Redirect tex.print output to a file

Question

I want to redirect tex.print output to a file to collect and reuse the produced LaTeX material :

the mwe :

\documentclass{article}

\begin{document}

essai

\directlua{require("essai.lua")}

\directlua{ a = "Hello word \\dots" }

\directlua{ to_file ( a ) }

\end{document}

and the lua script called essai.lua :

function to_file ( x )
    local file_name = "Foo"

    out = io.open( file_name .. ".tex", 'w' )

    tex.print = out:write

    out:write ( "\\documentclass{article}" )
    out:write ( "\\begin{document}")
    tex.print ( x )
    out:write ( "\\end{document}" )
    io.close( out )
end

The error message is :

(/usr/local/texlive/2022/texmf-dist/tex/latex/base/ts1cmr.fd)...2/texmf-dist/te x/luatex/lualibs/lualibs-basic-merged.lua:391: error loading module essai.lua f rom file ./essai.lua: ./essai.lua:9: function arguments expected near 'out'

Answer 1

You can overwrite the function definition (see https://stackoverflow.com/a/4510038/):

function to_file ( x )
    local file_name = "Foo"

    out = io.open( file_name .. ".tex", 'w' )

    tex.print = function(trp)
        out:write(trp)
        out:write("\n")
    end

    out:write ( "\\documentclass{article}\n" )
    out:write ( "\\begin{document}\n")
    tex.print ( x )
    out:write ( "\\end{document}\n" )
    io.close( out )
end

Resulting Foo.tex with your MWE:

\documentclass{article}
\begin{document}
Hello word \dots
\end{document}