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

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> 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?

You can use `commandp` to know if it's got an interactive form or not,
and `interactive-form` (the function) to extract it from a function.

> 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?)

Pretty much, yes (with the extra handling for the cases where the
function is not a `(lambda ...)` list but a byte-compiled object, or an
autoloaded function, or a function implemented in C).

>> 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?

That's indeed the convention for functions.  For commands we don't
really have a convention for "commands that should be hidden from `M-x".
We used to prevent completion from revealing obsolete commands.
It's been reverted recently (to my disappointment), but we could add
a similar feature for "commands not to be used via M-x" (could be used
for those commands that only work when bound to a mouse event, for
example).

Maybe `smex` offers something like that.  When I rewrote
`execute-extended-command' into Elisp, I hoped that it would encourage
people to hack on it and add features to it (like this one), but sadly
it doesn't seem to have had much effect of this kind so far.

We could add some property that holds a predicate function which
`execute-extended-command' could use to filter out commands which can't
be used in the current context (e.g. the predicate could check the
major mode, for those commands which only work in a given major 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?

No, and I don't recommend it.
E.g `C-h m` will then fail to show the proper docstring of the major mode.


        Stefan