How do I make an Emacs keybinding to set a variable?

Question

I've got a variable in Emacs called my-var that I'd like to set whenever I press C-v. How do I do that? I tried this:

(defun set-my-var (value)
  "set my var"
  (interactive)
  (defvar my-var value
    "a variable of mine")
)

(global-set-key "\C-v" 'set-my-var)

But that fails:

call-interactively: Wrong number of arguments: (lambda (value) "set my var"
(interactive) (defvar my-var value "a variable of mine")), 0

Answer 1

It's in the argument. Look over at the text I just posted about (interactive). When you bind set-my-var to a key, it's looking for an argument, but since you used (interactive) there's no argument to be had. What you wanted is something like (interactive "p") to get the CTRL-u argument, or (interactive "M") to get a string.

Read the EMACS Lisp manual on "Using Interactive."

Answer 2

A couple of other hints:

• CTRL-v is a standard binding and pretty heavily used (scroll-up). You'd be better off finding something that's not otherwise used. Canonically, those would be added to the CTRL-c keymap.
• Don't get in the habit of treating parens as if they were C braces. It's better (more customary) LISP style for the rest of us who might read your code to just close all the parens at the end.

Answer 3

Actually, defvar doesn't do what you think it does either: it only changes the value IF there was no value before. Here's a chunk that does what you're looking for, using the CTRL-u argument:

(defun set-my-var (value)
  "Revised version by Charlie Martin"
  (interactive "p")
  (setq my-var value))

and here's an example, code from the *scratch* buffer

(defun set-my-var (value)
  "Revised version by Charlie Martin"
  (interactive "p")
  (setq my-var value)) ; do ^J to evaluate the defun form, showing return in the buffer.
set-my-var

(global-set-key "\C-v" 'set-my-var)
set-my-var
 ;Another ^J

;;; Now, what's the value after ^U^V?
my-var
4
;;; Now ^U 8 ^V
my-var
8
;;; now ^U^U^V
my-var
16