True, but only if you don't mind possibly calculating the value more than
once since the update is not atomic.
On 30 August 2014 18:31, Eldar Gabdullin <eldar...@gmail.com> wrote:
> 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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