Does LaTeX3 address the subject of scope (i.e. grouping)?

Question

TeX scope is an overwhelmingly convoluted and opaque subject. It is impossible to tell by sight when a scope starts and ends, and what exactly is being affected by it. Since scope is one of the fundamental building blocks of every programming language, this is one aspect of TeX that, in my opinion, desperately needs an overhaul from the ground up. Does LaTeX3 address this subject?

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I was asked:

What do you mean by ”impossible to tell by sight“?

I'm not even talking about pathological examples like

\def\mymac{a}%
\input enigma %redefines \begingroup and \endgroup as \relax
\begingroup%
    \def\mymac{b}%
\endgroup%
\mymac%
\bye%

which typesets

b

(Could you tell it by sight?)

I'm talking about normal examples, like the fact that the {...} in \def<control sequence>{...} do not imply a grouping, but the {...} in \output{...} do imply a grouping, yet the {...} in \mymacro{...} don't imply grouping, whereas the {...} in \loop{...}\repeat do imply grouping.

I'm talking about the fact that \setbox<number> = ... is local, but \ht<number> = ... is global (could you tell it by sight?), and when you create a plain TeX box, the current font will be captured in the box, but the current color won't.

And this is just to give you a taste.

Answer 1

LaTeX3 (and in particular expl3, the programming layer) is written in TeX so ultimately cannot change the way TeX works. However, it can provide documented interfaces which detail how particular concepts are to be used.

For grouping, expl3 thus has to work with the various constructs TeX uses. The major ones are {/} (catcode 1/2 pair group) and \begingroup/\endgroup (semi-simple group): the latter in expl3 are renamed as \group_begin:/\group_end:. The way that these work is simple: they create a group in which local assignments will be 'trapped'. Reflecting this, expl3 provides a range of \<thing>_set:Nn and \<thing>_gset:Nn (or similar) macros which either locally or globally set the <thing>. The latter will usually be a variable such as \l_tmpa_tl (a token list to be set locally), \g_tmpb_clist (a comma list to be set globally), etc.

There are some TeX constructs which can only be set globally. However, these are largely low-level and are not generally exposed by expl3. Where they are, the documentation is clear that they must be handled using global interfaces: they will be given \g_... names. (For example, \prevgraf is not exposed by l3galley but there is a \g_galley_previous_par_lines_int variable which covers the same concept.)

As LaTeX3/expl3 is still in development, there are places where TeX primitives are not yet covered or where some changes may still be needed (box dimensions may well fall into the latter). However, the general approach is clear: document the scope, name appropriately and provide higher-level interfaces for a range of low-level concepts.

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One of the examples in the question is that { ... } may be used for (at least) two reasons:

• To create a balanced text argument
• For grouping

The two are linked as TeX determines something is a balanced text as it is enclosed as a group using begin-group and end-group tokens. However, for expl3 code we have favoured (as a convention) using \group_begin:/\group_end: only for grouping: any { ... } pairs appearing in code are therefore arguments.

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On the example of setting box height/depth/width, this is an area in expl3 which may still need attention. One may imagine dealing with the slightly odd behaviour by using appropriate wrappers to allow setting the box 'correctly'. That is not (yet) done, and has performance implications. On the other hand, the box code is relatively lightly reviewed at present: this may change. (I've raised with the team: I have a plan of sorts.)