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

[Alan Mackenzie]

Hello, Emacs.

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 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 .....))

..  This is not actually all that difficult to write.  My first attempt
uses a compiler-macro, and looks like this:

    (defun hash-if (condition if-part &rest else-part)
      "A compiler macro analogous to C's #if.
    CONDITION is evaluated at compile time.  If it is non-nil,
    IF-PART gets compiled.  Otherwise ELSE-PART (enclosed in a
    `progn') gets compiled."
      (declare (indent 2))
      (error "hash-if has been called directly"))

    (put 'hash-if 'compiler-macro
         (lambda (form condition if-part &rest else-part)
           (if (eval condition lexical-binding)
               if-part
             (cons 'progn else-part))))

..  I propose adding it to subr.el, just before (defmacro when ....).

What do people think about this?

-- 
Alan Mackenzie (Nuremberg, Germany).