Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 2 Sep 2023 15:06:46 +0000
> Cc: emacs-devel@gnu.org,
>  Mattias Engdegård <mattiase@acm.org>
> From: Alan Mackenzie <acm@muc.de>
> 
> +@node Conditional Compilation
> +@section Conditional Compilation
> +
> +  There will be times when you want certain code to be compiled only
> +when a certain condition holds.  This is particularly the case when
> +maintaining Emacs packages; to keep the package compatible with older
> +versions of Emacs you may need to use a function or variable which has
> +become obsolete in the current version of Emacs.
> +
> +  You could just use a conditional form to select the old or new form
> +at run time, but this tends to output annoying warning messages about
> +the obsolete function/variable.  For such situations, the macro
> +@code{static-if} comes in handy.  It is inspired by the conditional
> +compilation directives like @code{#if} in C like languages, and is
> +patterned after the special form @code{if} (@pxref{Conditionals}).
> +
> +  To use this facility for an older version of Emacs, copy the source
> +for @code{static-if} from the Emacs source file @file{lisp/subr.el}
> +into your package.

Thanks, but I think the references to #if make the documentation less
helpful than it could be.  This manual is for Lisp programmers, and
those are not necessarily familiar with C and its preprocessor
directives.  So I think it would be better if you removed the
references to cpp.  If you think removing that would make the
documentation less self-explanatory, I suggest to add explanations
that are based on Lisp and on typical situations while writing Lisp
programs, not on cpp.

> --- a/etc/NEWS
> +++ b/etc/NEWS
> @@ -855,6 +855,10 @@ Use 'define-minor-mode' and 'define-globalized-minor-mode' instead.
>  See the "(elisp) Porting Old Advice" node for help converting them
>  to use 'advice-add' or 'define-advice' instead.
>  
> ++++
> +** There is now conditional compilation, based on the C language's #if.
> +To use this, see the new macro 'static-if'.

Same here.  Here, it is actually worse: "based on C language's #if"
could be misinterpreted as meaning the implementation is based on #if
in some way.  I would suggest the following text in NEWS:

 ** New macro 'static-if' for conditional byte-compilation of code.
 This macro hides a form from the byte-compiler based on a
 compile-time condition.  This is handy for avoiding byte-compilation
 warnings about code that will never actually run under some
 conditions.

Thanks.