How to automatically add text immediately after \begin{document}

Question

I attempted to redefine the document environment as follows, but get a complaint that:

LaTeX Error: \begin{MYdocument} on input line 13 ended by \end{document}.

\documentclass[12pt]{article}
\newtoks{\SpecialText} 
\SpecialText={Text to be printed after begin document}

\newenvironment{MYdocument}{%
  \begin{document}{}
  Note: \the\SpecialText
}{%
  \end{document}
} 

\begin{MYdocument}
 ... file contents ...
\end{MYdocument}

I want this to produce the following output

Note: Text to be printed after begin document

... file contents ...

If I could do this by just slightly modifying the environment that would be preferable so that I could just use \begin{document} ... \end{document}.

Answer 1

You can use the \AtBeginDocument command before the \begin{document} line to queue up code (even multiple pieces of code) so that it it executed when that line is hit. Here's a reference.

Answer 2

As already stated by Aaron, \AtBeginDocument{<code>} can be used to place code "at the begin of the document". This hook is executed after the old .aux is read in and the new one is opened, but before the preamble is officially closed. Therefore the code can still load packages and add already type-settable material which writes into the .aux file, like sectioning commands. So it is actually executed with one foot in the preamble and the other one in the document body.

The etoolbox package provides \AfterEndPreamble{<code>} (and other useful hooks) which is executed after the preamble is closed, so e.g. all preamble-only macros are disabled etc. Actually it is executed at the very end of \begin{document}, only followed by an \ignorespaces. I personally would prefer \AfterEndPreamble to \AtBeginDocument (already heavily used by packages) to add code at the very beginning of the document body.

Answer 3

As of 2023 the answers suggesting to use \AtBeginDocument might be outdated. In fact, clsguide.pdf has on p. 16

The \AtBeginDocument hook should not be used for code that does any typesetting since the typeset result would be unpredictable.

Simplifying, the code \begin{document} expands to

\UseHook{env/document/before}
\begingroup
\def\@currenvir{document}
\UseHook{env/document/begin}
\UseOneTimeHook{begindocument/before}%
% ... global option check, size setting, aux file, font setup
\normalsize
\everypar{}%
% ... spacefactor codes, language, metadata, page size
\UseOneTimeHook{begindocument}%   <--- \AtBeginDocument goes in here
% ... other settings
\UseOneTimeHook{begindocument/end}%
\ignorespaces

I've marked where the content of \AtBeginDocument is inserted. While font sizes have been set up and \everypar has been "cleared" such that it doesn't throw the Missing \begin{document} error, other stuff goes on and, as the official guide tells, the result might be unpredictable. (Subjunctive: clearly you might also get lucky.) The very last hook executed by \begin{document}, just before \ignorespaces, is begindocument/end (see also lthooks-doc.pdf, p.25). Material to be typeset should thus come into this hook:

\documentclass{article}
\AddToHook{begindocument/end}{Hello world!\par}
\begin{document}
Hello world.
\end{document}

With a format later than 2020-10-01, etoolbox's own \AfterEndPreamble is simply mapped into \AddToHook{begindocument/end}. With older formats it can still be used safely, because the used hook is added at the very end of \document by something like this:

\edef\document{\unexpanded\expandafter{\document Hello world!\par}}

Doing this yourself is clearly evil. Fluffy little bunnies will die horribly any time you do so, unless you are a dark TeX wizard/witch who really knows what he/she is doing.