From: "Kjetil S. Matheussen" <k.s.matheussen@notam02.no>
To: guile-user@gnu.org
Subject: Re: Closure?
Date: Mon, 14 Jul 2008 18:30:47 +0200 (CEST) [thread overview]
Message-ID: <Pine.LNX.4.64.0807141815040.5269@ttleush> (raw)
In-Reply-To: <cmu-lmtpd-15105-1216051603-3@mail-imap1.uio.no>
"Maciek Godek"
>> I don't know exactly how it works out that using a define in
>> local-eval falls foul of the define placement rule, but it is not hard
>> to imagine that it could do.
>
> The other question is: is it really necessary to impose such
> limitations on "define". Why is it required to make its position
> inside let privileged?
>
The scheme standard is a bit pedantic. The above I would write
like this:
(let ()
(define a)
(display a)
(newline)
(let ()
(define b 2)
(+ a b)))
>>> Yes, since there's local-eval and the-environment, everything I've
>>> ever dreamed of is possible :)
>>> But as I've concluded from the discourse, neither of these is
>>> defined in R5RS (and it makes me wonder)
>>
>> Well I've never thought this through before, but perhaps that is
>> because in many cases it is equivalent to create a lambda at the point
>> where you would call the-environment, containing the code that you
>> would later pass to local-eval.
>>
>> For example, the ++ example then becomes:
>>
>> (define ++ (let ((c 0)) (lambda () (begin (set! c (+ c 1)) c))))
>>
>> - which is the traditional way of writing this example.
>
> You didn't focus :>
> The whole idea of accessing a closure environment
> was in fact to make scheme object oriented
> programming more intuitive.
>
> In guile info pages there's an oo closure example:
>
> (section 3.1.4.9 "Example 4: Object Orientation")
> "
> (define (make-account)
> (let ((balance 0))
> (define (get-balance)
> balance)
> (define (deposit amount)
> (set! balance (+ balance amount))
> balance)
> (define (withdraw amount)
> (deposit (- amount)))
>
> (lambda args
> (apply
> (case (car args)
> ((get-balance) get-balance)
> ((deposit) deposit)
> ((withdraw) withdraw)
> (else (error "Invalid method!")))
> (cdr args)))))
>
> (define my-account (make-account))
> "
> Notice the ugly "case" statement that requires
> the variables to be accessed in the following manner
> (the same example, a few lines later):
> "
> (my-account 'get-balance)
> =>
> 0
>
> (my-account 'withdraw 5)
> =>
> -5
>
> (my-account 'deposit 396)
> =>
> 391
>
> (my-account 'get-balance)
> =>
> 391
> "
>
> This is ugly as it requires doubling the names of functions.
> Perhaps it could be overcome with some sort of macro,
> but the "with" I proposed allows to avoid the whole "case"
> and to write (after slight modifications in the "let" form):
>
> (with my-account (get-balance))
>
> Or maybe I think wrong; I'm new in the world of lisp,
> so please forgive me my mistakes :)
>
I think local-eval is necessary for making
a namespace system of the below type without having to use
codewalking macros to expand the bodies of functions:
(define-namespace bank)
(def-bank sum 0)
(def-bank (add n)
(set! sum (+ n sum)) ;; Note that it's enough to write "sum".
(bank.add 50)
bank.sum
=> 50
But for implementing a message passing OO system,
it's easier to use macros and hash tables, plus
that it probably performs much better:
(def-class <bank>
(def-var sum 0)
(def-method (add n)
(set! sum (+ n sum)))
(define bank (new <bank>))
(-> bank add 50)
(-> bank sum)
=> 50
There's a bunch of these systems for scheme.
The syntax above is used from
http://snd.cvs.sourceforge.net/snd/cvs-snd/oo.scm?view=log
Guile's own OO system called GOOPS is also very nice.
GOOPS a quite verbose but very powerful and a lot
more interactive. It's similar to CL's CLOS, which
you should look at if you are not familiar with
already.
next parent reply other threads:[~2008-07-14 16:30 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <cmu-lmtpd-15105-1216051603-3@mail-imap1.uio.no>
2008-07-14 16:30 ` Kjetil S. Matheussen [this message]
2008-07-14 21:14 ` Closure? Maciek Godek
[not found] ` <e2ceda030807141414i5acef7d1h37d12d14e01cc1d@mail.gmail.com>
2008-07-14 21:41 ` Closure? Kjetil S. Matheussen
2008-07-14 22:46 ` Closure? Maciek Godek
[not found] <cmu-lmtpd-26382-1215792454-10@mail-imap1.uio.no>
2008-07-11 17:42 ` Closure? Kjetil S. Matheussen
2008-07-11 17:47 ` Closure? Kjetil S. Matheussen
2008-07-11 20:54 ` Closure? Maciek Godek
2008-07-12 11:47 ` Closure? Kjetil S. Matheussen
2008-07-13 6:59 ` Closure? Maciek Godek
2008-07-12 22:43 ` Closure? Neil Jerram
2008-07-15 7:59 ` Closure? Ludovic Courtès
2008-07-15 9:11 ` Closure? Andy Wingo
2008-07-16 16:42 ` Closure? Ludovic Courtès
2008-07-11 14:48 Closure? Maciek Godek
2008-07-11 15:01 ` Closure? Ludovic Courtès
2008-07-11 15:32 ` Closure? Maciek Godek
2008-07-12 22:57 ` Closure? Neil Jerram
2008-07-13 6:57 ` Closure? Maciek Godek
2008-07-13 22:56 ` Closure? Neil Jerram
2008-07-14 1:15 ` Closure? Maciek Godek
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