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

[Alan Mackenzie]

Hello, Ulrich.

On Mon, Aug 28, 2023 at 21:47:57 +0200, Ulrich Mueller wrote:
> >>>>> On Mon, 28 Aug 2023, Alan Mackenzie wrote:

> > In C, we have the very useful conditional compilation directives
> > introduced by #if or #ifdef, etc., which end at #end.

> > In Emacs Lisp we have no such construct.  This is a Bad Thing.

> > More and more, especially recently, irritating warning messages are
> > occurring for, for example, obsolete variables and functions inside
> > conditionals which ensure they aren't used.  For example:

> >     (when (< emacs-major-version 24)
> >       (defadvice .....))

> > produces the warning about defadvice being obsolete.  (I haven't actually
> > tested this example).  What we really want here is for the defadvice only
> > to be _compiled_ when (< emacs-major-version 24), rather than compiled
> > unconditionally and not run.

> I believe (eval-when-compile (< emacs-major-version 24)) would work?

I've twisted my head into knots in the past trying to work out how to
use eval-when/and-compile for this purpose.  The point is, I don't want
the defadvice to be evaluated at compile time; I just want it to be
compiled (or not).

> > I propose a new function, hash-if, which would do what we want.  The
> > above example could then be written something like:

> >     (hash-if (< emacs-major-version 24)
> >         (defadvice .....)
> >       (advice-add .....))

> But the old Emacs version wouldn't have the function, so it couldn't be
> used like this. What am I missing?

That's a very good point.  I would propose that we suggest to package
maintainers that they include the entire source of hash-if inside a
suitable conditional in their packages.  It's only 13 lines of code,
including a doc string.

It would appear that the compiler-macro mechanism existed at the latest
in Emacs 21.4.  I don't know how well it worked in these earlier
versions.

-- 
Alan Mackenzie (Nuremberg, Germany).