Re: Function passing flag

[Corwin Brust <corwin@bru.st>]

On Tue, Mar 8, 2022 at 2:28 PM goncholden <goncholden@protonmail.com> wrote:
>
> Would like to use arg as a switch so I can enable or disable some emacs modes (customarily using
> value of 1 to enable, and -1 to disable).  If no arg is supplied, I would like to enable the modes.
>
> How would I define the parameter outside the function, and then pass it through.  I might require
> a buffer local variable.
>

One typical way of doing this would be to use the "universal
argument", also known as a "prefix argument":

(info "(elisp)Prefix Command Arguments")

Here's an example that would create an interactive command that
enables tool-bars and scroll-bars or, with a prefix argument, disables
them.

(defun my:enable-bars (&optional arg)
  "Enable `tool-bar-mode' and `scroll-bar-mode'.

When ARG is non-nill, disable them."
  (interactive "P")
  (if arg
      (progn (tool-bar-mode -1)
     (scroll-bar-mode -1))
    (tool-bar-mode 1)
    (scroll-bar-mode 1)))

Once you evaluate this you can ensure both tool-bar and scroll bar
modes are active with:
  M-x my:enable-bar RET

Or, to ensure both tool-bar and scroll-bar modes are disabled:
  C-u M-x my:enable-bar RET