Re: [PATCH] Proposal to change cursor appearance to indicate region activation

[Kelly Dean <kelly@prtime.org>]

Stefan Monnier wrote:
> You still have a global minor mode (and its
>> variable), and a separately-named buffer-local variable, so it's no
>> simpler.
>
> It's a lot simpler because the buffer-local var is internal.

What do you mean by ‟internal”? Just the double-dash naming convention? If by ‟simpler” you mean that no ‟external” (no double-dash, and intended to be user-level) var has to be used buffer-locally, then your solution isn't simpler; you still need (setq-local dynamic-cursor-mode nil). See below.

>> If (global) dynamic-cursor-mode is enabled, your code provides no way
>> to prevent it from operating in a buffer in which cursor-type happens
>> to be t.
>
> Of course it does:
>
>    (add-hook 'foo-mode-hook (lambda () (setq-local dynamic-cursor-mode nil)))

I didn't know it was appropriate for any code other than the mode itself to set a mode's variable. I thought it was supposed to be treated as read-only by all other code.

>> (setq-default cursor-type 'bar)
>> A few days later in the same Emacs session...
>> (dynamic-cursor-mode)
>
> No, because dynamic-cursor-mode can do (setq-default cursor-type t).
> That's one of the advantages of a minor mode over a plain defcustom.

You mean do that when dynamic-cursor-mode turns on? In that case, what if I want to enable dynamic-cursor-mode buffer-locally, rather than globally?

If I were to turn it on globally by doing (dynamic-cursor-mode), then it would (once, at the time of turn-on) set the cursor type in all buffers that are using the global value of cursor-type (i.e. that have no buffer-local value for it), which is the wrong thing to do, since I was only trying to enable dynamic-cursor-mode buffer-locally.

If I were to instead do (setq-local dynamic-cursor-mode t), then would I lose the supposed advantage of a minor mode over a plain defcustom, and the original failure mode I described would therefore remain.

> The difference between a boolean defcustom and a global minor-mode is
> pretty much that the global minor mode offers the "set triggers".

But only in one direction: dynamic-cursor-mode can set cursor-type when the former is turned on (though this doesn't do what's needed, as described above), but setting cursor-type can't turn off dynamic-cursor-mode (which is what's actually needed to enable defaulting dynamic-cursor-mode to t without causing conflict).

It still seems my original patch, except with the ⌜-mode⌝ suffix removed and the default changed to nil, is the cleanest way to implement dynamic-cursor. Even if you want it to formally be a minor mode (even though it has no advantage in this case), the cleanest implementation is with the simple additions to de/activate-mark in my patches (all three patches have these same additions), and nothing in the minor mode itself except the variable, which I'll set buffer-locally when necessary since you say that's appropriate.