Re: Ligatures (was: Unify the Platforms: Cairo+FreeType+Harfbuzz Everywhere (except TTY))

[Clément Pit-Claudel]

On 23/05/2020 02.47, Eli Zaretskii wrote:
>> Cc: address@hidden, address@hidden, address@hidden
>> From: Clément Pit-Claudel <address@hidden>
>> Date: Fri, 22 May 2020 16:02:22 -0400
>>
>> What I don't understand is what it is about Emacs that means that we need 
>> special lists of regexps for each new font, while other editors don't need 
>> them.
> 
> Emacs doesn't need a special list for each font.  I already said that
> several times.  Please look at some examples of composition rules we
> already have, for example the Arabic rules at the very end of
> misc-lang.el.  Do you see any fonts mentioned there?  These rules work
> with any font that supports Arabic.

The only thing I'm talking about is symbol compositions in programming fonts, 
and for these, we *will* need a custom list for each font, right?

>> Each font offers a different set of symbol ligatures: there is no common 
>> superset that covers all fonts, except the ".+" regexp that Pip posted 
>> earlier.
> 
> I'm not yet sure this is indeed so.  I didn't see any reference which
> implies that any combination of 26 ASCII letters could become a
> ligature. 

I think that's where I'm confused.  I'm talking of ligatures like -> and =>, 
which do not involve the 26 ASCII letters.

> This is a discussion that didn't yet happen.  It is quite possible
> that in practice the list of ligatures we want to support is not very
> long.  E.g., the list in
> https://github.com/tonsky/FiraCode/wiki/Emacs-instructions is not
> long, and I doubt manu additions to it will ever make sense for us.

As I said, this list is incomplete and broken.

> And finally, if a given font doesn't support some ligature, the
> original characters will be displayed "normally", so nothing is lost,
> and there's no need to tune the list of ligatures to each and every
> font.  I said that as well several times already.

As long as you can produce a superset of all ligatures, yes.  My claim is that 
this superset is ".+".
Otherwise, how do you handle the fact that Fira Code handles arrows of 
arbitrary lengths?  Or is that different from ligatures?