[luatex] macro definition inside a callback

[Paul Isambert]

Arno Trautmann <Arno.Trautmann at gmx.de> a écrit:
> maybe a somewhat strange question again from me: Is it possible to 
> define a macro inside the *_linebreak_filters? I want to use a function 
> in the pre_linebreak_filter, and this function should define a macro. So 
> the macro should be present with a new definition at the beginning of 
> the next paragraph, but only if the previous one has been typeset with 
> that function registered, which removes itself after running.
> 
> I tried something like
> 
> tex.print("\\gdef\\something{}")
> 
> but this does not work as the effect of tex.print is not defined in the 
> *_linebreak_filters, if I remember correctly.
> 
> So, is there any way to do this?

I can see two solutions (both untested); the first is a ugly hack, I
wouldn't count much on it. In both, the target macro is \mymacro; in the
second solution, the definition of that macro is left untouched, since
it simply prints a Lua variable, whose contents changes instead.

However, with Lua under the hood, I'm sure there are other ways to
achieve what you want without any single macro.


% SOLUTION 1

-- Should be modified to take care of functions possibly registered in
-- process_input_buffer.
function redefine (str)
  callback.register("process_input_buffer",
    function (s)
      callback.register("process_input_buffer", nil)
      return "\\gdef\\mymacro{" .. str .. "}" .. s
    end)
end

-- Then, in whatever linebreak callback:
callback.register("pre_linebreak_filter",
  function (...)
    ...
    redefine(<something>)
    ...
  end)


% SOLUTION 2

% Tex:
\def\mymacro{\directlua{tex.print(macro_definition)}}

% Lua:
callback.register("pre_linebreak_filter",
  function (...)
    ...
    macro_definition = <something>
    ...
  end)

Best,
Paul