On inspecting the source for memoize, it would actually seem (to my eyes,
someone please correct me if I'm mistaken) to be using a similar approach
to what has been suggested earlier in the discussion, with the risk of
re-computing values at concurrent invocations with the same arguments. So
using memoize in this case wouldn't actually provide any benefit -
should've double checked before posting, sorry!
Den måndagen den 1:e september 2014 kl. 11:16:00 UTC+2 skrev Colin Fleming:
>
> Hi Marcus,
>
> That's an interesting approach. My first reaction was that the update is
> still not atomic, since you have:
>
> (when-not (contains? @cache k)
> (swap! cache assoc k (calc-value* k)))
>
> Which I thought could cause different clients to see different values
> depending on the race condition there. But since in theory calc-value*
> should always return the same value for the same key, at worst you'll just
> end up swapping the same value in unnecessarily. I'd need to think a bit
> about the implications of using a memoised function in an atom update to
> convince myself, but it's a neat idea - thanks!
>
> Cheers,
> Colin
>
>
>
> On 31 August 2014 04:25, Marcus Magnusson <mar...@gmail.com <javascript:>>
> wrote:
>
>> How about something like this? You can still keep your cache as an atom
>> that you can pass around, and you avoid unnecessary recomputation through
>> memoize:
>>
>> (def cache (atom {}))
>>
>> (def lookup
>> (let [calc-value* (memoize calc-value)]
>> (fn [cache k]
>> (when-not (contains? @cache k)
>> (swap! cache assoc k (calc-value* k)))
>> (@cache k))))
>>
>> Den lördagen den 30:e augusti 2014 kl. 12:11:58 UTC+2 skrev Colin Fleming:
>>>
>>> In my case I can't use memoize because I need to supply the cache map -
>>> that in turn is stored on another object so it can be invalidated by events
>>> outside my control.
>>>
>>>
>>> On 30 August 2014 20:00, Ray Miller <r...@1729.org.uk> wrote:
>>>
>>>> On 30 August 2014 06:26, Colin Fleming <colin.ma...@gmail.com> wrote:
>>>> >
>>>> > 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 ...)
>>>>
>>>> Why not just use memoize?
>>>>
>>>> (def calc-value (memoize (fn [key] ...))
>>>>
>>>> If you need more control over the cache, check out core.memoize (which
>>>> builds on top of core.cache).
>>>>
>>>> Ray.
>>>>
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