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Re: Set (global) function argument from inside function

From: Stefan Monnier
Subject: Re: Set (global) function argument from inside function
Date: Sun, 10 Aug 2014 10:38:14 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.4.50 (gnu/linux)

>  (defun foo (X)
>    (setq X (+ X 5)))
>  (setq Y 2)
>  (message "fooY: %s Y: %s" (foo Y) Y)

> #+results:
> : fooY: 7 Y: 2

IIUC you want to pass a variable by reference.  Elisp does not offer to
pass arguments by reference (contrary to, say, C++).  But just like C,
it lets you construct references and pass them:

  (defun foo (X-ref)
    (setf (gv-deref X-ref) (+ (gv-deref X-ref) 5)))

  (setq Y 2)
  (message "fooY: %s Y: %s" (foo (gv-ref Y)) Y)

Note that gv-ref only exists in Emacs>24.3 and only works with

If you only care about global variables (rather than arbitrary "place"s,
such as global vars, local vars, array elements, list elements, ...),
and you want it to work with older Emacsen, you can use the following

  (defun foo (X-name)
    (set X-name (+ (symbol-value X-name) 5)))

  (setq Y 2)
  (message "fooY: %s Y: %s" (foo 'Y) Y)

-- Stefan

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