From: David Kastrup <dak@gnu.org>
Cc: emacs-devel@gnu.org
Subject: Re: Is this a bug of Emacs-Lisp?
Date: Fri, 17 Feb 2006 18:20:33 +0100 [thread overview]
Message-ID: <85zmkpaom6.fsf@lola.goethe.zz> (raw)
In-Reply-To: <87slqipmde.fsf@emacsfans.org> (Zhang Wei's message of "Fri, 17 Feb 2006 13:47:41 +0800")
Zhang Wei <id.brep@gmail.com> writes:
> (defun dummy () '(1 . 2))
>
> (dummy)
> => (1 . 2)
>
> (setcdr (dummy) 3)
>
> (dummy)
> => (1 . 3)
>
> Modify the return value of dummy changed it's defination.
No, it didn't.
> Is this a bug of Elisp? If it's not. How does this happen?
'(1 . 2) is a cons cell. Cons cells are _always_ accessed by
reference, like arrays and strings.
So what is compiled into the function is a reference to a cons cell,
and this reference stays the same. setcdr however changes the cons
cell itself.
--
David Kastrup, Kriemhildstr. 15, 44793 Bochum
next prev parent reply other threads:[~2006-02-17 17:20 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-02-17 5:47 Is this a bug of Emacs-Lisp? Zhang Wei
2006-02-17 16:21 ` Giorgos Keramidas
2006-02-18 5:26 ` Drew Adams
2006-02-18 6:42 ` Drew Adams
2006-02-18 16:18 ` Andreas Schwab
2006-02-17 16:28 ` Andreas Schwab
2006-02-17 17:20 ` David Kastrup [this message]
2006-02-17 17:57 ` Luc Teirlinck
2006-02-17 18:18 ` Luc Teirlinck
2006-02-17 19:24 ` Alan Mackenzie
2006-02-17 19:29 ` Kevin Rodgers
2006-02-17 22:34 ` Sascha Wilde
2006-02-18 18:34 ` Richard M. Stallman
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