Check if parameter in command is declared/empty in LuaLaTeX

Question

I'm trying to check if a parameter is declared/empty using lua. So far I wrote the following MWE:

\documentclass{standalone}
\def\var#1{\def@var{#1}}
%\var{Hello world}
\begin{document}
    @var --
\directlua{if string.len("\@var") == 0 then tex.print("empty") else tex.print("not empty") end}

\end{document}

If \var{Hello world} is uncommented the code runs with no errors, but if it's commented then it always returns error.

Is there a way to make this checking on a parameter/variable with LuaLaTeX?

Answer 1

Your line

\@var --

prints var – because the macro \@ sets the spacefactor and then there are letters v a r and then space and the double -- which is converted to the ligature –.

Your \directlua line works like this:

\directlua{if string.len("\spacefactor \@m {}var") == 0 then tex.print("empty") else tex.print("not empty") end}

because the \directlua primitive fully expands its argument before the argument is used.

If you use your macro \var (which is unused in your example), then the macro \@ is redefined as a macro with mandatory separator var.

There are three cases described above, I mean that none of these case were your intention.

I can only guess your intend. Maybe, you want to define \@var macro when the \var macro is used and you want to test if the \@var macro is defined. You can't do this test using \directlua because you cannot put an undefined macro to the \directlua parameter because \directlua expands its parameter. You can do:

\catcode`\@=11 % `@` is letter
\def\var#1{\def\@var{#1}}
%\var{Hello world}  % this defines @var as a macro with body Hello world.
\ifx@var\undefined undefined var\else defined var \fi

Answer 2

As wipet wrote you are missing \makeatletter. Apart from this you can get the content of a macro in lua with token.get_macro. Side remark: do not use standalone for such small examples. That is a complicated class with various side-effects. article is normally much better.

\documentclass{article}
\makeatletter
\def\var#1{\def@var{#1}}
\makeatother
\var{Hello world}
\begin{document}
\makeatletter
    @var --
\makeatother
\directlua{
  a=token.get_macro("@var")
  print("XXXXXXXXXX @var is: ",a) %for debugging on the terminal
  if string.len(a) == 0 then 
   tex.print("empty") 
  else 
    tex.print("not empty " .. string.len(a)) 
  end}

\var{}

  \directlua{
  a=token.get_macro("@var")
  print("XXXXXXXXXX @var is: ",a) 
  if string.len(a) == 0 then 
   tex.print("empty") 
  else 
    tex.print("not empty " .. string.len(a)) 
  end}




\end{document}