Yes, in my case the update is atomic but doesn't strictly avoid
recomputation (since the Atom can call the calculation function many
times). I've given this some hammock time and I don't think that's a
problem. However on further investigation I do have to ensure that for each
of my keys the value that is returned is identical since it uses identity
equality semantics, so I might have to stick with my original solution.
Thanks for the input though, it's very interesting to see your solution.
On 30 August 2014 19:54, Eldar Gabdullin <eldar...@gmail.com> wrote:
> 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
>>>>
>>> --
>>> You received this message because you are subscribed to the Google
>>> Groups "Clojure" group.
>>> To post to this group, send email to clo...@googlegroups.com
>>>
>>> Note that posts from new members are moderated - please be patient with
>>> your first post.
>>> To unsubscribe from this group, send email to
>>> clojure+u...@googlegroups.com
>>>
>>> For more options, visit this group at
>>> http://groups.google.com/group/clojure?hl=en
>>> ---
>>> You received this message because you are subscribed to the Google
>>> Groups "Clojure" group.
>>> To unsubscribe from this group and stop receiving emails from it, send
>>> an email to clojure+u...@googlegroups.com.
>>>
>>> For more options, visit https://groups.google.com/d/optout.
>>>
>>
>> --
> You received this message because you are subscribed to the Google
> Groups "Clojure" group.
> To post to this group, send email to clojure@googlegroups.com
> Note that posts from new members are moderated - please be patient with
> your first post.
> To unsubscribe from this group, send email to
> clojure+unsubscr...@googlegroups.com
> For more options, visit this group at
> http://groups.google.com/group/clojure?hl=en
> ---
> You received this message because you are subscribed to the Google Groups
> "Clojure" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to clojure+unsubscr...@googlegroups.com.
> For more options, visit https://groups.google.com/d/optout.
>
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to clojure@googlegroups.com
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
---
You received this message because you are subscribed to the Google Groups
"Clojure" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to clojure+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.
