Re: How to define a (derived) minor mode inaccessible to the user

[Marcin Borkowski]

On 2021-01-22, at 15:19, Stefan Monnier <monnier@iro.umontreal.ca> wrote:

>> I'd like to define a special-purpose major mode, derived from
>> `special-mode', in such a way that the user cannot set it using M-x.
>
> Can you explain why?

Because I woulnd't like the user to accidentally turn it on?  (It might
be a bit of OCD on my part, too.)  Also, because I didn't know how to do
it and the very fact bugged me a bit;-).  Also, see below.

>> (It is to be called via another command.)
>
> That doesn't explain why it would be harmful to also expose the actual
> major mode command.

Maybe not harmful, but here's another argument: assume that I have this
new mode, let's call it "blue-mode", with all its commands named
"blue-...", and the entry point being the command "blue-display".  Now,
when the user says M-x blue- TAB, "blue-mode" will show up and pollute
the list (meaning: it will occupy some room on the list while being
useless, since blue-display is what user really wants to start using
blue-mode).

> E.g. You can do `M-x comint-mode` even though it likely won't do you
> much good since `comint-mode` needs to be used together with a process
> and is hence normally enabled by the command that launches the process.
>
>> It seems that `define-derived-mode' always sets the mode as
>> `interactive'.
>
> Indeed.
>
>> M-: (symbol-plist 'my--mode)
>> expecting an `interactive-form' entry on the plist
>
> The `interactive-form` property is virtually never used.  Instead, the
> interactive form is traditionally stored within the function.

So how does `execute-extended-command' know what to list, then?
A cursory look told me that it uses `read-extended-command', which uses
`commandp'.  So, how does `commandp' know?  (I'm not well versed in C,
but do I guess correctly that it checks the `interactive-form' property,
and if nil, it (somehow) looks for `(interactive ...)' in its
definition?)

> If you *really* care about hiding the major mode from the user
> I suggest you use an "obscure" or "scary" name.

Well, the convention is to use two dashes, but how can the end-user (not
knowing Elisp and the conventions) know that?

>> What's going on?  Is it possible to define a (derived) major mode so
>> that it can't be called interactively?
>
> Of course it is:
>
>     (define-derived-mode my-mode "My" "My doc.")
>     (defconst my-mode (symbol-function 'my-mode))
>     (fmakunbound 'my-mode)
>
> and then you can enable the mode with `(funcall my-mode)`.

So, basically you move the function definition from the function cell to
the value cell of `my-mode', right?  Clever!  Is it actually used
anywhere in Emacs?

Thanks,

--
Marcin Borkowski
http://mbork.pl