How to make the transition from LaTeX2e to expl3 syntax?

Question

I was peeking at the LaTeX3 Github repository and specifically l3auxdata.dtx. There was a surprise for me, as I have been practicing and studying the expl3 syntax and modules for the last couple of weeks. The surprise was code like:

\def\auxwrite{\auxdata_write:nn}
\def\auxshipout{\auxdata_write_shipout:nn}

I am not too sure if there was a reason for the code to use \def, but this reminded me of my own difficulties with the syntax.

There is a great temptation when starting with expl to bypass the verbosity of the expl language and use \def and TeX primitives. When I started with expl3 I was literally translating LaTeX2e code in my head and then writing it down. Pretty much like when you learn a foreign language.

I have by now mostly overcome this as I forced myself to do the following:

1. Avoided \defs.
2. As the too many underscores bothered me, at the beginning I wrote functions as \module_dosomething:nn. This way I was a bit quicker with debugging and typing. Only started adding l_ or g_ much later or on refactoring the code.
3. I mentally kept reading the commands together with the argument specifier until it started sinking in. Like module dosomething en en. The reason I kept forgetting to write the specifier.
4. When I used a tex_<cs>:D like a command, I went back and refactored the code later to remove it.
5. Write notes with examples as I practice the commands. This can be also useful to others.

Do you have any additional tips and tricks to share?

Answer 1

As the too many underscores bothered me, ...

+1 for too many underscores. I am really unable to use that syntax.

For people who don't like underscores in LaTeX3 programming layer (expl3), now there is a new functional package for you. It provides intuitive LaTeX2 interfaces for expl3.

It also emulates functional programming in other programming languages such as Lua. the evaluation of composite functions is from inside to outside. Here is a simple example:

\documentclass{article}

\usepackage{functional}
% make every function become a group
\Functional{scoping=true}

\begin{document}

\IgnoreSpacesOn
\PrgNewFunction \MathSquare { m } {
  \IntSet \lTmpaInt { \IntEval {#1 * #1} }
  \Return { \Value \lTmpaInt }
}
\IgnoreSpacesOff
\MathSquare{5}
\MathSquare{\MathSquare{5}}

\end{document}

Note that with this package, you need to pass return values of functions with \Return command.