Define command for which its arguments must be present and encased in (curly) brackets

Question

I want to create a command that explicitly requires its user to give the arguments in the curly brackets, or else throw an error.

For instance, take the following command definition:

\newcommand{\example}[1]{The argument is: "#1"}

Now, suppose a user uses it the following way:

\example{Hello}

... will result in:

The argument is: "Hello"

However, if the user uses it like this:

\example Hello

... will result in:

The argument is: "H"ello

This behavior is undesirable for the command I want to define -- how would it be possible to enforce this and return an error to the user? I've tried using ifthen constructs to test the arguments, but a single letter could potentially be a valid argument actually. Using \newcommand* only works if there is whitespace after the command, not if it is used mid-sentence.

Answer 1

You can "grab # as macro argument" (see Macros with # as the last parameter and "Grab to #{" macro arguments for explanation)

\documentclass[11pt]{article}

\newcommand*{\example}{}% test if undefined; better safe than sorry
\def\example#{\exampleaux}
\newcommand{\exampleaux}[1]{The argument is: "#1"}

\begin{document}

\example{Hello}

\example Hello

\end{document}

The second use will break down with the error

! Use of \example doesn't match its definition.
l.11 \example H
               ello

Answer 2

In the example below the macros \example␣␣ and \fetchbracedarg form a loop for accumulating brace-nested arguments in the token-register \toks@ in an amount which corresponds to the initial value of the count-register \@tempcnta—I suggest a token-register instead of a scratch-macro in order to avoid the halving of amounts of consecutive hashes (##→#) which takes place with hashes occuring in ⟨definition text⟩s of macros at the time of expanding the macros in question.

\documentclass{article}

\makeatletter
\newcommand\example{%
  \toks@\expandafter{%
        \expandafter\toks@
        \expandafter{\the\expandafter\toks@\expandafter}%
        \expandafter\@tempcnta\expandafter=\the\@tempcnta\relax
        \innerexample
  }%
  \@tempcnta=3 %
  \csname example\@firstofone{ } \endcsname
}%
\expandafter\@ifdefinable\csname example\@firstofone{ } \endcsname{%
  \@namedef{example\@firstofone{ } }#{\fetchbracedarg}%
}%
\newcommand\fetchbracedarg[1]{%
  \toks@\expandafter{\the\toks@{#1}}%
  \advance\@tempcnta -1 %
  \ifnum\@tempcnta=0 \expandafter\@firstoftwo\else\expandafter\@secondoftwo\fi
  {\the\toks@}{\csname example\@firstofone{ } \endcsname}%
}%
\newcommand\innerexample[3]{%
  \par This is argument 1: "#1"%
  \par This is argument 2: "#2"%
  \par This is argument 3: "#3"%
}%
\makeatother

\begin{document}

\example{A}{B}{C}

% \example{A}{B}C % Triggers error: ! Use of \example   doesn't match its definition.

% \example{A}B{C} % Triggers error: ! Use of \example   doesn't match its definition.

% \example A{B}{C} % Triggers error: ! Use of \example   doesn't match its definition.

\end{document}