Custom error message for bad length value?

Question

Using LuaLaTeX in texlive2017, but I assume that this would apply generally.

Question: Is it possible to set a custom error message, that will be shown when a bad value is used as the argument to a length? The custom message would be defined as a string, which can be changed at various places in the document.

The usual error message is ``Missing number, treated as zero''. In my case, using a custom document class, I want to be able to guide the user to a specific place in the documentation. MWE:

\documentclass{article}
\newlength\mylength
\def\notalengthvalue{Hello!}
\def\isalengthvalue{20.6pt}
\begin{document}
\def\myerrmsg{Read documentation section 1.2.3.}
\setlength\mylength{\notalengthvalue} % error message is \myerrmsg
\def\myerrmsg{Read documentation section 3.7.2.}
\setlength\mylength{\notalengthvalue} % error message is \myerrmsg
\setlength\mylength{\isalengthvalue} % no error
\end{document}

If TeX had a Try/Catch routine, that would be easy. Lacking Try/Catch, is there another way to do it? That is, one that does not involve brute force pre-parsing of the argument for all possible strings that might be a length.

Answer 1

This is a low level tex parse error the wording comes from tex-the-program, traditionally it would have been visible in the pool file, but modern implementations do not use a separate pool file).

As such it can not be changed from tex code, all you could do is modify (say) \setlength to detect non-length input and avoid attempting a primitive length assignment.

---

For luatex you can do this:

\documentclass{article}

\directlua{
function my_error ()
local e =status.lasterrorstring
if(e=='! Missing number, treated as zero') then
  texio.write_nl ('! Missing number,\string\n\space\space perhaps you should look at page 6')
else if(e=='! Illegal unit of measure (pt inserted)') then
  texio.write_nl ('! Illegal unit of measure\string\n\space\space you really should look at page 6 of the documentaion')
else
  texio.write_nl(e)
end
end
end
luatexbase.add_to_callback('show_error_message',my_error,'error message hook')
}
\begin{document}

\setlength\oddsidemargin{x}

\newcommand\cos{\sin}

\def  x {y}

\end{document}

which produces a terminal output of:

! Missing number,
  perhaps you should look at page 6.
<to be read again> 
x
l.19 \setlength\oddsidemargin{x}

? 
! Illegal unit of measure
  you really should look at page 6 of the documentaion.
<to be read again> 
x
l.19 \setlength\oddsidemargin{x}

? 

! LaTeX Error: Command \cos already defined.
               Or name \end... illegal, see p.192 of the manual.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.21 \newcommand\cos{\sin}

?