Something like the following would be fine for me
(def cache (atom {}))
(defn lookup [cache k]
(let [v (get @cache k ::nil)]
(if (= v ::nil)
(let [v (calc-value k)]
(swap! cache assoc k v)
v)
v)))
(let [value (lookup cache k)]
; use value and @cache here
)
суббота, 30 августа 2014 г., 9:27:05 UTC+4 пользователь Colin Fleming
написал:
>
> Hi all,
>
> I want to use a map to cache values based on a key. I'm planning to use an
> atom for this. My basic operation is "give me the value for this key" - if
> the value exists in the map then that value should be returned, otherwise a
> new value should be calculated, inserted in the map and then returned. My
> plan is to implement something like the following:
>
>
> (defn ensure [cache key] (if (contains? cache key) cache (assoc cache
> key (calc-value key))))(let [value (get (swap! cache ensure key) key)] ...
> do my thing with value ...)
>
>
> So 'ensure' ensures that the cache contains the value for key, the swap!
> operation returns the cache with the value and then I get it out. This
> works but feels a little clumsy, is there a better way to do this?
>
> Also, looking at the Atom source code, I see that this will cause a CAS
> operation even if the value returned from swap! is identical to the
> original value. It seems like a reasonable optimisation would be to check
> if the values are identical and not update if so - is there a reason this
> might not be a good idea?
>
> Thanks,
> Colin
>
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