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From: Alan Mackenzie <acm@muc.de>
To: Philip Kaludercic <philipk@posteo.net>
Cc: emacs-devel@gnu.org
Subject: Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp?
Date: Tue, 29 Aug 2023 13:23:59 +0000	[thread overview]
Message-ID: <ZO3xb8wr9rFsLlO_@ACM> (raw)
In-Reply-To: <877cpeko06.fsf@posteo.net>

Hello, Philip.

On Tue, Aug 29, 2023 at 12:54:17 +0000, Philip Kaludercic wrote:
> Alan Mackenzie <acm@muc.de> writes:

> > 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.

> In this specific case, would it be possible to use the nadvice
> compatibility package on GNU ELPA?

I suspect it would be, yes.

> > 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))))

> Would something like work as well:

> --8<---------------cut here---------------start------------->8---
> (defmacro cif (test then &rest else)
>   "Evaluate TEST during macro-expansion and return THEN or ELSE."
>   (if (eval test t) then else))
> --8<---------------cut here---------------end--------------->8---

Hah!  Trust me to build something twisted and complicated when there's a
simple solution which will do just as well!

I think the `else' needs a 'progn, though, like this:

    (if (eval test t) then (cons 'progn else))

, and the eval form probably wants to go into a condition-case for
Emacsen lacking the second parameter.  But I appreciate you put the code
together quickly, rather than working out every last detail.  Thanks!

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

> > What do people think about this?

Yes, what do people think about the idea?

-- 
Alan Mackenzie (Nuremberg, Germany).



  reply	other threads:[~2023-08-29 13:23 UTC|newest]

Thread overview: 66+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-08-28 19:37 Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp? Alan Mackenzie
2023-08-28 19:47 ` Ulrich Mueller
2023-08-28 20:06   ` Alan Mackenzie
2023-08-28 21:01     ` Ulrich Mueller
2023-08-28 21:46       ` Alan Mackenzie
2023-08-31  2:07         ` Richard Stallman
2023-08-31  7:50           ` Alan Mackenzie
2023-09-04  1:34             ` Richard Stallman
2023-09-04 10:50               ` Alan Mackenzie
2023-09-04 11:02                 ` tomas
2023-09-04 15:19                   ` Emanuel Berg
2023-09-04 18:57                     ` tomas
2023-09-06  0:58                   ` Richard Stallman
2023-09-06  0:58                 ` Richard Stallman
2023-09-06  7:28                   ` Andreas Schwab
2023-09-06  9:31                     ` Alan Mackenzie
2023-09-06  9:56                   ` Alan Mackenzie
2023-09-09  0:39                     ` Richard Stallman
2023-09-09 10:32                       ` Ihor Radchenko
2023-09-10  0:22                         ` Richard Stallman
2023-09-10  8:36                           ` Ihor Radchenko
2023-09-13 23:53                             ` Richard Stallman
2023-09-20 12:59                               ` Ihor Radchenko
2023-09-05  0:30           ` Why have a #if .... #else .... #endif construct in Emacs Lisp, when we could make the existing code DTRT unchanged? Richard Stallman
2023-09-05  0:41             ` Emanuel Berg
2023-09-08 17:54               ` Emanuel Berg
2023-09-05  4:37             ` tomas
2023-09-05  5:53               ` Ihor Radchenko
2023-09-05  6:28                 ` tomas
2023-09-05 11:06             ` Adam Porter
2023-09-05 11:26             ` Lynn Winebarger
2023-09-05 14:11             ` Alan Mackenzie
2023-09-08  1:01               ` Richard Stallman
2023-09-08  2:45                 ` Po Lu
2023-09-10  0:24                   ` Richard Stallman
2023-09-05  8:14         ` Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp? Ulrich Mueller
2023-08-28 19:53 ` Emanuel Berg
2023-08-29  9:19   ` Alan Mackenzie
2023-08-29 10:36   ` João Távora
2023-08-29 11:09     ` Ihor Radchenko
2023-08-29 11:20       ` João Távora
2023-08-30 20:48         ` Sean Whitton
2023-08-30 20:59           ` João Távora
2023-09-02 23:12     ` Stefan Monnier via Emacs development discussions.
2023-09-03  0:18       ` Emanuel Berg
2023-09-03 12:27       ` João Távora
2023-08-29 12:54 ` Philip Kaludercic
2023-08-29 13:23   ` Alan Mackenzie [this message]
2023-09-02 23:09   ` Stefan Monnier via Emacs development discussions.
2023-08-29 16:28 ` LdBeth
2023-08-29 20:09 ` Stefan Kangas
2023-08-30 10:31   ` Alan Mackenzie
2023-08-30 17:36     ` Stefan Kangas
2023-08-30 18:03       ` Alan Mackenzie
2023-08-30 18:17         ` Stefan Kangas
2023-09-02 15:06           ` Alan Mackenzie
2023-09-02 15:17             ` Eli Zaretskii
2023-09-02 19:43               ` Alan Mackenzie
2023-09-03  4:42                 ` Eli Zaretskii
2023-09-03 10:48                   ` Alan Mackenzie
2023-09-03 11:02                     ` Eli Zaretskii
2023-09-03 13:24                       ` Alan Mackenzie
2023-09-02 19:20             ` Philip Kaludercic
2023-09-02 19:37               ` Stefan Kangas
2023-09-02 19:58               ` Alan Mackenzie
2023-09-04 11:12                 ` Lynn Winebarger

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