[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Why shouldn't we have a #if .... #else .... #endif construct in Emac
From: |
Philip Kaludercic |
Subject: |
Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp? |
Date: |
Tue, 29 Aug 2023 12:54:17 +0000 |
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 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---
> .. I propose adding it to subr.el, just before (defmacro when ....).
>
> What do people think about this?
- Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp?, (continued)
Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp?, Emanuel Berg, 2023/08/28
Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp?,
Philip Kaludercic <=
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