Is there a "do nothing" command in LaTeX?

Question

I'm defining it like this for myself:

\newcommand\nothing[1]{#1}

Maybe the same already exists in LaTeX and I can just use it?

PS. The purpose of this is to highlight certain parts of the .tex document for another post-processor (spell checker). In reality, my command's name is \nospellcheck.

Answer 1

Any time you absorb an argument using the standard approach, you are potentially losing information...for example, a leading space before the absorbed argument. Another example is if the argument absorbed is a single token, you won't know if it originally had braces on it or not. As a result of these uncertainties, I tend not to consider argument absorption as a "do nothing" command.

In the MWE below, the active-Q is set to the OP's definition of \nothing. It is employed twice in the 2nd line. In the first instance, one loses the space because of argument absorption. In the 2nd instance, one loses the braces around a.

\documentclass{article}
\usepackage[T1]{fontenc}
\newcommand\nothing[1]{#1}
\begin{document}
\catcode`\Q=\active
\letQ\nothing
a b\detokenize{{a}}
aQ b\detokenize\expandafter{Q{a}}
\end{document}

A better "do nothing" command would be

\newcommand\nothing{}

or, equivalently,

\let\nothing\empty

However, even that is not foolproof. Any new token introduced, even \empty, can interfere if it becomes the target of an expansion or absorption.

Here's such an example where the introduction of the simplified \nothing still changes the result (though, in this case, the \nothing improves the result):

\documentclass{article}
\usepackage[T1]{fontenc}
\let\nothing\empty
\begin{document}
\def\z|#1|{\detokenize\expandafter{#1}}
\z|{abc}|
\z|\nothing{abc}|
\end{document}

Note: this exact situation was a bug I just discovered yesterday in my tokcycle package when text is escaped from the cycle when placed between | delimiters. It lost the braces if the escaped text was a single brace group, in the manner shown in the first line above. My fix, which I am about to submit to CTAN, amounts to using the technique with \empty, shown on the second line, which will retain those braces.