Starred macro with delimited arguments?

Question

My goal is to define a starred and a non-starred version of a macro \foo that will be called as \foo arg {actual argument} or \foo* arg {actual argument}.

So I tried the following:

\makeatletter
\def\foo{\@ifstar\withstar\withoutstar}
\makeatother

\def\withstar arg #1{%
  With star: #1
}

\def\withoutstar arg #1{%
  Without star: #1
}

\foo* arg {actual argument}
\foo arg {actual argument}

but I get the error

! Use of \withstar doesn't match its definition.
l.67 \foo* 
           arg {actual argument}

However, if I change the definition of \withstar and \withoutstar to

\def\withstar #1{%
  With star: #1
}

\def\withoutstar #1{%
  Without star: #1
}

Then the calls \foo* {actual argument} and \foo {actual argument} work as expected. Why does this happen?

Answer 1

There is a space between the start * and arg:

\foo* arg {actual argument}

But the parameter text of \withstar directly starts with arg:

\def\withstar arg #1{...}

because the space inbetween ends the command sequence \withstar.

Fix:

\makeatletter
\def\foo{\@ifstar\withstar\withoutstar}

\@firstofone{\def\withstar} arg #1{%
  \typeout{With star: #1}%
}
\makeatother

\def\withoutstar arg #1{%
  \typeout{Without star: #1}
}

\foo* arg {actual argument}
\foo arg {actual argument}

% End TeX job
\makeatletter\@@end

The space after curly braces is not ignored. \@firstofone is defined in the LaTeX kernel as such:

\long\def\@firstofone#1{#1}

It is used as trick to insert the space after the command sequence \withstar.

Result:

With star: actual argument
Without star: actual argument