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I want to do something VERY simple. I want to create a minor mode, which when enabled sets a buffer-local variable (say left-margin-width, for instance) with a specific value.

I can do this:

(define-minor-mode margin-mode
  (setq left-margin-width 3))

Now, I want the variable to take back it's global value when margin-mode is disabled. To me this seems like a very natural use-case, but apparently I can't find a simple and easy way to do his yet! Once enabled, the variable retains it's value until the buffer is closed. Which is very undesirable.

Help!

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Minor modes are toggles. Defining a minor mode also creates a variabel with the same name. This variable is toggled when enabling/disabling the minor mode.
So following code works:

(define-minor-mode margin-mode
  (if (not margin-mode)
      (setq left-margin-width 3)
    (setq left-margin-width (default-value 'left-margin-width))))

Elisp is a Lisp-n type language. This means there exist separate namespaces for function names and variable names. You can have a variable and a function with the same name. This is used here.

(instead of resetting the local variable with (setq left-margin-width (default-value 'left-margin-width)) you could also use (kill-local-variable 'left-margin-width).

To get more info on how to define minor modes, read the documentation. you can access the documentation with C-h f define-minor-mode <RET>

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  • Thanks. I kinda knew this. I was wondering if it was workable without using an explicit condition check.
    – sixter
    Dec 15 '20 at 9:53
  • Also this wouldn't work with the margins cause apparently setting the variable isn't enough.
    – sixter
    Dec 15 '20 at 10:07
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    C-h v left-margin-width tells you that you have to call set-window-buffer to make changes to this variable take effect. But this was not what you asked about, so I expected, you already know this.
    – jue
    Dec 15 '20 at 21:14
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    Have a look at public source code minor modes, most use a if or when or unless and the minor-mode variable to act according.
    – jue
    Dec 15 '20 at 21:17
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See C-hf kill-local-variable for reverting a buffer-local variable to the global state.

If you just set the buffer-local value to the variable's current default value, then it's still a buffer-local value (i.e. it won't reflect future changes to the default).

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