On 7 December 2014 at 05:13, Andy L <core.as...@gmail.com> wrote:
>
>
>
>> The SoftCache uses a ConcurrentHashMap, but that caching option isn't
>> used in core.memoize. Are you building a custom memoizer?
>>
>>
> WRT ConcurrentHashMap, it was an incorrect conclusion on my part. In any
> case, I fail to see "thread safety" in the cache implementation, but again
> I could be wrong. Or it might not be needed for 99.99% cache use cases.
> Also, at that point I would like to avoid to "defcache" my own version.
>
Most of the cache implementations in core.cache have no side-effects. They
simply return a new cache rather than overwriting the old one. The memoize
library places the cache in an atom, so it's guaranteed to change
atomically.
> So I ended up with this (this is my first LISP macro ever, so please be
> gentle :-) :
>
> (defmacro when-map-future-swap! [a k f]
> `(locking ~a
> (when (not (contains? @~a ~k))
> (swap! ~a assoc ~k nil)
> (future (swap! ~a assoc ~k (~f ~k)))
> )
> )
> )
>
You could write this as a function. There's nothing in there that requires
a macro.
(defn when-map-future-swap! [a k f]
(locking a
(when-not (contains? @a k)
(swap! a assoc k nil)
(future (swap! a assoc k (f k))))))
But I'd personally just use a delay rather than "locking" for this purpose.
- James
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