alter`.
>
> Keep in mind that *any* code in a Ref transaction has the potential to be
> called more than once if there's a conflict.
>
> All this doesn't mean that it's impossible to avoid the duplicate
> computation on `commute`. The code to study would be here:
>
>
> https://github.com/clojure/clojure/blob/8fda34e4c77cac079b711da59d5fe49b74605553/src/jvm/clojure/lang/LockingTransaction.java#L459
>
> -S
>
>
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