write to file full expansion of command

Question

I have an issue I want to resolve, maybe you can help me.

When I compile the mwe the following happens:

In the PDF you can see: (X/A)(Up/Down)

In the file.txt write: (\frac {a}{x})(\frac {u}{d})

How can I write to the file what is seen in the PDF?

mwe:

\documentclass[border=1cm]{standalone}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{expl3,xparse,xstring}
\ExplSyntaxOn
\RenewDocumentCommand{\frac}{mm}{
\IfStrEqCase{#1}{{a}{X}}[Up]
/
\IfStrEqCase{#2}{{x}{A}}[Down]
}
\NewDocumentCommand{\Write}{+m}{
\iow_new:N \tdm_iow
\cs_gset:Npn \my_wrr:x ##1{
 \iow_open:Nn \tdm_iow { file.txt }
 \iow_now:Nx \tdm_iow{##1}
}
\my_wrr:x {#1}
}
\ExplSyntaxOff
\begin{document}
\def\str{(\frac{a}{x})(\frac{u}{d})}
\str
\Write{\str}
\end{document}

Thanks for your help!

you can't typically. The typeset version is a combination of glyphs from fonts, rules drawn by TeX and positioning commands, it does not have a text version that you can write to a text file. — David Carlisle, May 17 '22 at 14:58
The issue its from xstring or xparse? because with
\def\frac#1#2{#1/#2}

write to file

(a/x)(u/d) — Alexander Rodríguez, May 17 '22 at 15:14
both. Neither \RenewDocumentCommand nor \IfStrEqCase are expandable. — Ulrike Fischer, May 17 '22 at 15:18
well xstring commands are not expandable and xparse/lcmd ones aren' unless you use \NewExpandableDocumentCommand but that isn't really relaed to "what appears in the pdf" — David Carlisle, May 17 '22 at 15:24
Here it's meant "you can't generally". expansion - LaTeX macro expander - TeX - LaTeX Stack Exchange — user202729, May 17 '22 at 15:53
But... if \Write and \frac are things you define yourself, you can. (I make execinside package for such purpose and other purposes, although... currently undocumented and not on ctan...) — user202729, May 17 '22 at 15:54
To be fair... unravel can probably do what you want (emulate TeX within TeX to get the characters that is going to be executed) but that's most likely overkill. — user202729, May 17 '22 at 15:56
By the way if you learn expl3 you should also learn how expansion work properly... — user202729, May 17 '22 at 15:58

score 0 · Accepted Answer · answered May 17 '22 at 16:35

Instead of xstring's non-expandable \IfStrEqCase use expl3's expandable \strcase:nnTF.
Instead of \RenewDocumentCommand use \RenewExpandableDocumentCommand.

(With an up-to-date TeX-installation you need neither \usepackage[utf8]{inputenc} nor \usepackage{expl3,xparse}.)

\documentclass[border=1cm]{standalone}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{expl3,xparse}
\ExplSyntaxOn
\RenewExpandableDocumentCommand{\frac}{mm}{
  \str_case:nnTF{#1}{{a}{X}}{}{Up}
  /
  \str_case:nnTF {#2}{{x}{A}}{}{Down}
}
\NewDocumentCommand{\Write}{+m}{
\iow_new:N \tdm_iow
\cs_gset:Npn \my_wrr:x ##1{
 \iow_open:Nn \tdm_iow { file.txt }
 \iow_now:Nx \tdm_iow{##1}
}
\my_wrr:x {#1}
}
\ExplSyntaxOff
\begin{document}
\def\str{(\frac{a}{x})(\frac{u}{d})}
\str
\Write{\str}
\end{document}

file.txt:

(X/A)(Up/Down)

score 0 · Answer 2 · answered May 17 '22 at 16:57

I'm not sure why you would redefine \frac, so I'll use a different name.

\iow_new:N should be outside \Write
There is no need to set a function inside \Write
\IfStrEq is not fully expandable.

\documentclass{article}
%\usepackage{xparse} % uncomment for older LaTeX
\ExplSyntaxOn
\NewExpandableDocumentCommand{\FRAC}{mm}
  {
    \str_case:nnF { #1 } { {a}{X} } { Up }
    /
    \str_case:nnF { #1 } { {x}{A} } { Down }
  }
\iow_new:N \rodriguez_out_iow
\NewDocumentCommand{\Write}{O{\c_sys_jobname_str.txt}m}
  {
    \rodriguez_write:nn { #1 } { #2 }
  }
\cs_new:Nn \rodriguez_write:nn
  {
    \iow_open:Nn \rodriguez_out_iow { #1 }
    \iow_now:Nx \rodriguez_out_iow { #2 }
  }
\ExplSyntaxOff
\begin{document}
\def\str{(\FRAC{a}{x})(\FRAC{u}{d})}
\str
\Write{\str}
\end{document}

The file \jobname.txt will contain

(X/Down)(Up/Down)

The PDF will be

write to file full expansion of command

2 Answers2