Generate file write handles on the fly

Question

I am experimenting a little bit with automatic generation of file handles via \newwrite.

I want to achieve following behaviour:

• A user provides a list of file categories, for example Hints, Solutions, Problems etc. and a list of corresponding extensions, say .hints, .sol, .prb in the correct order (how should TeX know which category belongs to which extension and vice versa), e.g. as 1st and 2nd arguments to a command, \RegisterOutputFiles.

• The connection between the category and the extension is stored in two text labels with different names but same counter number, the basics are described in another question of mine(already solved, see details here: Writing text content as labels and refer to them with \nameref*)

• \RegisterOutputFiles should create the file handles with some nice name, to remember them, say \HintsFile, \SolutionsFile etc. and open them for writing via \immediate\openout\HintsFile=\jobname.hints etc., all in a loop.

Now in the loop comes the failure, when the selfdefined command \NewWrite shows up the error message

 ! Missing \endcsname inserted. <to be read again> 
               \protect  l.84 ....prb,.hints,.sol,.explain,.idea,.concepts}

I am pretty sure, that I miss some expansion or similar, but I can not figure out, since \NewWrite works with direct injection of words like Hints etc.

Here is the (usual non-working) MWE

\documentclass{article}

\usepackage{ifthen}
\usepackage{etoolbox}
\usepackage{blindtext}
\usepackage{morewrites}
\usepackage{xcolor}
\usepackage{hyperref}%



\newrobustcmd{\NewWrite}[1]{%
\expandafter\newwrite\csname#1\endcsname%
}%


%% Later on... Create a handle from #1 and directly combine
%% it with filename #2 with \immediate\openout%
\newrobustcmd{\NewWriteOpenOut}[2]{%
\expandafter\newwrite\csname#1\endcsname%
\immediate\openout\csname #1\endcsname=#2%
}%


\makeatletter
\newcounter{pc@@filecounter}%      
\newcounter{pc@@totalfilecounter}%
\newcounter{pc@@loopcounter}%


%% Hopefully only text is in #1%
\newrobustcmd{\WriteOutputFileLabel}[1]{%
\refstepcounter{pc@@filecounter}%
\immediate\write\@auxout{%
\string\newlabel{pc::outputfile::\number\value{pc@@filecounter}}{{\thesection}{\thepage}{#1}{}}%
}% End of writing to AUX file
}%

\newrobustcmd{\WriteOutputFileNameLabel}[1]{%
\refstepcounter{pc@@filecounter}%
\immediate\write\@auxout{%
\string\newlabel{pc::outputfilename::\number\value{pc@@filecounter}}{{\thesection}{\thepage}{#1}{}}%
}% End of writing to AUX file
}%


\newrobustcmd{\RegisterOutputFiles}[2]{%
%%% Other code before
%
\setcounter{pc@@filecounter}{0}%
\forcsvlist{\WriteOutputFileLabel}{#1}%      Store the output file `categories` to label names
\setcounter{pc@@totalfilecounter}{\number\value{pc@@filecounter}}%
\setcounter{pc@@filecounter}{0}%
\forcsvlist{\WriteOutputFileNameLabel}{#2}%  Store the extensions to label 'names'
%
\noindent%
\textbf{\Large \textcolor{blue}{There are \number\value{pc@@totalfilecounter} file categories!}}
%
\setcounter{pc@@filecounter}{1}%
\setcounter{pc@@loopcounter}{\number\value{pc@@totalfilecounter}}
\addtocounter{pc@@loopcounter}{1}%
%
%
\begin{center}
\textbf{\textcolor{red}{Diagnostics}}
\end{center}
%
\whiledo{\number\value{pc@@filecounter} < \number\value{pc@@loopcounter}}{%
\noindent \nameref*{pc::outputfile::\number\value{pc@@filecounter}}  \(\longrightarrow \)  \jobname\nameref*{pc::outputfilename::\number\value{pc@@filecounter}}%
\stepcounter{pc@@filecounter}%
%% Creating the file handles file with error message `\protect` 
\NewWrite{\nameref*{pc::outputfile::\number\value{pc@@filecounter}}}%  %Creating file handle name `\anyname` 
\newline%
} %
%%% Other code after
}%

\makeatother


\begin{document}


\RegisterOutputFiles{Problem,Hints,Solution,Explanation,Ideas,Concepts}{.prb,.hints,.sol,.explain,.idea,.concepts}%

\blindtext

\end{document}

Notes:

You have to remove the % before \NewWrite in the loop and run pdflatex twice to see the final error message, the first one complains about missing references.

The code is not finished and not polished, it is pre-stage of some concept study for another package.

Answer 1

You can't use \nameref* inside \csname...\endcsname, because it's not fully expandable. The association category-extension is best done with a different syntax, where the two parts are next to each other, so errors are less likely.

File hupfer-handle.tex

\documentclass{article}

\usepackage{etoolbox}
\usepackage{morewrites}
\usepackage{xcolor}

\makeatletter
%% create a list of the open streams
\listadd{\cs@stream@list}{}% initialize

%% a unique command for allocating a stream and opening it if the
%% optional name argument is given    
\newrobustcmd{\NewWrite}[2][]{%
  \listadd{\ch@stream@list}{#2}%
  \expandafter\newwrite\csname#2\endcsname
  \ifblank{#1}{}{\immediate\openout\@nameuse{#2}=#1 }%
}

%% a helper macro for counting the number of items in the csv argument
\newcommand{\ch@count@streams}[1]{\advance\@tempcnta\@ne}
%% a helper macro for separating the two parts at the slash
\newcommand\ch@open@stream[1]{\ch@open@stream@aux#1\@nil}
\def\ch@open@stream@aux#1/#2\@nil{%
  %% allocate a stream
  \NewWrite[\jobname.#2]{#1}%
  %% save the extension
  \@namedef{ch@stream@@#1}{.#2}%
}

\newrobustcmd{\RegisterOutputFiles}[2]{%
  %%% Other code before
  %
  \@tempcnta=\z@
  \forcsvlist{\ch@count@streams}{#1}%
  \forcsvlist{\ch@open@stream}{#1}%
  \noindent
  \textbf{\Large\textcolor{blue}{There are \number\@tempcnta\space file categories!}}%
  \begin{center}
  \textbf{\textcolor{red}{Diagnostics}}
  \end{center}
  \begingroup\parindent=\z@
  \renewcommand{\do}[1]{Handle ``##1'' with extension \texttt{\@nameuse{ch@stream@@##1}}\par}
  \dolistloop{\ch@stream@list}
  \endgroup
  %%% Other code after
}

\makeatother


\begin{document}

\RegisterOutputFiles{
  Problem/prb,
  Hints/hints,
  Solution/sol,
  Explanation/explain,
  Ideas/idea,
  Concepts/concepts
}

\end{document}

Output

File list

> ls hupfer-handle.*
hupfer-handle.aux       hupfer-handle.hints hupfer-handle.mw    hupfer-handle.sol
hupfer-handle.concepts  hupfer-handle.idea  hupfer-handle.pdf   hupfer-handle.tex
hupfer-handle.explain   hupfer-handle.log   hupfer-handle.prb

Answer 2

Here is another more straightforward solution:

\documentclass{article}

\newcount\tmpnum
\def\RegisterOutputFiles#1#2{\tmpnum=0 \def\listsummary{}%
   \registeroutputfilesA #1,,\relax #2,,\relax
}
\def\registeroutputfilesA #1,#2\relax #3,#4\relax{%
  \if^#1^The number of categories: \the\tmpnum{\tt\listsummary}\else
     \if^#3^\errmessage{?? less suffixes than categories.}\skiploop \fi
     \advance\tmpnum by1
     \csname newwrite\expandafter \endcsname \csname#1\endcsname
     \immediate \expandafter\openout \csname#1\endcsname =\jobname#3
     \edef\listsummary{\listsummary\endgraf
        The \jobname#3 is opened as \expandafter\string\csname#1\endcsname}%
     \registeroutputfilesA #2\relax #4\relax
  \fi\relax
}
\def\skiploop #1\fi\relax{\fi\fi}

\begin{document}

\RegisterOutputFiles {Problem,Hints,Solution,Explanation,Ideas,Concepts}
                     {.prb,.hints,.sol,.explain,.idea,.concepts}

\end{document}

I've added it here in order to show that usage of pure TeX can be more compact often.