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

[Alan Mackenzie]

Hello, Ulrich.

On Mon, Aug 28, 2023 at 23:01:53 +0200, Ulrich Mueller wrote:
> >>>>> On Mon, 28 Aug 2023, Alan Mackenzie wrote:

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

> >> 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).

> Sorry, I should have been clearer. The full construct would look like
> this:

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

Ah!  So the eval-when-compile gets compiled to a constant nil or t, and
the byte compiler optimises the test of this out, either compiling or
not compiling the (defadvice .....).  Thanks!  I hadn't been aware of
that little detail.

> So only the test would be evaluated at compile time, the defadvice
> itself would be compiled normally (or not, if the test fails).

However, when evaluating the containing defun/defmacro rather than byte
compiling it, the irritating warning message will appear with the e-w-c
strategem, but not with hash-if.  ;-)

-- 
Alan Mackenzie (Nuremberg, Germany).