Delimiter error using nested commands

Question

I'm not an expert on TeX and I've run into a problem. I've been able to condense it into the following minimal working (or rather not working) example:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amssymb}
\makeatletter
\newcommand{\E}{
    \@ifnextchar[{\Ea}{\mathbb{E}}
}
\newcommand{\Ea}[1][]{
    \@ifnextchar\bgroup{\Eb{#1}}{\mathbb{E}\left[#1\right]}
}
\newcommand{\Eb}[2]{
    \mathbb{E}\left[#1\,\middle|\, #2\right]
}
\makeatother
\begin{document}
\begin{equation}
    \E[\E[X]{Y}]{Y}=\E[X]{Y}
\end{equation*}
\end{document}

I want this command \E to be usable as just \mathbb{E}. I want the first optional argument to be in square brackets and the second (even more optional) argument to be in curly brackets. I don't quite understand, why the above example doesn't work. I get the error "Missing delimiter (. inserted)." and the output looks like: E[E[X|Y]]Y=E[X|Y] where it is supposed to look like this E[E[X|Y]|Y]=E[X|Y] I also had some problems passing

\E[X_{n+1}]{X_1,\dots,X_n}

where I could identify \dots as the problem. This was fixable (why, I don't entirely understand) by writing

\newcommand{\Eb}[2]{
    \mathbb{E}\left[\nonexpanded{#1\,\middle|\, #2\right}]
}

However, this also doesn't solve the nesting-issue. I'm completely lost at this point, as to what is happening. Is there some way to solve this?

nesting - Bug? LaTeX misparses nested optional arguments - TeX - LaTeX Stack Exchange — user202729, Nov 07 '21 at 15:09
Does this answer your question? ] inside an optional argument — user202729, Nov 07 '21 at 15:11
(... if you don't care about the internal details, search for "summary" and "conclusion" in the answer.) — user202729, Nov 07 '21 at 15:11
Hm. Actually the first linked question above would be better as a duplicate target, as in the second case the OP already know about the workaround. — user202729, Nov 07 '21 at 15:14
you could define this in one line using xparse but note that even though xparse is now built in to latex you need to explicitly load the old version to enable the g type for optional {} arguments. g is deprecated and not supported built in format version of xparse as it breaks all latex syntax conventions. — David Carlisle, Nov 07 '21 at 15:23

score 3 · Accepted Answer · answered Nov 07 '21 at 15:18

Don't use optional `{`...`}`-delimited arguments. Those are ugly!

_{^{_{but if you must...}}}

Either wrap the inner argument in braces :

\begin{equation}
  \E[{\E[X]{Y}}]{Y}=\E[X]{Y}
%    ^        ^
\end{equation*}

or use xparse:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{xparse}
\NewDocumentCommand \E { o !g }
  {%
    \IfValueTF{#1}%
      {%
        \mathbb{E}\left[#1%
          \IfValueT{#2}{,\middle|,#2}%
        \right]%
      }%
      {\mathbb{E}\IfValueT{#2}{{#2}}}%
  }
\begin{document}
\begin{equation}
    \E[\E[X]{Y}]{Y}=\E[X]{Y}
\end{equation}
\begin{equation}
    \E[\E[X]]=\E[X]
\end{equation}
\begin{equation}
    \E\E{Y}=\E[X]
\end{equation}
\end{document}

Thank you very much, that really helped! For some reason I never came across the xparse package. Very nice. — Stocced, Nov 07 '21 at 15:42

Delimiter error using nested commands

1 Answers1

Don't use optional {...}-delimited arguments. Those are ugly!

Don't use optional `{`...`}`-delimited arguments. Those are ugly!