Re: [Chicken-users] Memoizing a procedure

[Stephen Eilert]

On Mon, Nov 29, 2010 at 10:29 AM, Jörg F. Wittenberger
<address@hidden> wrote:
> Am Montag, den 29.11.2010, 13:53 +0100 schrieb Jörg F. Wittenberger:
>> Hi Hugo,
>>
>> Am Sonntag, den 28.11.2010, 17:27 +0100 schrieb Peter Bex:
>> > On Sun, Nov 28, 2010 at 12:53:53PM -0300, Hugo Arregui wrote:
>> > > Hi guys,
>> (define (make-memoized proc)
>>   (let ((memo (make-hash-table))
>>         (missing (list 'missing)))
>>     (lambda args
>>       (let ((result (hash-table-ref memo args
>>                                   (lambda ()
>>                                     (let ((x (cons missing (make-mutex))))
>>                                       (hash-table-set! memo args x)
>>                                       x)))))
>>         (when (and (pair? result) (eq? (car result) missing))
>>           (let ((mux (cdr result)))
>>             (mutex-lock! mux)
>>           (when (and (pair? result) (eq? (car result) missing))
>>               (set! result (apply proc args))
>>             (hash-table-set! memo args result))
>>             (mutex-unlock! mux)))
>>         result))))
>>
>> Note, however that this will not work, if your memoized procedure
>> terminates by exception!
>
> Note also, that hash tables in chicken are not thread safe, hence you'd
> need some more locking anyway.  Left as an exercise.
>
> Be warned however, that you don't want to grow that code too much.
> "Recently" - that is I'm still kind of screwing with it - wrote some
> code, which handles such cases, plus some optional features like
> timeout's for the result being taken to be valid (which comes in handy,
> when you want to memoize procedures, which return temporary valid values
> (e.g., network connections), cache-cleanup's.  It can easily outgrow the
> scope of an educational exercise.

So it is not only "memoization", but a caching system. I was going to
write something similar, to cache webservice responses.


--Stephen

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