blank lines break xparse or ifthen macro

Question

Leaving blank line break simple macro but % terminating it fixes it. WTH is going on? I know one is suppose to use % BUT I can't have any blank lines in the macro without it complaining. A blank line essentially terminates the macro and TeX complains about a mixing }. Using etoolbox also produces the same problem so it seems it is an issue with xparse.

Is there any way to fix xparses package so I don't have to litter my code with %s?

\documentclass{book}

\usepackage{xparse, ifthen, etoolbox}


\DeclareDocumentCommand\iftb{m m G{\relax}}{\ifthenelse{\boolean{#1}}{#2}{#3}}

%\DeclareDocumentCommand\iftb{m m G{\relax}}{\ifbool{#1}{#2}{#3}}
\newboolean{A}\setboolean{A}{true}
\newboolean{B}\setboolean{B}{true}


\begin{document}

\iftb{A}{%
% <- remove all text on this line breaks macro (keep blank line)
    \iftb{B}{%
        asdffdsasdff
        % <- same here
        asdf
    }
}

End
\end{document}

Answer 1

Macros defined with \DeclareDocumentCommand don't accept \par (or empty lines, which is the same) in their arguments.

You can overcome this behavior by adding + in front of an argument specifier that can possibly contain a \par:

\NewDocumentCommand\iftb{m +m +G{\relax}}{\ifthenelse{\boolean{#1}}{#2}{#3}}

or

\NewDocumentCommand\iftb{m +m +G{\relax}}{\ifbool{#1}{#2}{#3}}

The macros on which \ifthenelse and \ifbool are built already accept \par.

---

The question why \par is not accepted in macro arguments comes from the fact that this is the default behavior for macros, as Knuth imagined them.

A macro defined with

\def\macro#1{...}

doesn't accept \par in its argument. Knuth thought that this can catch the common error of forgetting the closing brace and wanted to avoid that such a mistake could lead to reading in all the current file looking for the closing brace.

However a macro definition can be prefixed by \long: so

\long\def\macro#1{...}

allows empty lines in the argument of \macro.

LaTeX's \newcommand is of course built on \def and by default prefixes it with \long; the *-form \newcommand* doesn't.

The functions \NewDocumentCommand and \DeclareDocumentCommand (by the way, use the former, as it provides a check against redefinitions) take the \newcommand* approach, but in a slightly different way: each argument can be declared "long" or not (by default it isn't), while with \newcommand and \newcommand* all arguments are treated equal.