From: jack-mac <duthen.mac@gmail.com>
To: gnu.emacs.help@googlegroups.com
Cc: help-gnu-emacs@gnu.org
Subject: Re: "defmacro" and "local variables" and "let" and "nconc" strange behaviour...
Date: Mon, 14 Jan 2013 01:04:47 -0800 (PST) [thread overview]
Message-ID: <13970288-575d-4593-a694-0c3f09c3844b@googlegroups.com> (raw)
In-Reply-To: <mailman.17291.1358086805.855.help-gnu-emacs@gnu.org>
Le dimanche 13 janvier 2013 15:18:46 UTC+1, Oleksandr Gavenko a écrit :
> On 2013-01-13, Oleksandr Gavenko wrote:
>
>
>
> > I wrote simple macro:
>
> >
>
> > (defmacro my-filter (pred list)
>
> > "Construct list with elements from LIST which satisfy PRED."
>
> > (let ( (r (make-symbol "r_")) )
>
> > `(let ( (,r '(nil)) )
>
> > (mapc (lambda (item)
>
> > (when (,pred item)
>
> > (nconc ,r (cons item nil))))
>
> > ,list)
>
> > (cdr ,r))))
>
> >
>
> > When I evaluate several times:
>
> >
>
> > (my-filter (lambda (x) x) '(1 nil "a"))
>
> >
>
> > I get sequentially:
>
> >
>
> > (1 "a")
>
> > (1 "a" 1 "a")
>
> > (1 "a" 1 "a" 1 "a")
>
> > (1 "a" 1 "a" 1 "a" 1 "a")
>
> >
>
> > When I eval:
>
> >
>
> > (pp (macroexpand '(my-filter (lambda (x) x) '(1 nil "a"))))
>
> >
>
> > I get:
>
> >
>
> > (let
>
> > ((r_
>
> > '(nil 1 "a" 1 "a" 1 "a" 1 "a" 1 "a" 1 "a" 1 "a" 1 "a")))
>
> > (mapc
>
> > (lambda
>
> > (item)
>
> > (when
>
> > ((lambda
>
> > (x)
>
> > x)
>
> > item)
>
> > (nconc r_
>
> > (cons item nil))))
>
> > '(1 nil "a"))
>
> > (cdr r_))
>
> >
>
> > Why instead of '(nil) I get something else?
>
>
>
> I found fix by changing '(nil) to (list nil). Please explain why first variant
>
> fail?
>
>
>
> --
>
> Best regards!
To understand it, just try:
(defun foo (x)
(let ((l '(0)))
(nconc l (list x))))
(foo 1)
(foo 2)
This looks like a closure!
next prev parent reply other threads:[~2013-01-14 9:04 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-01-13 14:15 "defmacro" and "local variables" and "let" and "nconc" strange behaviour Oleksandr Gavenko
2013-01-13 14:18 ` Oleksandr Gavenko
[not found] ` <mailman.17291.1358086805.855.help-gnu-emacs@gnu.org>
2013-01-14 9:04 ` jack-mac [this message]
[not found] <mailman.17290.1358086558.855.help-gnu-emacs@gnu.org>
2013-01-13 14:21 ` Barry Margolin
2013-01-13 15:00 ` Oleksandr Gavenko
2013-01-13 16:45 ` Le Wang
2013-01-13 17:25 ` Oleksandr Gavenko
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