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bug#35230: char-displayable-p return code is conflictingly documented
From: |
Robert Pluim |
Subject: |
bug#35230: char-displayable-p return code is conflictingly documented |
Date: |
Sun, 14 Apr 2019 11:51:36 +0200 |
>>>>> On Fri, 12 Apr 2019 15:51:48 +0300, Eli Zaretskii <eliz@gnu.org> said:
>> From: Robert Pluim <rpluim@gmail.com> Date: Thu, 11 Apr 2019
>> 16:49:29 +0200
>>
>> (This comes as a result of the stackexchange question at
>> https://emacs.stackexchange.com/questions/48810)
>>
>> char-displayable-p docstring says
>>
>> Return non-nil if we should be able to display CHAR.
>>
>> The elisp manual says something subtly different:
>>
>> This function returns ‘t’ if Emacs ought to be able to display
>> CHAR. More precisely, if the selected frame’s fontset has a
>> font to display the character set that CHAR belongs to.
>>
>> The function itself is more in line with the docstring:
Eli> Yes, the manual is wrong and should be fixed. Non-nil is
Eli> exactly right, and callers should not depend on any finer
Eli> definition of the return value, as it could be many different
Eli> non-nil objects.
That I can do easily enough in emacs-26
>> (char-displayable-p #xE01EF) => unicode
>>
>> since I donʼt have a font with a glyph for that character, so
>> it ends up displayed as a box with the unicode code point
>> inside it. The code that results in 'unicode has the comment
>>
>> ;; On a text terminal without glyph codes, CHAR is displayable
>> ;; if the coding system for the terminal can encode it.
>>
>> but Iʼm very much on a graphical terminal here, not a text
>> terminal.
Eli> This is a (known) deficiency in char-displayable-p, but one
Eli> which is not easy to fix: no one says that every call to this
Eli> function asks about the selected frame, so we cannot
Eli> unconditionally disable the TTY branch when on GUI frames.
Eli> Perhaps an optional argument could be added for that purpose,
Eli> which is the frame for which to make the test. At least
Eli> some, if not most, of the calls will still need to omit that
Eli> argument, though, because we currently need to know that up
Eli> front to set up the quote-style, for example.
Eli> Perhaps mentioning this caveat in the manual would be good.
Iʼve made an attempt below. Do we want a cross reference to 'Glyphless
Chars' as well?
diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index b07999432c..fac883ae16 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -3507,12 +3507,16 @@ Fontsets
@end defun
@defun char-displayable-p char
-This function returns @code{t} if Emacs ought to be able to display
-@var{char}. More precisely, if the selected frame's fontset has a
-font to display the character set that @var{char} belongs to.
+This function returns non-@code{nil} if Emacs ought to be able to
+display @var{char}. More precisely, if the selected frame's fontset
+has a font to display the character set that @var{char} belongs to.
Fontsets can specify a font on a per-character basis; when the fontset
does that, this function's value may not be accurate.
+
+This function may return non-@code{nil} even when there is no font
+available, since it also checks if the coding system for the text
+terminal can encode the character.
@end defun
@node Low-Level Font