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Re: Closure?

From: Maciek Godek
Subject: Re: Closure?
Date: Tue, 15 Jul 2008 00:46:57 +0200

>> This certainly looks like trashing global namespace
>> (which isn't good in the long run) and doesn't allow you
>> to have object handlers (like many variables referring
>> to the same object) without additional quirks.
> It's just an example of what local-eval can
> be good for. It's an example of a _namespace
> system_, and not an _OO system_.

OK, sorry, I didn't understand at first.

>>> But for implementing a message passing OO system,
>>> it's easier to use macros and hash tables, plus
>>> that it probably performs much better:
>> Pefrorms better than local-eval or better than
>> define-namespace?
> It's much faster than local-eval since it's not
> calling eval for each method call. The overhead of calling
> a method in an OO system, based on hash tables
> and macros, is basically just the time it takes looking
> up the address of the function from
> a hash-table. For guile, which has a large
> overhead already, your program probably won't
> go any slower doing this compared to calling
> functions directly:

Is local-eval much slower than the usual eval?
It is very probable that I get something wrong
right now, but according to my imagination of
a possible implementation :D of eval it could be
the same function  (and the closure would
actually be implemented as a hash table or
something like that).
And eval is actually what lisp is doing all the
time, so the only thing that changes here
is the scope (=reference to a hash table)
That's how, I believe, it could be implemented.
(but I've never tried to do that nor read it in the code,
so I'm just being a smart ass)

> (define (make-bank sum)
>  (let ((attributes (make-hash-table)))
>    (define dispatcher
>      (lambda (which . rest)
>        (apply (hashq-ref attributes which) rest)))
>    (hashq-set! attributes 'sum (lambda ()
>                                  sum))
>    (hashq-set! attributes 'add (lambda (n)
>                                  (set! sum (+ n sum))))
>    dispatcher))
> (define bank (make-bank 0))
> (bank 'add 2)
> (bank 'sum)
> => 2
> And by using some macros, you can make the syntax look
> much prettier.

To me it already looks very pretty (and works just
as efficiently as my imagination of local-eval operation :>).
This might be just what I've been looking for.
Thanks a lot.

>> Well, I've read some documentation of GOOPS and then
>> I took a glimpse at its source. It has at least a few
>> disadvantages, for it is an object system implemented in
>> scheme -- it is therefore hard to access its objects from C
>> (while closures are easily accessible through
>> scm_local_eval) and it probably won't run as fast as
>> local-eval, at least conceptually.
> local-eval is forced to interpret its argument, and is therefore, at least
> conceptually, very slow. I very much doubt GOOPS is that slow,
> but I don't know how GOOPS is implemented though. :-)

Neither do I (I just saw a lot of code, many many layers of code
so I've concluded that it is a complex multi-layer system I don't
want to get into). I never realized that eval causes such terrible
overheads ((procedure-source eval) doesn't show any of the
mysteries of the universe :P)

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