Lua code to recognize texlua versus luatex

Question

I have some foo.lua code that needs to behave slightly differently when loaded from luatex or texlua. Here is my example document.

%!TeX Program=LuaLaTeX
\documentclass{article}
\RequirePackage{luacode}
\begin{document}
\begin{luacode}
require("foo.lua")
\end{luacode}
\end{document}

and here is foo.lua

if is_luatex then
  tex.sprint("Hello LuaTeX World!")
else
  print("Hello TeXLua World!")
end

How to define is_luatex? My first idea is to parse arg[0] but I really need a very strong solution that covers the widest range of situations (99.999%).

Answer 1

You can just test for the function you are using

if tex.sprint then
  tex.sprint("Hello LuaTeX World!")
else
  print("Hello TeXLua World!")
end

luatex typesets

texlua does

$ texlua foo.lua 
Hello TeXLua World!

If you first tested for the tex table, you could also detect stock Lua rather than texlua

Answer 2

There is no documented straightforward flag to make the difference between LuaTeX and TeXLua, but we can take advantage of the fact that LuaTeX defines command sequences whereas TeXLua does not. The number of command sequences defined is available through status.cs_count:

local is_luatex = status.cs_count > 0
if is_luatex then
  tex.sprint("Hello LuaTeX World!")
else
  print("Hello TeXLua World!")
end