Passing text containing linebreaks as command or environment parameter?

Question

I just tried to write some commands which used its argument to check whether they were empty (isempty{...} from package ifthen) or not. The problem was that the text to check contained a linebreak, and that screwed up the whole system and caused it to produce very strange error messages.

Is there a way to work around this and make it safe to pass text containing linebreaks as parameter to a command or environment?

Answer 1

The etextools package provides an expandable \ifempty{<text>}{<true>}{<false>} macro which works with newlines.

If you don't need the test to be expandable you can also use the following code:

\newcommand{\mymacro}[1]{%
   \begingroup
   \def\temp{#1}%
   \ifx\temp\empty
      \endgroup
       % empty
   \else
      \endgroup
       % not empty
   \fi
}

Answer 2

The \isempty test expands its argument; you may try

\ifthenelse{\isempty{\unexpanded{#1}}}{<true>}{<false>}

or resort to a different test like

\ifx\relax\detokenize{#1}\relax
  <true>%
\else
  <false>%
\fi

In both cases #1 stands for the argument you want to test.