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Change a symbol's plist member with (let ...)?

From: Thorsten Jolitz
Subject: Change a symbol's plist member with (let ...)?
Date: Fri, 14 Mar 2014 22:17:18 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Hi List, 

the title says it all - is it possible to temporarily change a symbol's
plist member within (let ...)?

#+begin_src emacs-lisp
  (plist-get (symbol-plist 'outline-mode)

: text-mode

For the sake of this example, say I want to "con" the function
`derived-mode-p' and make it believe outline-mode is actually derived
from lisp-mode (or so ...).

How to do this with working code (not pseudo-code)?

#+begin_src emacs-lisp
  (defadvice derived-mode-p' (around derived-mode-p-around)
    "Con `derived-mode-p' about the true parent-mode of outline-mode."
    (when (eq major-mode 'outline-mode)
      (let ((...<plist-key derived-mode-parent> 'lisp-mode ...)) ; pseudo-code

Or do I have to do it the pedestrian way: do plist-get first, store the
original value, do plist-put with the new value, call ad-do-it, and then
restore the original value again with plist-put?


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