Token read/write turns # into ##

Question

When I try to write an input given by a user in a token list to a file, it turns # into ##, so if a users wants to write \def\hello#1{Hello #1!}, instead of writing:

\def\hello#1{Hello #1!}

as expected I get:

\def \hello ##1{Hello ##1!}

Any idea how to make it work? Ideally I'd like to avoid asking the user to change their input.

MWE

\documentclass{article}
\newtoks\mytokens
\newwrite\myWrite
\NewDocumentCommand{\writeInFile}{m}{%
  \mytokens={#1}% We put the input in a token list (that token list might actually be populated by another script before)
  \immediate\openout\myWrite myfile.tex\relax% We open the output file
  \immediate\write\myWrite{\the\mytokens}% We write to the output file
  \immediate\closeout\myWrite% We close the file
}
\begin{document}
%% I don't want to change the syntax in this part of the document:
\writeInFile{\def\hello#1{Hello #1!}}
\input{myfile.tex}
\hello{Alice}
\hello{Bob}
\end{document}

Answer 1

Here I define the command with two arguments, the first being the file name.

You want to change the category code of # before reading.

\documentclass{article}
\newwrite\myWrite
\newtoks\mytokens
\newcommand{\writeInFile}{%
  \begingroup\catcode`#=12 \writeInFileAux
}
\newcommand{\writeInFileAux}[2]{% #1 = file name, #2 = what to write
  \immediate\openout\myWrite #1\relax% We open the output file
  \mytokens={#2}%
  \immediate\write\myWrite{\the\mytokens}% We write to the output file
  \immediate\closeout\myWrite% We close the file
  \endgroup
}
\begin{document}
%% I don't want to change the syntax in this part of the document:
\writeInFile{\jobname-out}{\def\hello#1{Hello #1!}}
\input{\jobname-out}
\hello{Alice}
\hello{Bob}
\end{document}

This will be the written out file

\def \hello #1{Hello #1!}

Of course, you cannot have \writeInFile as the argument to another command.

You can remove this limitation by doing a string replacement.

\documentclass{article}
\ExplSyntaxOn
\iow_new:N \g_bora_write_iow
\str_new:N \l_bora_write_str
\NewDocumentCommand{\writeInFile}{mm}
 {
  % #1 = file name, #2 = what to write
  \str_set:Nn \l_bora_write_str { #2 }
  \str_replace_all:NnV \l_bora_write_str { ## } \c_hash_str
  \iow_open:Nn \g_bora_write_iow { #1 }
  \iow_now:NV \g_bora_write_iow \l_bora_write_str
  \iow_close:N \g_bora_write_iow
 }
\cs_generate_variant:Nn \iow_now:Nn { NV }
\cs_generate_variant:Nn \str_replace_all:Nnn { NnV }
\ExplSyntaxOff
\begin{document}
%% I don't want to change the syntax in this part of the document:
\textit{xx \writeInFile{\jobname-out}{\def\hello#1{Hello #1!}}}
\input{\jobname-out}
\hello{Alice}
\hello{Bob}
\end{document}

The \textit bit is just for demonstration purpose.