From: "Maciek Godek" <pstrychuj@gmail.com>
To: "Neil Jerram" <neiljerram@googlemail.com>
Cc: guile-user@gnu.org
Subject: Re: Closure?
Date: Mon, 14 Jul 2008 03:15:32 +0200 [thread overview]
Message-ID: <e2ceda030807131815v4ddea089v6b19b546fe68a745@mail.gmail.com> (raw)
In-Reply-To: <49dd78620807131556u78b6a3bah9f34e12a5c60c41f@mail.gmail.com>
>> As the practise shows, although guile documentation says something
>> different. In section 3.1.4.7 (A Shared Persistent Variable)
>>
>> "An important detail here is that the `get-balance' and `deposit'
>> variables must be set up by `define'ing them at top level and then
>> `set!'ing their values inside the `let' body. Using `define' within
>> the `let' body would not work: this would create variable bindings
>> within the local `let' environment that would not be accessible at top
>> level."
>>
>> So one might conclude that it _is_ possible to use define inside
>> a 'let' form.
>
> Which would be correct! For example:
>
> (let ((a 1))
> (define b 2)
> (+ a b))
> =>
> 3
>
> Whereas:
>
> (let ((a 1))
> (display a)
> (newline)
> (define b 2)
> (+ a b))
> =>
> ERROR: Bad define placement
>
> The "special rules" are just that any defines have to come before
> anything else in the body of the let.
Yeah, guess you're right (under certain circumstances :P)
> 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?
>> 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 :)
Best regards,
M
next prev parent reply other threads:[~2008-07-14 1:15 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
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 ` Maciek Godek [this message]
[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
[not found] <cmu-lmtpd-15105-1216051603-3@mail-imap1.uio.no>
2008-07-14 16:30 ` Closure? Kjetil S. Matheussen
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
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