What is the right way to set a dimension parameter?

Question

In the TeXbook Knuth set parameters for dimensions mostly as follows:

 \parskip=3pt

Other package authors define such parameters without using the equal sign

 \parskip30pt

Many later authors use the LaTeX \setlength

 \setlength{\parskip}{30pt}

You can even define a constant somewhere and write:

  \parskip\parsep

Even \parskip30PT or even \parskip10Pt works!

I favour \parskip30pt for macros in a package and \newlength for preamble commands in a document. Is there a right way to set a dimension parameter?

Note: here is a short minimal example of all the above commands for anyone wishing to try them out.

\documentclass[11pt]{article} 
\usepackage{lipsum}
\begin{document}
  \parskip30pt
  \lipsum[1-2]
  \parskip=3pt
  \lipsum[1]
  \setlength{\parskip}{30pt}
  \lipsum[1]
 \parskip\parsep
  \lipsum[1]
 \parskip25Pt
 \lipsum[3]
 \parskip30PT
 \lipsum[4]
\texttt{\meaning\parsep}

\texttt{\the\parsep}
\end{document}

Answer 1

At the TeX level there are two register types to worry about, dimensions and skips. The former only take a dimension (not surprisingly), whereas the later also have a rubber component. LaTeX's 'lengths' are TeX skips and so can have a rubber component. This is important when you consider how to terminate the setting instructions. For example:

\newskip\myskip
\def\setmyskip#1{\myskip #1} % Bad!
\setmyskip{10 pt} plus some other context

Here, TeX will think that 'plus' is part of the skip information and will complain about a missing number. So LaTeX includes a \relax in the definition of \setlength to avoid this:

> \setlength=macro:
#1#2->#1#2\relax .

This termination question does not apply in the case where you are setting one length from another:

\parskip\parsep plus 

will not treat the plus as part of the setting. Whenever you set one register from another TeX 'knows' that there is nothing else to look for. (Note: if you try the same with a constant stored as a macro, you are back needing the \relax.)

Now, on the 'right' way to do this I'm not aware of a case where the = is vital (contrast the situation with \let, where sometimes that optional = is significant). So I would not use one.

As LaTeX provides proper termination of the setting, I would prefer it's method over the plain TeX approach if we are talking about a LaTeX document. That said, if I want a fixed length I will tend to prefer a dimension, and use \newdimen followed by the plain TeX setting method with \relax inserted as appropriate.