How does one make the latex compiler stricter with '\item's missing an enumerate or itemize environment?

Question

When \item is inside a theorem-type environment, but not inside an enumerate or itemize environment, LaTeX does not give an error. Is there a way to make compiling stricter so that one receives an error when this happens?

Here is a minimal working example:

\documentclass{amsart}
\usepackage{enumitem}
\theoremstyle{plain}
\newtheorem{lemma}{Lemma}
\begin{document}
\begin{lemma}
  Hello
\item abcd 
  \item This compiles without error.
\end{lemma}
\end{document}

Here is the version of TeX that I'm using, if it's important:

This is pdfTeX, Version 3.141592653-2.6-1.40.24 (TeX Live 2022) (preloaded format=pdflatex
restricted \write18 enabled.
entering extended mode
Processing: test.tex
LaTeX2e <2022-11-01> patch level 1
L3 programming layer <2023-02-22>

The LaTeX kernel defines quite a few (dozens, probably) user-level environments -- including most (all?) theorem-like environments and the enumerate, itemize, and description environments -- in terms of the very-low-level list and trivlist environments. That's why placing an \item instruction inside a theorem or lemma environment doesn't cause an error. What you're asking for basically amounts to a major re-write of large parts of the LaTeX kernel. — Mico, Mar 18 '23 at 19:18
Related question: errors - Why is \item allowed in proof environments? - TeX - LaTeX Stack Exchange — user202729, Mar 24 '23 at 08:34

score 2 · Accepted Answer · answered Mar 24 '23 at 08:26

2

If you try hard enough, you can probably do this:

\documentclass{amsart}
\usepackage{enumitem}
\theoremstyle{plain}
\newtheorem{lemma}{Lemma}
\begin{document}
\ExplSyntaxOn
\makeatletter
\AddToHook{cmd/item/before}{
  \str_if_eq:VnT @currenvir {lemma} {\errmessage{oh~no}}
}
\makeatother
\ExplSyntaxOff
\begin{lemma}
  Hello
\item abcd 
  \item This compiles with error.
\end{lemma}
\end{document}

Just a proof of concept, documentation in texdoc lthooks.

You may want to use \str_case or something else (e.g. use TeX's hash table for "constant"-time checking) if you want to check multiple environments etc. but compiling a whitelist (or a blacklist) appears to be unavoidable to me.

answered Mar 24 '23 at 08:26

user202729

7,143

It's a bit slower to do this however. – user202729 Mar 24 '23 at 08:33
Thank you! I will try this next week. – Anthony Pulido Mar 25 '23 at 10:55
This works for me! Thank you! – Anthony Pulido Apr 03 '23 at 00:20

How does one make the latex compiler stricter with '\item's missing an enumerate or itemize environment?

1 Answers1