From: "Neil Jerram" <neiljerram@googlemail.com>
To: "Maciek Godek" <pstrychuj@gmail.com>
Cc: guile-user@gnu.org
Subject: Re: Closure?
Date: Sun, 13 Jul 2008 23:56:48 +0100 [thread overview]
Message-ID: <49dd78620807131556u78b6a3bah9f34e12a5c60c41f@mail.gmail.com> (raw)
In-Reply-To: <e2ceda030807122357p33ed8cbr78728754dba73872@mail.gmail.com>
2008/7/13 Maciek Godek <pstrychuj@gmail.com>:
>> except that the last line fails with a "Bad define placement" error.
>> That's because there are special rules for defines inside lexical
>> scopes.
>
> 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.
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.
> 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.
Regards,
Neil
next prev parent reply other threads:[~2008-07-13 22:56 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 ` Neil Jerram [this message]
2008-07-14 1:15 ` 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
[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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