Re: Use of `window-system' as a predicate is deprecated. Why?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 9 Jul 2022 12:22:41 +0000
> From: Alan Mackenzie <acm@muc.de>
> 
>    Do _not_ use `window-system' and `initial-window-system' as predicates
>    or boolean flag variables, if you want to write code that works
>    differently on text terminals and graphic displays.  That is because
>    `window-system' is not a good indicator of Emacs capabilities on a
>    given display type.  Instead, use `display-graphic-p' or any of the
>    other `display-*-p' predicates described in Display Feature Testing.
> 
> ..  I am amending minibuf.c and wish to know only whether the current
> terminal->focus_frame_hook is validly set.
> 
> The manual entry seems to be to be unnecessarily patronising.  It
> insinuates that the only possible "capabilities" a user could be
> interested in are graphic capabilities.

The ELisp reference manual is written for Lisp programmers, not for
people who work on the C sources.

To check whether terminal->focus_frame_hook is set, test it for being
non-NULL.  More generally, if you want to know whether a frame is on a
window-system, use the FRAME_WINDOW_P macro.