Latex3: Declare variable in aux file

Question

My question is a follow-up to this question.

I want to write variables to aux file and also declare them there because the number of variables varies and their csnames are defined in the document body automatically during build process. I have a control sequence that iterates through generated variables and writes declare and store commands (using c-type argument instead of N) to aux file. Everything works well except that I get errors similar to this:

Control sequence \g_test_int already defined.

The aux file itself looks correct (this is from MWE):

\relax

\ExplSyntaxOn

\int_new:N \g_test_int

\ExplSyntaxOff

MWE:

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn
\AtEndDocument{
    \iow_now:cx{@auxout}{
        \token_to_str:N \ExplSyntaxOn
        ^^J
        \int_new:N\g_test_int
        ^^J
        \token_to_str:N \ExplSyntaxOff
    }
}
\ExplSyntaxOff


\begin{document}
Hello
\end{document}

Answer 1

There are two problems with your code:

1. TeX tries to interpret \g_test_int, because you're doing \iow_now:cx

2. When TeX rereads the .aux file, \int_new:N will raise an error.

Solution:

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn
\AtEndDocument{
    \iow_now:cn{@auxout}{
        \ExplSyntaxOn
        ^^J
        \int_zero_new:N \g_test_int
        ^^J
        \ExplSyntaxOff
    }
}
\ExplSyntaxOff


\begin{document}
Hello
\end{document}

At rereading, \g_test_int will be zeroed, but it's unimportant as all typesetting has already been done.

On the other hand, it's unclear why you would declare a variable in the .aux file: if this file does not yet exist, no macro in the document body can use the variable.