On Fri, Mar 19, 2010 at 06:56, Greg Fodor <[email protected]> wrote:
> I would like to memoize bar such that the memory used for memoization
> is GC'ed at the end of the call to foo, and additionally the cache
> used for memoization is thread local (so no need for heavyweight
> synchronization tools like atoms, etc.) In Ruby, I would implement
> this as a simple local hash with the ||= operator through each
> iteration of a loop inside foo that calls bar.
;; the "trick" I found is to explicitly deref the var binding the
;; function to be memoized. This way fib's recursive calls will use
;; the memoized binding established in
;; use-fib-memoized-thread-locally.
(defn fib [n]
(if (> 2 n) n
(+ (@#'fib (dec n))
(@#'fib (dec (dec n))))))
(defn use-fib-memoized-thread-locally [n]
(binding [fib (memoize fib)]
(fib n)))
;; user> (time (fib 32))
;; "Elapsed time: 1755.796366 msecs" ;; SLOW
;; 2178309
;;
;; user> (time (use-fib-memoized-thread-locally 32))
;; "Elapsed time: 1.514927 msecs" ;; FAST
;; 2178309
;;
;; user> (time (fib 32))
;; "Elapsed time: 2024.836838 msecs" ;; SLOW, again
;; 2178309
// Ben
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