Why doesn't \makeatletter work inside \newcommand?

Question

After hours of banging my brain against my skull, I narrowed down an issue I'm having to the fact that \makeatletter doesn't work inside \newcommand. Commands that I call after \makeatletter are registered as undefined, as if \makeatletter didn't do anything.

Why is this the case?

In the example below, I define two commands. Calling one of them from the document body causes the undefined error, while the other doesn't.

\documentclass[11pt]{book}
\usepackage{kvsetkeys}

\newcommand{\commandThatFails}{
    \makeatletter \comma@parse{}{} \makeatother
}

\makeatletter
    \newcommand{\commandThatWorks}{
        \comma@parse{}{}
    }
\makeatother

\begin{document}

Hi there.

\commandThatFails
\commandThatWorks

\end{document}

Answer 1

When \makeatletter is executed, it changes the catcode of @ to 11 (letter) so it can be part of a macro name. However, when defining a macro or using an argument of a macro, the tokens are stored without being executed.

So here,

\newcommand{\commandThatFails}{
    \makeatletter \comma@parse{}{} \makeatother
}

\makeatletter is stored as a token in the command rather than being executed. Because @ is still a non-letter, the command parses as the tokens \comma, @, p, a, r, s, and e, which is not \comma@parse

However, in your second example, \makeatletter is executed before the command is defined, so that \comma@parse{}{} is parsed correctly:

\makeatletter
    \newcommand{\commandThatWorks}{
        \comma@parse{}{}
    }
\makeatother

This is for the same reason that \verb|...| fails within arguments: it’s not executed, just stored, causing any strange characters to cause all sorts of errors.

As noted by @percusse, in this cases where you need @-macros just once or twice, you can use \csname comma@parse\endcsname which expands all the macros and then converts every token between \csname and \endcsname to a macro name. So this would also work:

\newcommand{\commandThatDoesntFail}{\csname comma@parse\endcsname{}{}}

Where the command resolves to \comma@parse{}{} (note that the tailing {}{} is treated as arguments to \comma@parse, not \endcsname).

Further reading:

• What catcodes are within LaTeX.
• More on \csname ... \endcsname.
• A similar problem related to changing catcodes within a command definition.