this post was submitted on 13 Nov 2023
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I'm wondering if there's any way to make a variable automatically save its old value, so that I can, e.g., write a minor mode that will automatically restore the old values of variables when it's deactivated (without having to declare a bunch of -old-value variables).

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[–] 7890yuiop@alien.top 2 points 1 year ago

You don't have to declare a new variable each time you want to remember an old value. You could, e.g., put a custom symbol property on the variable symbol with the old value (or list of old values) you wanted to store. Or maintain a single variable mapping all your variable symbols to their old values.

[–] Qudit314159@alien.top 1 points 1 year ago

The function add-variable-watcher registers a callback that is invoked whenever the variable's value changes. You could use it to save old values in a list or other data structure.

[–] pwnedary@alien.top 1 points 1 year ago

There is buffer-local-set-state at least, not sure if it is any good.

[–] arthurno1@alien.top 1 points 11 months ago

There is no built-in automatic way; you will have to backup your old values any way you want; create a bunch of new symbols, put them into a hash table, a list, or whichever else strategy you prefer.

[–] SnooPets20@alien.top 1 points 11 months ago

I think themes could be used for this.

[–] fuzzbomb23@alien.top 1 points 11 months ago

Have a look at the approach taken by the logos.el package. There's a section in the manual called Leverage logos-focus-mode-hook.

The idea there is that when you enable the logos-focus-mode, you'll often want to change some other minor modes and variables too. But when you disable logos-focus-mode, you'll want those other minor modes to revert to their previous value. The Logos package provides a few convenience functions to remember the previous values, and they are automatically restored when logos-focus-mode is turned off.

Perhaps the code for that could be made generic, instead of being coupled to the logos-focus-mode?

[–] JDRiverRun@alien.top 1 points 1 year ago

A variable watcher maybe.

    (add-variable-watcher
     'myvar
     (lambda (sym val op buf)
       (when (eq op 'set)
         (push val (get sym 'prior-values)))))