writing to \jobname.aux using LaTeX3

Question

In LaTeX2e, one can write to the standard auxiliary file via the \@auxout file handle. Are there dedicated LaTeX3 functions (planned) that facilitate writing to \jobname.aux or is there an equivalent \@auxout file handle in LaTeX3 we can use with the functions from the l3file package?

Answer 1

At present the answer here is 'no', as we do not have structures for the higher-level idea of 'an auxiliary file'. Clearly we will need to address this along with a number of other file-related areas (for example a native mechanism for loading files, akin to \usepackage).

Answer 2

This probably doesn't conform to the guidelines, but \iow_shipout:Nn, \iow_shipout_x:Nn, and \iow_now:Nn understand the usual file handler:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\makeatletter
\NewDocumentCommand{\writeaux}{ s m }
 {
  \IfBooleanTF{#1}
   { \iow_shipout:Nn \@auxout { #2 } }
   { \iow_shipout_x:Nn \@auxout { #2 } }
 }
\makeatother
\ExplSyntaxOff
\begin{document}

a\writeaux{\noexpand\typeout{foo}}\writeaux*{\typeout{bar}}

\end{document}

For internal commands to be used in packages, one can think to

\cs_new_protected:Npn \iowaux_now:n #1 { \iow_now:Nn \@auxout { #1 } }
\cs_new_protected:Npn \iowaux_shipout:n #1 { \iow_shipout:Nn \@auxout { #1 } }
\cs_new_protected:Npn \iowaux_shipout_x:n #1 { \iow_shipout_x:Nn \@auxout { #1 } }

and similarly for the x variant. Thus usage can become independent on the actual implementation; just modifying the four definitions will be sufficient to keep packages otherwise unchanged.