Snippet you showed before is also not an atomic. If you want strictly to
avoid recomputations you need something more elaborate.
May be this
(def cache (atom {}))
(defn unwrap [v]
(if-let [a (get v ::atom)]
@a
v))
(defn compute [cache k]
(let [p {::atom (atom nil)}
c (swap! cache assoc k p)
val (get c k)]
(when (identical? val p)
(swap! (::atom p) (fn [_] (calc-value k)))
(swap! cache assoc k @(::atom p)))
(unwrap val)))
(defn lookup [cache k]
(let [v (get @cache k ::nil)]
(if (= v ::nil)
(compute cache k)
(unwrap v))))
суббота, 30 августа 2014 г., 11:18:51 UTC+4 пользователь Colin Fleming
написал:
>
> 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 <[email protected] <javascript:>>
> 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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