Checking in Lisp if a given symbol has its original value

Checking in Lisp if a given symbol has its original value

[Bruno Félix Rezende Ribeiro]

What’s the canonical way of checking if a given symbol has its
original value, as reported by ‘describe-variable’?

I’ve come up with:

#+BEGIN_SRC elisp
  (defun standard-value-p (symbol)
    "Return non-nil if SYMBOL has its original value."
      (equal (eval (car (get symbol 'standard-value)))
             symbol))
#+END_SRC

-- 
Bruno Félix Rezende Ribeiro (oitofelix) [0x28D618AF]
<http://oitofelix.freeshell.org/>

RE: Checking in Lisp if a given symbol has its original value

[Drew Adams]

> What’s the canonical way of checking if a given symbol has its
> original value, as reported by ‘describe-variable’?
> I’ve come up with:
>   (defun standard-value-p (symbol)
>     "Return non-nil if SYMBOL has its original value."
>       (equal (eval (car (get symbol 'standard-value)))
>              symbol))

That's only for user options, aka customizable variables,
aka variables defined with defcustom.  See the Elisp
manual, node Standard Properties.

https://www.gnu.org/software/emacs/manual/html_node/elisp/Standard-Properties.html

There's no way to check other variables, in general.

For any given variable whose creation you control or
monitor, you can of course record its initial value
in some way.

You can also use function `add-variable-watcher' to
monitor changes to a variable's value.  But again, you
would want to do this from the outset, when it is
created.  See the Elisp manual, node Watching Variables.

https://www.gnu.org/software/emacs/manual/html_node/elisp/Watching-Variables.html