newcommand with two arguments, one optional

Question

I'm struggling with a command that doesn't do what I expect. This is my code

\newcommand{\expectation}[2][]{\mathrm{E}_{#1}\left[#2\right]}

\begin{document}
I get $\expectation{A}{B}$, but I expect $\mathrm{E}_{A}\left[B\right]$\\
Optional arguments, I get $\expectation{B}$, and I expect $\mathrm{E}\left[B\right]$
\end{document}

and this is the output

This does not make a lot of sense to me. If I skip the newcommand and directly plug in what I want, everything looks fine. But the newcommand version with both inputs does something different. What am I missing?

(Even worse: I'm sure everything worked as intended yesterday. Today it doesn't anymore)

Edit: turns out this was due to a fixed bug in Texstudio. Optional arguments have to be called with brackets

Answer 1

I would use the xparse package as it keeps the default behaviors for an optional argument very manageable.

\documentclass[10pt]{article}

\usepackage{xparse}

%\ExplSyntaxOn
    \DeclareDocumentCommand{\expectation}{ O{} m }{%
        $\mathrm{E}_{#1}\left[#2\right]$%
    }
%\ExplSyntaxOff

% choose a different way of producing subscripts if you need ExplSyntax

\begin{document}
    I get \expectation[A]{B}, but I expect $\mathrm{E}_{A}\left[B\right]$\\
    Optional arguments, I get \expectation{B}, and I expect $\mathrm{E}\left[B\right]$
\end{document}