How to understand correctly scope of local variable in LaTeX3?

Question

Common programming languages (C).

// foo is global scope
int foo = 0;

int main() {
    // bar is local scope within main
    int bar = 1;
}

Let's consider LaTeX3

\documentclass[]{article}

\ExplSyntaxOn
\cs_set:Nn \create_list:n
{
    \tl_set:Nn \l_my_tl {(#1)}
}

\NewDocumentCommand\uselist{m}
{
    \create_list:n {#1}
     \tl_use:N \l_my_tl
}
\ExplSyntaxOff

\begin{document}

\uselist{2}

\end{document}

In LaTeX3 I expected the scope of \l_my_tl is a body of \create_list:n. But \l_my_tl visible also from \uselist{m} macro. Why is that? What am I misunderstanding here?

Answer 1

Scope in TeX, and thus in expl3, is not defined by macros: they are simply replaced by their definition when 'used'. Scope is therefore provided using explicit groups:

• \group_begin:/\group_end: (expl3) - \begingroup/\endgroup (classical TeX names)
• { ... } or \bgroup/\egroup

The brace-based version potentially impacts on math mode spacing, and in expl3 we always use the first type of grouping.

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There are some 'hidden' scopes, such as table cells, but for the bulk of programming these are not what we use.