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Re: Why shouldn't we have a #if .... #else .... #endif construct in Emac
From: |
Alan Mackenzie |
Subject: |
Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp? |
Date: |
Tue, 29 Aug 2023 13:23:59 +0000 |
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).
Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp?, Philip Kaludercic, 2023/08/29
- Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp?,
Alan Mackenzie <=
Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp?, Stefan Kangas, 2023/08/29
Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp?, LdBeth, 2023/08/29