Modify / Add to TeX-Ligatures?

Question

I have finally caved and made the switch to LuaLaTeX, allowing me the proper use of UTF-8 everywhere and, of course, the use of \usepackage{fontspec}:

\usepackage{fontspec}
\setmainfont[Numbers={OldStyle, Proportional}, Ligatures={TeX}]{Minion Pro}

My question is: Where are those common “TeX-Ligatures” defined, and how do I modify (read: add to) them? (I am not above hacking some core (Lua)(La)TeX files, mind you.)

-- becomes an n-dash, --- an m-dash: that is all good and proper, but there are a few others, most pressingly ... (three dots) becoming … (proper ellipsis).

Answer 1

It is not really visible, but if you copy&paste you can see that it works:

\documentclass{article}
\usepackage{fontspec}
\directlua{
fonts.handlers.otf.addfeature{
    name = "dotlig",
    type = "ligature",
    data = {
      [0x2026] = { ".", ".", "." },
    },
  }

}
\setsansfont{texgyreheros}[RawFeature=+dotlig;]
\setmainfont{texgyretermes}[RawFeature=+dotlig;]
\begin{document}
some text .... some text

\sffamily 

some text .... some text 
\end{document}

Answer 2

There are several ways to accomplish your objective. One of them -- I trust additional methods will be posted by other readers -- is to employ LuaTeX's process_input_buffer callback to replace all instances of ... with \dots. This approach ensures that text-style ellipses will be used in text mode and math-style ellipses will be used in math mode. If you want text-mode ellipses throughout the entire document, you should replace

  s = s:gsub ( '%.%.%.', '\\dots{}' ) 

with

  s = s:gsub ( '%.%.%.', '\\char"2026{}' )

in the Lua function shown below.

An advantage of this preprocessor-based approach is that you have to run the code just once, for the entire document. A potential disadvantage could be that you can't disable or enable this feature selectively for some fonts but not for others.

\documentclass{article}
\usepackage{fontspec} % 'Ligatures={TeX,Common}' is enabled by default
\setmainfont{Minion Pro}[Numbers={OldStyle,Proportional}]
\setsansfont{Myriad Pro}[Numbers={OldStyle,Proportional}, Scale=MatchLowercase]

\usepackage{luacode}
\begin{luacode}

function dots2ellipsis ( s ) 
  s = s:gsub ( '%.%.%.', '\\dots{}' ) 
  return s
end

\end{luacode}

\AtBeginDocument{\directlua{luatexbase.add_to_callback ( 
   "process_input_buffer" , dots2ellipsis , "dots2ellipsis" )}}
   

\begin{document}
abc123 -- --- ... | $...$

\sffamily
abc123 -- --- ... | $...$
\end{document}