Re: New feature: displaying ligature characters in the buffer

[Eli Zaretskii <eliz@gnu.org>]

On January 25, 2022 6:16:33 AM GMT+02:00, Richard Stallman <rms@gnu.org> wrote:
> 
> When the buffer contains a ligature character, it would be a good thing for
> Emacs to determine that the terminal doesn't support ligatures, and in
> that case to arrange to display those ligature characters using two letters.
> This should happen by default.
> 
> Other pre-composed characters could likewise be displayed in two columns
> using non-composed characters.
> 
> Emacs needs to know which compositions the terminal can display.  I
> expect it will handle a fairly limited set.  So a new TERMINFO field
> could specify which characters work, and Emacs could convert that into
> a binary array for quick lookup.  TERM=linux could have a TERMINFO
> field to say that ligatures don't work.
> 

This is supposed to be working already, up to a point, see terminal_glyph_code in terminal.c.  I'm guessing that the diamond glyphs you see for some ligatures is the way your terminal "supports" these characters.  Or maybe it lies to Emacs about which characters it supports, or maybe the code which queries the terminal about supported characters doesn't work in your case for some other reason.

I don't think I agree that this must work by default.  That's certainly the desire, but the capabilities of the linux terminal and the way they are reported are a mess, and the use case is quite marginal nowadays.  Ligatures are no different for this purpose from any other non-ASCII character that the console cannot display. We have the latin1-display feature that you can turn on if your console doesn't cope well enough with non-ASCII characters.  And if you want to set up display of ASCII equivalents for just a small set of characters, you can use the latin1-display-char function todo that in your .emacs, in a way that suits the capabilities of your particular type and version of the linux console.