Re: Handling extensions of programming languages

[haj@posteo.de (Harald Jörg)]

Clément Pit-Claudel <cpitclaudel@gmail.com> writes:

> On 3/20/21 7:40 PM, Harald Jörg wrote:
>> Well, I have my doubts that Perl is a good candidate for SMIE, and
>> trying to use SMIE in CPerl mode would be a major rewrite anyway.  I
>> guess the Emacs Losp basics (font-lock-add-keyword, hooks) will have to
>> do the job.
>
> I'm pretty sure SMIE would work wonderfully for Perl, but I'm also not
> sure how it relates to font-lock-add-keywords and hooks, so maybe
> we're not thinking of the same thing?

I admit that I don't know much about SMIE, so maybe I'm wrong here.
Most of Perl is pretty similar to C or Java, but there are cases where
Perl's syntax just can't be parsed statically.

About the relation to font-lock-add-keywords - let me show an example.
"Traditional" Perl has no keywords for object oriented programming, but
there are dozens of extensions which add them.  For example, with
Object::Pad you can write (I apologize for the nonsensical example):

  class Coffee::Machine extends Lawn::Mower
  {
     has $grinder :reader :writer(replace_grinder)
     method grind { ...; }
  } 

If I want to support that with CPerl mode, I need to:

 - highlight class, extends, method and some more I haven't included in
   that example as keywords.  That's where font-lock-add-keywords comes
   into play.  Also, "Dishwasher" and "clean_up" should be highlighted
   like package and sub names.

 - add "Dishwasher" and "clean_up" to the imenu index.
 
 - make sure that indentation recognizes that the closing braces end a
   statement after "class" and "method".  Perl syntax has various cases
   where it doesn't.  I guess this is the part where SMIE would help.

For the latter two tasks, I need to "hook" the logic somehow into
CPerl's implementations of `imenu-create-index-function' and the various
indentation functions.  The current indentation code in CPerl mode
is... a bit messy, and some old bugs call for attention anyway.

If, however, that same class would be defined using the Dios extension,
it would look like this:

  class Coffee::Machine is Lawn::Mower
  {
     has $.grinder is rw
     method grind { ... }
  } 

...and also offer the keywords "func" and "submethod" for stuff that
should go into imenu, and "lex" for declaring variables.  There's a
dozen or more other extensions providing OO frameworks for Perl.

So, if the Emacs support for an extension could be done by a separate
.el file, these could be developed within GNU Emacs, in GNU ELPA, but
also contributed via NonGNU elpa, MELPA or GitHub.

I am aware that probably as soon as such an extension mechanism is
available, _someone_ will publish a Perl extension which can't be
covered :)
-- 
Cheers,
haj