Re: Activating a function when a variable is set

[Vinicius Jose Latorre]

Davis Herring wrote:
> > Does anybody know if there is a way to activate a function when a
> > variable is set to t and also when the same variable is set to nil?
> >     
>
> As in, react immediately to (setq trapped-variable t)?  No.  With custom
> variables you can react to them being customized by giving them a :set
> attribute.  But AFAIK there's no way to "detect" a change to a Lisp
> variable.
>   


There is a way to react to (setq trapped-var t) by programming a minor 
mode, like:

(defvar trapped-var nil)

(add-to-list 'minor-mode-alist '(trapped-var :eval (trapped-fun)))

(defun trapped-fun ()
  (message "trapped-var is t")
  " TRAP")

When the command (setq trapped-var t) is executed, the modeline will 
show " TRAP" and the minibuffer will display "trapped-var is t".

When the command (setq trapped-var nil) is executed, the modeline will 
not show " TRAP" but trapped-fun will not be invoked.

The question is if there is a way to react also when (setq trapped-var 
nil) is executed.