\newif conditional causing problem in \typeout or \edef

Question

The following code produces result as expected, i.e.,

False True

on the typeset document.

\documentclass{minimal}

\newif\iftest

\newcommand{\test}[2]{
\iftest
  #1
\else
  #2
\fi
}
\newcommand{\ex}{\test{True}{False}\testtrue\test{True}{False}}

\begin{document}
\ex
\end{document}

However, if I wrap \ex with \typeout, or try to \edef\ex instead of \newcommand I get:

! Incomplete \iffalse; all text was ignored after line...

Answer 1

The problem here is that \testtrue is not expandable and not protected and therefore fragile. It is defined as \let\iftrue\iftrue. In an expandable context like \edef or \typeout the \let assignment is ignored, and both if-switches are expanded. Because there is only one \fi (which is taken as part of \iftrue) the compiler complains about the missing \fi.

You can use \test{<true>}{<false>} in this context, but not \testtrue or \testfalse. So you would need to do the following:

\typeset{\test{True}{False}}
\testtrue
\typeset{\test{True}{False}}

---

I like to give you one more advice. I would define the \test macro as follows:

\makeatletter
\newcommand{\test}{%
  \iftest
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
}
\makeatother

This way the if-statement is fully processed before one of the two arguments is processed. This avoid some issues with can arise when code is used inside and if-switch. For example defining a new if-switch with \newif and using it inside such an argument will break, if that argument is to be skipped. The \@firstoftwo and \@secondoftwo macros simply take two arguments and expand to either the first or the second. The \expandafter expands (i.e. removes, in this case) the \else or \fi first. Also note the % after the { which is required to avoid a space being added there by the line break.