Oh, forget to wait for value in unwrap. Here is a better version
(defn unwrap [v]
(if-let [p (get v ::promise)]
@p
v))
(defn compute [cache k]
(let [p {::promise (promise)}
c (swap! cache assoc k p)
val (get c k)]
(when (identical? val p)
(let [result (calc-value k)]
(deliver (::promise p) result)
(swap! cache assoc k result))
(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., 11:54:59 UTC+4 пользователь Eldar Gabdullin
написал:
>
> 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 <elda...@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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