bug#9261: `let' behavior is strange

bug#9261: `let' behavior is strange

[小江沈]

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(progn (put 'defun 'x "out")
       (let ((old (plist-member (symbol-plist 'defun) 'x)))
         (message "old: %s." old)
         (put 'defun 'x "in")
         (message "old: %s." old)
         nil))

When I eval this form, I get something like this in *message* buffer:
old: (x out).
old: (x in).
nil

I thought it should be:
old: (x out).
old: (x out).
nil

If it is not a bug, how can I save the prev symbol property?

Thank you.



In GNU Emacs 23.3.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.4)
 of 2011-04-11 on brahms, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11002000
configured using `configure  '--build' 'x86_64-linux-gnu' '--build'
'x86_64-linux-gnu' '--prefix=/usr' '--sharedstatedir=/var/lib'
'--libexecdir=/usr/lib' '--localstatedir=/var/lib'
'--infodir=/usr/share/info' '--mandir=/usr/share/man' '--with-pop=yes'
'--enable-locallisppath=/etc/emacs23:/etc/emacs:/usr/local/share/emacs/23.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.3/site-lisp:/usr/share/emacs/site-lisp:/usr/share/emacs/23.3/leim'
'--with-x=yes' '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars'
'build_alias=x86_64-linux-gnu' 'CFLAGS=-DDEBIAN -g -O2' 'LDFLAGS=-g'
'CPPFLAGS=''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.utf8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
M-x r e p o <tab> r <tab> <return>

Recent messages:
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Load-path shadows:
/usr/share/emacs/23.3/site-lisp/debian-startup hides
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/usr/share/emacs/23.3/site-lisp/dictionaries-common/flyspell hides
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/usr/share/emacs/23.3/lisp/textmodes/ispell

Features:
(shadow sort mail-extr message sendmail regexp-opt ecomplete rfc822 mml
mml-sec password-cache mm-decode mm-bodies mm-encode mailcap mail-parse
rfc2231 rfc2047 rfc2045 qp ietf-drums mailabbrev nnheader gnus-util
netrc time-date mm-util mail-prsvr gmm-utils wid-edit mailheader canlock
sha1 hex-util hashcash mail-utils emacsbug help-mode easymenu view
tooltip ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd
font-setting tool-bar dnd fontset image fringe lisp-mode register page
menu-bar rfn-eshadow timer select scroll-bar mldrag mouse jit-lock
font-lock syntax facemenu font-core frame cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese hebrew
greek romanian slovak czech european ethiopic indian cyrillic chinese
case-table epa-hook jka-cmpr-hook help simple abbrev loaddefs button
minibuffer faces cus-face files text-properties overlay md5 base64
format env code-pages mule custom widget hashtable-print-readable
backquote make-network-process dbusbind system-font-setting
font-render-setting gtk x-toolkit x multi-tty emacs)

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bug#9261: `let' behavior is strange

[Andreas Schwab]

小江沈 <xiaojiang@siteshen.com> writes:

> (progn (put 'defun 'x "out")
>        (let ((old (plist-member (symbol-plist 'defun) 'x)))
>          (message "old: %s." old)
>          (put 'defun 'x "in")
>          (message "old: %s." old)
>          nil))
>
> When I eval this form, I get something like this in *message* buffer:
> old: (x out).
> old: (x in).
> nil

This has nothing to do with let.  plist-member returns a tail of the
property list, and when the value of an existing property is changed
only the cdr of the cons cell is overwritten by put, so the reference to
the cons cell in `old' will follow the change.

> If it is not a bug, how can I save the prev symbol property?

Use plist-get or get to extract the property value, or make a copy.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

bug#9261: `let' behavior is strange

[小江沈]

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Thanks, `plist-get' does work.


2011/8/9 Andreas Schwab <schwab@linux-m68k.org>

> 小江沈 <xiaojiang@siteshen.com> writes:
>
> > (progn (put 'defun 'x "out")
> >        (let ((old (plist-member (symbol-plist 'defun) 'x)))
> >          (message "old: %s." old)
> >          (put 'defun 'x "in")
> >          (message "old: %s." old)
> >          nil))
> >
> > When I eval this form, I get something like this in *message* buffer:
> > old: (x out).
> > old: (x in).
> > nil
>
> This has nothing to do with let.  plist-member returns a tail of the
> property list, and when the value of an existing property is changed
> only the cdr of the cons cell is overwritten by put, so the reference to
> the cons cell in `old' will follow the change.
>
> > If it is not a bug, how can I save the prev symbol property?
>
> Use plist-get or get to extract the property value, or make a copy.
>
> Andreas.
>
> --
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
> "And now for something completely different."
>

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