Create new environment with options

Question

I'm new to creating new environments and even though I searched at the xparse*documentation I couldn't figured out how to solve my specific problem. I would like to create a new environment, say concept, and I would like it to take an optional argument (I don't know if it has to be a boolean or what). This optional argument will make the text to be in italics. If the argument is not declared, it will simply print normal font. It's as simple as that. So

\begin{concept}[i] % I just invented a option named "i"
  This text will be in italics
\end{concept}

or without an option

\begin{concept}
  This text will be in normal font.
\end{concept}

I thought that may be using the xparse package would make the task easier. Sorry to ask such a silly question but I couldn't come to a solution.

Answer 1

Normal LaTeX environments can do this without the need for the xparse package:

Notes:

• I have used the xstring package for string comparison as I prefer it's syntax, but this could be done without that package if needed.

References:

Relevant for using \itshape:

• Is there a difference between \textit and \itshape?
• Why Different Commands for Seemingly Similar Tasks?
• Does it matter if I use \textit or \it, \bfseries or \bf, etc

Code:

\documentclass{article}
\usepackage{xstring}
\newenvironment{concept}[1][]{%
    \IfStrEq{#1}{i}{\itshape}{}%
}{%
}%
\begin{document}
\begin{concept}[i]% I just invented a option named "i"
  This text will be in italics
\end{concept}
\begin{concept}
  This text will be in normal font.
\end{concept}
\end{document}