Re: Macros considered harmful

["Basil L. Contovounesios" <contovob@tcd.ie>]

Stefan Monnier [2022-09-06 00:44 -0400] wrote:

> Clément Pit-Claudel [2022-09-05 19:53:26] wrote:
>> On 8/20/22 07:14, Stefan Monnier wrote:
>>> FWIW, I think specifying the highlighting rules with something akin to:
>>>      (defvar <foo> '<rules>)
>>> is a mistake.  It should go through some kind of macro, such as (maybe):
>>>      (defvar <foo> (tree-sitter-rules <rules>))
>>> which can thus do any preprocessing we may want, such as pre-compiling
>>> queries.  It also helps evolve the syntax since we can more easily warn
>>> about obsolete uses, etc...
>>> I've had a "rewrite font-lock.el so the rules go through a macro" in my
>>> todo list for ages.
>>
>> We do things this way in Flycheck, but we've been bitten a few times by the
>> way this pattern interacts with `with-eval-after-load`, so I would be
>> careful about adopting this pattern in tree-sitter (unless we expect it to
>> be preloaded).
>
> Very good point.  It would introduce a strong compile-time dependency on
> the tree-sitter package, which could be a serious annoyance for random
> `foo-mode` packages which want to keep working in the absence of
> tree-sitter.
>
> Hmm...
>
> [ I guess we'd have to make that macro lightweight and independent from
>   tree-sitter itself and put it into a separate package distributed via
>   GNU ELPA, so packages can feel free to depend on it.
>   Hmm...  Another problem.  ]
>
>> In fact, I think your suggestion back then was to *not* use a macro?
>
> Indeed,
>
> [ Lua eshews macros altogether for those kinds of reasons, AFAIU.  ]
>
> Admittedly, another way around these kinds of problems is to teach the
> compiler how to deal with an unknown macro.  I.e. something like
> (declare-macro my-foo ...) so that if the compiler see (my-foo ...) but
> `my-foo` can't be macroexpanded (because the macro is not yet defined),
> it doesn't incorrectly compile it into a function call, but instead
> residualizes it into something like a call to `eval`.  Making it
> interact correctly with lexical scoping could be tricky (I guess the
> simplest solution would be to residualize the whole toplevel expression
> in which the macro call was found).

Another downside of macros not directly addressed by this approach is
that packages using them may have the outrageous desire to both support
older Emacsen and build cleanly, at the same time!  Recall, for example,
this unresolved shortdoc thread:
https://lists.gnu.org/r/emacs-devel/2021-09/msg01719.html

I suppose in the general case this can be alleviated only through
'compat'-like ELPA dependencies (or by expecting each package to write
its own compatibility shims, of course).

-- 
Basil