Is there a way of defining a command which is automatically converted to a no-op after its first execution?

Question

Is there a way of defining a command that can be executed only once and is then redefined to be a no-op?

Related:

• Is there a way of defining a command \nullifyifmoved{arg} that will normally act like its argument but act like a no-op if the macro is “moved”?
• Macro to be used in a \section command that has no effect in the table of contents

Answer 1

An easy way is to just nullify the macro within the body so that the next time it does not have an effect if it is called again via either

\global\let\FirstTime\relax%

or

\global\def\FirstTime{}%

As per David Carlisle's comment, we need to add a \global to ensure that the macro is nullified for subsequent use in the case the first use was within a group (as per updated test case below)

References:

• A similar thing can be done for environments. For instance in Adjust environments's behaviour when nested, I wanted to disable the environment if it was ever nested.
• What is the difference between \relax and {}?

Code:

\documentclass{article}
\newcommand*{\FirstTime}{%
    \gdef\FirstTime{}%
    \textbf{FirstTime: }%
}%
\begin{document}
\begingroup
    abcd \FirstTime defg \FirstTime xys
\endgroup
abcd \FirstTime defg \FirstTime xys

abcd \FirstTime defg \FirstTime xys

\end{document}