How to redefine symbols inside my custom environment?

Question

I'm trying to create my own foo environment, which would be used like this:

\documentclass{article}
\begin{document}
\begin{foo}
a -> b [
  c => d ]
\end{foo}
\end{document}

I'm expecting it to render as:

\begin{equation}
\begin{split}
& a \mapsto b \llbracket \\[1pt]
& \quad c \to d \rrbracket
\end{split}
\end{equation}

Is it at all possible? I need to tell TeX somehow that ->, new line, leading spaces, =>, [, and some other symbols and their combinations must be "mapped" to something else.

To start with, do you guarantee that the foo will not be nested inside something else i.e. catcode changes are effective? — user202729, Oct 06 '22 at 04:51
Yes, I will use foo only at the top level, never nesting into something else — yegor256, Oct 06 '22 at 05:06

user202729 · Accepted Answer · 2023-08-19T02:04:27.757

Here's an approach that uses the filecontentsdef package to store the contents of the environment in a macro so that we can modify it:

\documentclass{article}
\usepackage{filecontentsdef}
\usepackage{amsmath}
\usepackage{stmaryrd} % Provides \llbracket and \rrbracket.
\ExplSyntaxOn
\cs_generate_variant:Nn \tl_replace_all:Nnn {Nx}
\NewDocumentEnvironment {foo} {} {
    % Use filecontentsdef to store everything until "\endfilecontentsdefmacro" 
    % into a macro named "\l__envcontents_tmp_tl".
    \filecontentsdefmacro \l__envcontents_tmp_tl
} {
    \endfilecontentsdefmacro
    \str_set:NV \l__envcontents_tmp_tl \l__envcontents_tmp_tl
 % Drop the last newline (^^M) character
\str_set:Nx \l__envcontents_tmp_tl {\str_range:Nnn \l__envcontents_tmp_tl {1} {-2}} 

%%%% Do Replacements %%%%
\tl_replace_all:Nxn \l__envcontents_tmp_tl {\tl_to_str:n{-&gt;}} {\mapsto}
\tl_replace_all:Nxn \l__envcontents_tmp_tl {\tl_to_str:n{=&gt;}} {\to}
\tl_replace_all:Nxn \l__envcontents_tmp_tl {[} {\llbracket}
\tl_replace_all:Nxn \l__envcontents_tmp_tl {]} {\rrbracket}

% Replace &quot;(newline)(space)(space)&quot; with &quot;\\[1pt] &amp; \quad&quot;. This must be done after
%  the replacement of &quot;[&quot; and &quot;]&quot;, above, otherwise &quot;[1pt]&quot; gets replaced by &quot;\llbracket 1pt \rrbracket&quot;.
\tl_replace_all:Nxn \l__envcontents_tmp_tl {\cs_to_str:N \^^M \c_space_tl \c_space_tl} {\\[1pt] &amp; \quad}  

% Replace &quot;(newline)&quot; with &quot;\\[1pt] &amp;&quot;.
% This must be done after the replacement of &quot;(newline)(space)(space)&quot;
\tl_replace_all:Nxn \l__envcontents_tmp_tl {\cs_to_str:N \^^M} {\\[1pt] &amp;}  

% Wrap contents in &quot;equation&quot; and &quot;split&quot; environment.
\tl_put_left:Nn \l__envcontents_tmp_tl {\begin{equation} \begin{split} &amp;}
\tl_put_right:Nn \l__envcontents_tmp_tl {\end{split} \end{equation}}

% Typeset the result.
\l__envcontents_tmp_tl

}
\ExplSyntaxOff
\begin{document}
Example, using custom environment:
\begin{foo}
a -> b [
  c => d ]
\end{foo}
Example, entered explicitly:
\begin{equation}
\begin{split}
& a \mapsto b \llbracket \[1pt]
& \quad c \to d \rrbracket
\end{split}
\end{equation}
\end{document}

You did not specify explicitly when you need a \quad exactly, so it's just a guess.

Strictly speaking, some \tl_to_str:n is not needed, but I put it there just in case.

There's some subtlety in the order of replacement needed; see the comments in the code.

Variables are named according to expl3 naming convention. Edit if you prefer.

There's some additional subtlety with filecontentsdef if you want to use a literal tab character in the environment.

What does \cs_generate_variant:Nn \tl_replace_all:Nnn {Nx} do? — Paul Wintz, Aug 16 '23 at 16:37
@PaulWintz It generates a command named \tl_replace_all:Nxn which does what it says. Refer to texdoc interface3 for the documentation. — user202729, Aug 19 '23 at 01:33

How to redefine symbols inside my custom environment?

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