bug#57087: 29.0.50; (face-at-point nil t) does not return all faces when hl-line-mode is active

bug#57087: 29.0.50; (face-at-point nil t) does not return all faces when hl-line-mode is active

[dalanicolai]

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Open some elisp (.el) buffer, place the cursor on some fontified keyword
(e.g. 'defun') and do `M-: (face-at-point nil t)`. Now activate `(M-x)
hl-line-mode` and again do `M-: (face-at-point nil t)`. The list shows
only the `hl-line` face, while the function its docstring says it should
return multiple faces.

When edebugging `face-at-point`, even though `hl-line-mode` is active,
the function returns only the char its face (and not `hl-line`).

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.34,
cairo version 1.17.6)
 of 2022-06-12 built on fedora
Repository revision: c1829b307cffce046bec6fcbdff03dbab9f4b562
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: Fedora Linux 36 (Workstation Edition)

Configured using:
 'configure --with-modules --with-cairo --with-native-compilation
 --with-json'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBSELINUX LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM
XINPUT2 XPM GTK3 ZLIB

Important settings:
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  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: ELisp/d

Minor modes in effect:
  hl-line-mode: t
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  eldoc-mode: t
  show-paren-mode: t
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Load-path shadows:
None found.

Features:
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Memory information:
((conses 16 89094 14677)
 (symbols 48 7738 0)
 (strings 32 21901 915)
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bug#57087: 29.0.50; (face-at-point nil t) does not return all faces when hl-line-mode is active

[Eli Zaretskii]

tags 57087 notabug
thanks

> From: dalanicolai <dalanicolai@gmail.com>
> Date: Tue, 9 Aug 2022 20:55:13 +0200
> 
> Open some elisp (.el) buffer, place the cursor on some fontified keyword
> (e.g. 'defun') and do `M-: (face-at-point nil t)`. Now activate `(M-x)
> hl-line-mode` and again do `M-: (face-at-point nil t)`. The list shows
> only the `hl-line` face, while the function its docstring says it should
> return multiple faces.

This is a misunderstanding of what the doc string means when it says
"faces".  It doesn't mean that you should see more than one face in
the above situation.

This is not a bug, it's just that your expectations from what
face-at-point can do are incorrect.

bug#57087: 29.0.50; (face-at-point nil t) does not return all faces when hl-line-mode is active

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

> This is a misunderstanding of what the doc string means when it says
> "faces".  It doesn't mean that you should see more than one face in
> the above situation.
>
> This is not a bug, it's just that your expectations from what
> face-at-point can do are incorrect.

Then I think this doc string needs clarification, at least:

---
face-at-point is a byte-compiled Lisp function in faces.el.

(face-at-point &optional THING MULTIPLE)

Return the face of the character after point.
If it has more than one face, return the first one.
If THING is non-nil try first to get a face name from the buffer.
IF MULTIPLE is non-nil, return a list of all faces.
Return nil if there is no face.
---

I think it sounds like it would be more useful if it did indeed return
all the faces at point instead of just the face(s) from either the
overlay or the face(s) from the text property.

bug#57087: 29.0.50; (face-at-point nil t) does not return all faces when hl-line-mode is active

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: dalanicolai <dalanicolai@gmail.com>,  57087@debbugs.gnu.org
> Date: Tue, 09 Aug 2022 21:35:25 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > This is a misunderstanding of what the doc string means when it says
> > "faces".  It doesn't mean that you should see more than one face in
> > the above situation.
> >
> > This is not a bug, it's just that your expectations from what
> > face-at-point can do are incorrect.
> 
> Then I think this doc string needs clarification, at least:

Yes, probably.  However, "return the first one" doesn't tell which one
this would be.  Also "character has more than one face" is inaccurate,
we should say "more than one source of face information" or somesuch.

> I think it sounds like it would be more useful if it did indeed return
> all the faces at point instead of just the face(s) from either the
> overlay or the face(s) from the text property.

AFAICT, there's only one user of MULTIPLE, and that is org.el, so we
should ask them what they expect.  There's always a possibility to add
a new function, say faces-at-point.

bug#57087: 29.0.50; (face-at-point nil t) does not return all faces when hl-line-mode is active

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

> Yes, probably.  However, "return the first one" doesn't tell which one
> this would be.  Also "character has more than one face" is inaccurate,
> we should say "more than one source of face information" or somesuch.

Yup.  And `thing' is an unfortunate argument name, since people might
interpret that as the face property should be gotten from that thing (as
with get-text-property and friends with `object').

>> I think it sounds like it would be more useful if it did indeed return
>> all the faces at point instead of just the face(s) from either the
>> overlay or the face(s) from the text property.
>
> AFAICT, there's only one user of MULTIPLE, and that is org.el, so we
> should ask them what they expect.  There's always a possibility to add
> a new function, say faces-at-point.

Yes, I think perhaps adding a new function like that might make more
sense, because `face-at-point' seems like a very DWIM-ish function with
unclear semantics (like preferring the `read-face-name' over `face'
property, etc).

So I've now just explained this in the doc string.  I think the function
is basically fine as is -- it works well as a prompt default function.