Re: [RFC PATCH] Per-window face support

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 16 Jun 2018 08:30:52 -0700
> From: "Daniel Colascione" <dancol@dancol.org>
> Cc: "Daniel Colascione" <dancol@dancol.org>,
>  emacs-devel@gnu.org
> 
> > That's not the Emacs way, IMO.  Being able to use only integers or
> > symbols as filtering parameters sounds too restrictive to me, and I
> > see no reason for such restrictions.
> 
> There's no loss of generality in allowing only eq at face filtering time:
> you can set an integer or symbol to reflect the result of any other
> computation

That's exactly the point: one needs to modify window parameters,
instead of just using what's already there.  So this will restrict
applications that want to modify a window's faces based on existing
window parameters whose values are neither symbols nor integers.  Such
applications will have to add their own parameters to windows.

> Contractually allowing something more powerful than eq here can really
> screw us over later: what happens when we equal-compare large data
> structures?

Same thing that happens when an :eval form in the mode-line-format
causes Emacs to solve partial differential equations in 20 dimensions:
they get what they deserve.  Emacs never prevented legitimate uses of
power features just because those features give us enough rope to hang
ourselves, should we be so stupid.