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 <javascript:>>
> wrote:
>
>> On 30 August 2014 06:26, Colin Fleming <colin.ma...@gmail.com
>> <javascript:>> 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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