Behavior of environment with optional argument changes when optional argument is not passed

Question

Recently, when I've been working with environments with an optional argument, I've noticed that the behavior changes when I introduce a set of square braces, even when there is nothing inside them. For instance:

\documentclass{article}
\usepackage{fancyvrb}

\newenvironment{exer}[1][]
{
    do the following exercise from #1
    \VerbatimOut{\jobname.exer}%
}
{
    \endVerbatimOut%
    \input{\jobname.exer}%
}

\begin{document}
\begin{exer}[]
some text
\end{exer}
\end{document}

It will compile a pdf with "some text" as well as a .exer file, but if I replace the exer environment with

\begin{exer}
some text
\end{exer}

it will throw the following error:

! FancyVerb Error:
  Extraneous input `some text' between \begin{exer}[<key=value>] and line end
.
\FV@Error ... {FancyVerb Error:
\space \space #1
}
                                                  
l.16 some text

So I have two questions: first, why does the environment behave differently even when I pass in the default optional argument, and second, is there any way to patch or address this behavior?

Many thanks.

Answer 1

I'm not sure what would be the usefulness of this. Anyway, you have to announce fancyvrb that this is a \VerbatimEnvironment.

\documentclass{article}
\usepackage{fancyvrb}

\newenvironment{exer}[1][]{%
  \VerbatimEnvironment
  do the following exercise\if\relax\detokenize{#1}\relax\else\space from~#1\fi
  \begin{VerbatimOut}{\jobname.exer}%
}{%
  \end{VerbatimOut}%
  \par\input{\jobname.exer}%
}

\begin{document}

\begin{exer}[]
some text
\end{exer}

\begin{exer}[the book]
some text
\end{exer}

\end{document}

Answer 2

\documentclass{article}
\usepackage{fancyvrb}

\makeatletter
\def\exer{\@ifnextchar[\exer@i\exer@i[]}
\def\exer@i[#1]{do the following exercise #1
\VerbatimOut{\jobname.exer}}
\def\endexer{\endVerbatimOut\par
\input{\jobname.exer}}
\makeatother

\begin{document}
\begin{exer}
some text
\end{exer}
\end{document}