LaTeX3 and document environments

There was a question on the {TeX} Q&A site recently about compiling a document reading

\documentclass{minimal}
\document
a
\enddocument

This doesn’t work: I’ve explained why as my answer to the question. I also mentioned that in LaTeX3 I’d expect this won’t work. Here, I want to look at that in a bit more detail.

First, the background. When LaTeX2e creates an environment foo, what actually happens is two macros called \foo and \endfoo are defined (hence the original question). When you do

\begin{foo}
...
\end{foo}

LaTeX looks for the \foo macro, and if it finds it starts a group and inserts \foo. At the end of the environment, LaTeX will use \endfoo if it exists, but this is not required.

There are some problems with this scheme. First, it’s possible to abuse the system, as something like

\begin{emph}
...
\end{emph}

will not raise an error even though \emph was never intended to be used as the start of an environment. (LaTeX2e does not require that \endemph exists here: it’s only the start of an environment which must be defined.) Secondly, due to this mixing it’s not possible to have a foo environment and independent \foo macro: the two are tied together. Finally, as \endfoo is ‘special’ \newcommand won’t let you create macros which start \end....

For LaTeX3, we want the approach to be different. Document commands and document environments should be independent of one another, and so xparse defines a pair of special internal macros when you use

\NewDocumentEnvironment{foo}...

This is totally independent of a macro called \foo, so the plan is that you’ll be able to have an environment called foo and a separate \foo command, or indeed one called \endfoo.

At present, most people will be using xparse with LaTeX2e. That means we still need to follow what LaTeX2e does. So at the moment you’re not going to see the change: messing with the LaTeX2e mechanism looks like a very bad idea. However, for a native LaTeX3 format the clash will disappear.