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Re: Should mode commands be idempotent?

From: Stefan Monnier
Subject: Re: Should mode commands be idempotent?
Date: Wed, 20 Sep 2017 18:30:47 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.0.50 (gnu/linux)

> Also, is there a motivation for introducing this new requirement, seeing as
> how we've never had such a restriction in the past?

In the case of minor modes, I think a good implementation of a minor
mode should be idempotent, because it can easily happen that a minor
mode is enabled twice via different hooks, e.g.:

    (add-hook 'text-mode-hook #'visual-line-mode)
    (add-hook 'xml-mode-hook  #'visual-line-mode)

If you know that xml-mode runs text-mode-hook, you can drop the second
line, but it's good if the second line is just redundant rather
than harmful (after all, some people prefer xml-mode not to run

For major modes, I don't know what is the intention exactly, because I'm
not sure exactly what kind of non-idempotence there is out there.
I think for major modes a more significant issue is to make sure that

    (with-temp-buffer (insert text) (funcall mode))

doesn't do anything undesirable (and currently, we're pretty far from
having such a guarantee, although we do have code like the above at
various places).


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