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

[LdBeth <andpuke@foxmail.com>]

>>>>> In <ZOz3lR_4Al88SEJ_@ACM> 
>>>>>	Alan Mackenzie <acm@muc.de> wrote:
acm> Hello, Emacs.

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

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

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

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

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

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

Have a look at `static.el' from APEL

https://github.com/wanderlust/apel

It provides `static-if' and other useful macros.

(defmacro static-if (cond then &rest else)
  "Like `if', but evaluate COND at compile time."
  (if (eval cond)
      then
    `(progn ,@ else)))