Re: Macros considered harmful

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

Stefan Monnier [2022-09-06 12:01 -0400] wrote:

>>> 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).
>
> A low-tech way to do it is to let the programmer do it by hand, e.g.:
>
>     (defmacro smalltalk--when-fboundp (sym exp)
>       (declare (indent 1) (debug (symbolp form)))
>       (if (fboundp sym)
>           exp
>         ;; `sym' is not defined during compilation, but keep the test at run-time,
>         ;; in case we use the compiled file on a newer Emacs.
>         `(eval '(if (fboundp ',sym) ,exp))))
>
> It can still break if you use in `exp` lexically scoped vars declared in
> the context, but that's considered a "programmer's problem" :-(
>
>> 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
>
> Would this kind of `<foo>--when-fboundp` help there?

Yes, thanks, it allows the macro to be used within with-eval-after-load
across all Emacs versions.  Sadly eval-after-load is still needed to
account for Emacs 28 not autoloading the shortdoc entrypoint macro, but
that's down to the package's API/hooks, not the use of macros.

-- 
Basil