How to \read inside a command and return the result?

Question

As a followup of How to use file content as a numeric value for a length?, I'm trying to do this (I really need to read the data.txt file inside this new command and use the value later, when the command is called):

\documentclass{article}
\usepackage{calc}
\begin{document}
\newcommand\x{
  \newread\foo
  \openin\foo=data.txt
  \read\foo to \temp
  \temp
}
\setlength\parskip{1pt * \x}
\end{document}

I'm getting:

! Missing number, treated as zero.
<to be read again>
                   \global
l.35 \setlength\parskip{1pt * \x}

What's wrong?

Answer 1

You can do this, but it requires LuaTeX. There you can use \beginlocalcontrol/\endlocalcontrol to define a block of code which gets fully executed (including opening file) during TeX's expansion stage:

\documentclass{article}
\usepackage{calc}
% First define \beginlocalcontrol based on tex.runtoks
\newluafunction\beginlocalcontrol
\directlua{lua.get_functions_table()[\the\allocationnumber] = function() return tex.runtoks(token.get_next) end}
\luadef\beginlocalcontrol\allocationnumber
\begin{document}
\newcommand\x{%
  \beginlocalcontrol % This block gets executed during expansion
    \begingroup % Let's keep our definitions local to avoid confusion macros which might not expect definitions to change during expansion
    \newread\foo
    \openin\foo=data.txt
    \read\foo to \temp
    \closein\foo
    \expandafter\endgroup % The two \expandafter's ensure that the final \temp gets the value from inside of the \endgroup and \endlocalcontrol blocks.
  \expandafter\endlocalcontrol
  \temp
}
\setlength\parskip{1pt * \x}
\end{document}

I would recommend though to move at least the \newread \foo outside of the command, otherwise you waste one input file every time the macro gets expanded.

Answer 2

Actually this question happens to be solvable:

\documentclass{article}
\begin{document}
\makeatletter
\setlength\parskip{@@input{b.tex}pt}
\typeout{\the\parskip}
\setlength\parskip{\dimexpr 1pt*@@input{b.tex}\relax}
\typeout{\the\parskip}
\makeatother
\end{document}

Assuming b.tex contains 123, both will print out 123.0pt.

Answer 3

\read is not expandable but is an assignment-command itself.
Thus it cannot be carried out in the course of carrying out another assignment-command like \setlength.
As you intend to define a scratch-macro \x you can do s.th. like this:

\begin{filecontents*}{mydata.txt}
8
\end{filecontents*}
\documentclass{article}
\usepackage{calc}
\newread\foo
\begin{document}
\immediate\openin\foo=mydata.txt
\immediate\read\foo to \x
\immediate\closein\foo
%\setlength\parskip{1pt * \x}
\setlength\parskip{\x pt}
\showthe\parskip
\end{document}

\@@input is LaTeX 2ε's copy of the expandable TeX-primitive \input.
\input/\@@input cannot be carried out in the course of directly obtaining a set of tokens that is to form a macro argument because in any case the end of the file will be encountered which is treated like an \outer token.
But you can arrange things in a way where they are not processed as macro arguments:

\begin{filecontents*}{mydata.txt}
8
\end{filecontents*}
\documentclass{article}
\begin{document}
\parskip=\csname @@input\endcsname mydata.txt pt %
%\parskip=\dimexpr1pt*(\csname @@input\endcsname mydata.txt )\relax
\showthe\parskip
\end{document}

Answer 4

Since you want to do an assignment, expandability is not a factor.

The simplest method is to define a suitable command, say

\setlengthfromfile{<length>}{<file>}{<expression>}

where <length> is the register you want to set, <file> is the file you want to read a value from and <expression> contains #1 to denote the file contents, so your case would be

\setlengthfromfile{\parskip}{data.txt}{1pt * #1}

Code with examples:

\begin{filecontents*}{\jobname.dat}
20
\end{filecontents*}
\documentclass{article}
\ExplSyntaxOn
\NewDocumentCommand{\setlengthfromfile}{mmm}
 {% #1 = length parameter
  % #2 = file name
  % #3 = expression
  \yegor_setlengthfromfile:nnn { #1 } { #2 } { #3 }
 }
\cs_new:Nn __yegor_setlengthfromfile_aux:n { } % initialize
\cs_generate_variant:Nn __yegor_setlengthfromfile_aux:n { V }
\tl_new:N \l__yegor_setlengthfromfile_tl
\cs_new_protected:Nn \yegor_setlengthfromfile:nnn
 {
  \file_get:nnN { #2 } { } \l__yegor_setlengthfromfile_tl
  \cs_set:Nn __yegor_setlengthfromfile_aux:n { #3 }
  \skip_set:Nn #1 { __yegor_setlengthfromfile_aux:V \l__yegor_setlengthfromfile_tl }
 }
\ExplSyntaxOff
\setlengthfromfile{\parskip}{\jobname.dat}{1pt * #1}
\showthe\parskip
\setlengthfromfile{\parskip}{\jobname.dat}{1pt * #1-12pt}
\showthe\parskip
\setlengthfromfile{\parskip}{\jobname.dat}{#1\parskip}
\showthe\parskip
\setlengthfromfile{\parskip}{\jobname.dat}{#1pt plus 0.1pt}
\showthe\parskip
\stop

The console will show

(./yegordim.dat
)
> 20.0pt.
l.31 \showthe\parskip
?
(./yegordim.dat)
> 8.0pt.
l.34 \showthe\parskip
?
(./yegordim.dat)
> 160.0pt.
l.37 \showthe\parskip
?
(./yegordim.dat)
> 20.0pt plus 0.1pt.
l.40 \showthe\parskip

In the third example we reused the previous value of \parskip.