How to 'robustly' use file contents in command arguments?

Question

I have the following code which sets up a XeLaTeX document (to be run with --enable-write18) and defines two commands, \showme and \prRaise:

\documentclass[a4paper,oneside]{memoir}
\usepackage{polyglossia}
\setdefaultlanguage{english}
\usepackage{catchfile}

% ----------------------------------------------------------------------------------------------------------
\newcommand*{\showme}[1]{The value is #1.}

% ----------------------------------------------------------------------------------------------------------
\makeatletter%
\newdimen\pr@Height%

\newcommand{\prRaise}[2]{%
  \setlength{\pr@Height}{\f@size pt}%
  \raisebox{#1\pr@Height}{#2}%
  }%

\makeatother%

% ----------------------------------------------------------------------------------------------------------
\def\cxltxReadA{\CatchFileEdef{\cxltxR}{/tmp/CXLTXtempout.tex}{}}
\def\cxltxReadB{\CatchFileEdef{\cxltxR}{/tmp/CXLTXtempout.tex}{}\cxltxR}

% ----------------------------------------------------------------------------------------------------------
\begin{document}

\showme{foobar}% => ”The value is foobar”.

\showme{\cxltxReadB}% => ”The value is -0.2 .”

\showme{\input{/tmp/CXLTXtempout}}% => ”The value is -0.2 .”

\showme{\immediate\input{/tmp/CXLTXtempout}}% => ”The value is -0.2 .”

A\prRaise{-0.2}{B}C% => "ABC" with B lowered

A\prRaise{-0.2 }{B}C% => "ABC" with B lowered (trailing space shouldn't be a problem)

A\prRaise{\input{/tmp/CXLTXtempout}}{B}C% !!! => Missing number, treated as zero.

A\prRaise{\immediate\input{/tmp/CXLTXtempout}}{B}% !!! => Missing number, treated as zero.

A\prRaise{\cxltxReadB}{B}C% !!! => Missing number, treated as zero.

\cxltxReadA A\prRaise{\cxltxR}{B}C% works

\end{document}

The thing is, trying to use the file contents with \input, \immediate\input and \cxltxReadA ... \cxltxR works for both commands, but, crucially, all attempts to read the file contents within the argument to prRaise fail. It'd be great if it did work, because what i really want to do is call some external program to do a contextual computation, so i want to call that external program at exactly the point where i need it.

Why do the two commands behave so differently? What in the definition of \prRaise causes these troubles?

Answer 1

As explained in Why is \input not expandable?, the LaTeX version of \input is not expandable. To use \input in the context you'd like to, as part of setting a length, you need everything to work by expansion. There are good reasons why LaTeX's \input is defined the way it is, but if you really need expansion use the saved TeX primitive:

\begin{filecontents*}{\jobname.tmp}
-0.2
\end{filecontents*}
\documentclass{article}
\makeatletter
\newlength\pr@Height
\newcommand\prRaise[2]{%
  \setlength\pr@Height{\f@size pt}%
  \raisebox{#1\pr@Height}{#2}%
}
\let\normalinput\@@input
\makeatother
\begin{document}
A\prRaise{\normalinput \jobname.tmp }{B}C
\end{document}

Note that the same issue applies to e.g.

A\prRaise{\cxltxReadB}{B}C%

as \cxltxReadB is trying to carry out a definition, which is again not expandable.

Answer 2

First of all let's get rid of \immediate, which makes sense only before \write, \openout and \closeout, because these operations are normally performed during shipout and with the \immediate prefix they are performed “immediately”.

So \immediate\input has just one effect, that of expanding \input until the first unexpandable token is found. Since this token is \let (resulting from \@ifnextchar), TeX ignores \immediate and moves on.

I would distinguish your calls of \prRaise between an “explicit” argument and a file to be read in.

\begin{filecontents*}{\jobname.tmp}
-0.2
\end{filecontents*}
\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand\prRaise{smm}
 {
  \dim_set:Nn \l_flow_pr_height_dim { \use:c { f@size } pt }
  \IfBooleanTF{#1}
   {% \tl_set_from_file:Nnn is the same as \CatchFileDef
    \tl_set_from_file:Nnn \l_flow_pr_factor_tl { } { #2 }
   }
   {
    \tl_set:Nn \l_flow_pr_factor_tl { #2 }
   }
  \flow_pr_raise:n { #3 }
 }

\dim_new:N \l_flow_pr_height_dim
\tl_new:N \l_flow_pr_factor_tl

\cs_new_protected:Npn \flow_pr_raise:n #1
 {
  \raisebox{ \l_flow_pr_factor_tl \l_flow_pr_height_dim } { #1 }
 }

\ExplSyntaxOff

\begin{document}
A\prRaise{-0.2}{B}C

A\prRaise*{\jobname.tmp}{B}C
\end{document}