Allowing first undelimited argument to be empty

Question

Consider the following macro definitions:

\def\mymacro#1{\mymacroii#1\nil}
\def\mymacroii#1#2\nil{%
  \ifx\\#1\\%
    empty%
  \else
    #1%
  \fi
}%
\mymacro{foo}% -> f
\mymacro{}
\bye

This is supposed to extract the first character of the parameter to \mymacro. This works, but only if the argument is not empty.

How must I change my macros, so that the call \mymacro{} works as expected, i.e. outputs "empty"? Note, that is is only a simplified version. In reality I have several checks in \mymacroii for the first character, if it is a number, an opening brace, and so on.

This question is for sure related to Why does TeX remove braces around delimited arguments?, but I couldn't apply those informations to my case.

Answer 1

\def\mymacro#1{\mymacroii#1{empty}\nil}
\def\mymacroii#1#2\nil{%
    #1%
}%

\mymacro{foo}% -> f

\mymacro{}
\bye

In your version, in the empty case, the inner call is

\mymacroii\nil

so #1 is \nil and then it starts looking for the next \nil (which isn't there) to make #2 and things go wrong.

Answer 2

Just check for emptyness first.

\long\def\firstoftwo#1#2{#1}
\long\def\secondoftwo#1#2{#2}
\def\mymacro#1{%
  \ifx\\#1\\%
    \expandafter\firstoftwo
  \else
    \expandafter\secondoftwo
  \fi
  {empty}%
  {\mymacroii#1\nil}%
}
\def\mymacroii#1#2\nil{#1}

Just to get the flavor with LaTeX3, here's a version that allows you to define a function that examines the first token of the argument provided it was not empty in the first place (in this case it would output empty). It works also in expanded definitions, as shown below.

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\mymacro}{ m }
 {
  \tl_if_empty:nTF { #1 }
   { empty }
   { \christoph_examine_first:f { \tl_head:n { #1 } } }
 }
\cs_new:Npn \christoph_examine_first:n #1
 {
  Whatever~you~want~with~#1~which~is~one~token
 }
\cs_generate_variant:Nn \christoph_examine_first:n { f }
\ExplSyntaxOff
\begin{document}

\mymacro{foo}

\mymacro{}

\edef\x{\mymacro{foo}\mymacro{}}\x

\end{document}