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Re: Should mode commands be idempotent?

From: Herring, Davis
Subject: Re: Should mode commands be idempotent?
Date: Tue, 10 Oct 2017 23:15:57 +0000

[Sorry this is so late: I only caught up on this upon seeing the manual patch.]

> The statement I made only applies if the command is called in immediate
> succession. That is, the function is *itself* idempotent; it doesn't guarantee
> that intervening effects are somehow negated.

There is a reasonable candidate condition, at least for minor modes, that is 
broader than this: if a minor mode has state, that state should not be 
reinitialized if the minor mode was already enabled before it is "enabled 
again".  For example, according to this principle compilation-minor-mode should 
not discard its markers if it is called again -- precisely because of other 
changes made between the calls.

For major modes, kill-all-local-variables tends to prevent this sort of 
idempotence.  This is not really a problem: since only one major mode can be 
active at a time, it makes sense to consider a second call to actually 
_replace_ the previous "instance" of the major mode.  There also isn't much 
reason to call major mode functions from places like hooks that might 
unexpectedly re-execute them.


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