Ensure `description` or `itemize`

Question

Is there a way to make sure that a macro can only be used inside a specific environment, say description?

I am looking for something like the \ensuremath macro.

Answer 1

You can modify the definition of the environment in order to set the command to either work or not. For the description environment, you could use the following code for example:

\documentclass{article}
\let\olddesc\description
\def\myrealcmd#1{\textbf{#1}}
\def\myfakecmd#1{#1}
\let\mycmd\myfakecmd
\def\description{\let\mycmd\myrealcmd\olddesc}
\begin{document}
  \noindent This is a \mycmd{test} outside of description.
  \begin{description}
    \item This is a \mycmd{test} inside of one description.
    \begin{description}
      \item This is a \mycmd{test} inside of two descriptions.
    \end{description}
    \item This is a \mycmd{test} after a nested description.
  \end{description}
  And a \mycmd{test} outside again.
\end{document}

With the following result:

Answer 2

The etoolbox package provides hooks for the environments; so you can define a "hidden" macro and then activate it only in selected environments

\usepackage{etoolbox}
\makeatletter % protect the definition of \mymacro
\newcommand{\@mymacro}{whatever}
\AtBeginEnvironment{description}{\let\mymacro\@mymacro}
\makeatother

% now we help users
\newcommand{\mymacro}{%
  \PackageError{mypackage}
    {\protect\mymacro\space outside `description'}
    {You can use \protect\mymacro\space only inside `description'}%
}

You can add other environments and also provide a different meaning in different environments:

\usepackage{etoolbox}
\makeatletter % protect the definition of \mymacro
\newcommand{\description@mymacro}{whatever}
\AtBeginEnvironment{description}{\let\mymacro\description@mymacro}
\newcommand{\itemize@mymacro}{whatever}
\AtBeginEnvironment{itemize}{\let\mymacro\itemize@mymacro}
\makeatother

Answer 3

The current environment name is stored in \@currenvir so you could test that that was description or whatever, but nested environments would mask that. If you need your macro to be allowed anywhere in description, even in a nested tabular or itemize, say, then you would need to save \@currenvir into another macro at the start of description (either explicitly or by patching the definition of \description then you can test this local macro without it being over-written by nested environments.