Re: cc-mode: Make all parameters introduced in Emacs 26 optional

[Alan Mackenzie <acm@muc.de>]

Hello, Jostein.

On Mon, Jan 22, 2018 at 09:09:21 +0100, Jostein Kjønigsen wrote:
> Hey everyone.

> While cc-mode seems to be a foundation for lots of the major-modes
> shipped with Emacs, it's also used by third-party packages.
> For major-modes shipped with Emacs, changes to the core cc-mode
> functions is not that big of a problem, since they can be changed in
> tandem with the changes to cc-mode itself.

> For third party modules (like csharp-mode, which I maintain), changes to
> cc-mode core-functions are more problematic due to Emacs lacking
> reliable introspection capabilities.

There's a convention in CC Mode that functions, macros, and variables
with a doc string are regarded as part of an "API" to derived modes, but
objects with merely a "doc comment" are regarded as internal to CC Mode,
and _much_ less secure against random changes.

> As an example in the Emacs 26 branch c-font-lock-declarators is now
> declared like this:
> (defun c-font-lock-declarators (limit list types not-top &optional template-
> class)  ...)
> While in Emacs 25.3 and earlier it's declared like this:

> (defun c-font-lock-declarators (limit list types)
>   ...)

> Basically the number of mandatory parameters has been bumped from 3 to
> 4, with another optional parameter added.

c-font-lock-declarators is one of these functions intended to be
"internal".  If a derived mode like csharp-mode is using it directly,
one of the following is true:
(i) There's a need for functionality which is currently lacking in CC
Mode.
(ii) The maintainer of the derived mode is unaware of existing CC Mode
functionality which would satisfy his need.
(iii) ???

> These kinds of changes makes it harder for third party modules to
> maintain compatibility across Emacs-versions.

Why is csharp-mode using c-font-lock-declarators at all?  Could it be
you're wanting to do something which currently can't be done with the
"API" functions/macros/variables?  If so, it might well be better to
amend CC Mode to provide this functionality.

> Wouldn't it be better to make *all *the new parameters optional and thus
> maintain compatibility? Are there any good reasons not to do so?

Well, to work properly, the caller of c-font-lock-declarators will need
to determine the `not-top' argument rather than just relying on a
randomish default.  The meaning of the function has changed.  `not-top'
doesn't seem suitable for being &optional.

Again, why is csharp-mode using this function?  Are there any other
"internal" functions/macros/variables it is using?

> --
> Regards
> Jostein Kjønigsen

> jostein@kjonigsen.net 🍵 jostein@gmail.com
> https://jostein.kjonigsen.net

-- 
Alan Mackenzie (Nuremberg, Germany).