# Tips for LaTeX3 beginners

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Last updated August 7, 2022
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While current stable version is LaTeX2e, LaTeX3 is already delivered as expl3 as the “L3 programming layer”. It provide a unified programming layer for package author and experienced user.
This blog post will not be the thorough tutorial, but rather a complement.

### Where to Start?

Before using LaTeX3, you should have a basic idea in LaTeX such as syntax, toolchains, packages et al. Then, following materials are what you need now.

#### Documentations

• expl3: Refered when needed.
• xprase: Help to define user command like \newcommand.

### Minimal Example for Class Author

% main.tex \documentclass{test} \begin{document} \hello \end{document}
% test.cls \RequirePackage{expl3} \ProvidesExplClass{test}{2022-01-01}{0.1.0}{Minimal Example Class} % Expl3 syntax start from here. \NewDocumentCommand { \hello } { } { hello, world! \par 你好，世界！ } % For CJK user, or you may load book \LoadClass{ctexbook}

### Global Variables and Local Variables

Global variables should be assigned globally and may get the proper value whenever you want.
Local variables does not have this assumption.

### Equivlent to \renewcommand

Although xparse provides \RenewDocumentCommand as equivlent to renew a user command. But I suggest you only use it in global level and with \NewDocumentCommand, in order to provide user interface. In the other circumstances, for example, to redefine a variable provided by other package, then \cs_set:Npn should be your choice.
\RenewDocumentCommand sometimes does work as expect, while \cs_set:Npn always does.

### Generate Variation

Usually, expl3 only ship functions with common variants: there is \tl_if_empty:NTF and also \tl_if_empty:nTF . But if you have want to expand the first variable, then you don’t have \tl_if_empty:xTF by default.
\cs_generate_variant:Nn is the one you need to generate these variations.
\tl_new:N \tmp_a_tl \tl_new:N \tmp_b_tl \tl_set:Nn \tmp_a_tl { \tmp_b_tl } \cs_generate_variant:Nn \tl_if_empty:nTF {x} \tl_if_empty:nT { \aaa } {hi} \tl_if_empty:xT { \aaa } {hello} % The output will be hello.

#### l3keys usage

In expl3 , package l3keys provide a consistent approach to define and process the key-value interface.