\IfBlankF in \NewDocumentCommand removes content leading up to argument

Question

I'm trying to define a command with \NewDocumentCommand that create a subsection with a given name (mandatory) and optionally some descriptive paragraph. Since I don't want these paragraph to be printed out when they're empty, I've tried taking advantage of the \IfBlankF functionality by excluding the paragraph altogether if their respective content is empty.

The command I've defined is as follows:

\documentclass{article}

\begin{document}
    \NewDocumentCommand{\newrule}{ m O{} O{} O{} O{} }{ 
        \subsubsection{#1}
        \IfBlankF{#2}{
            \paragraph{Addendum}
            #2
        }
        \IfBlankF{#3}{
            \paragraph{Rationale}
            #3
        }
        \IfBlankF{#4}{
            \paragraph{Examples}
            #4
        }
        \IfBlankF{#5}{
            \paragraph{Exceptions}
            #5
        }
    }

    \section{Section}
    \subsection{Subsection}
    \newrule{Rule 1}{blah blah}{}{}{}
 
\end{document}

This works almost as intended, except that whenever the arguments are non empty, the paragraph name leading up to them are not present. The same happens with any other kind of content replacing the \paragraph{...} part.

However, if I invert the behavior by e.g. writing

\IfBlankF{#2}{
    \paragraph{Addendum} abcdef
    #2
}

both the paragraph names and subsequent content works. What am I doing wrong?

Answer 1

You specified four optional arguments that should go in brackets [].

Anyway, I don't think it's a good interface.

\documentclass{article}

\NewDocumentCommand{\newrule}{ m O{} O{} O{} O{} }{%
    \subsubsection{#1}
    \IfBlankF{#2}{%
        \paragraph{Addendum}
        #2%
    }
    \IfBlankF{#3}{%
        \paragraph{Rationale}
        #3%
    }
    \IfBlankF{#4}{%
        \paragraph{Examples}
        #4%
    }
    \IfBlankF{#5}{%
        \paragraph{Exceptions}
        #5%
    }%
}

\begin{document}

\section{Section}

\subsection{Subsection}

\newrule{Rule 1}[blah blah]
 
\newrule{Rule 2}[][blah blah]
 
\newrule{Rule 3}[][][blah blah]
 
\newrule{Rule 4}[][][][blah blah]
 
\end{document}

I'd use a key-value interface:

\documentclass{article}

\ExplSyntaxOn

\keys_define:nn { brusamento/rule }
 {
  addendum .tl_set:N = \l_brusamento_rule_addendum_tl,
  rationale .tl_set:N = \l_brusamento_rule_rationale_tl,
  examples .tl_set:N = \l_brusamento_rule_examples_tl,
  exceptions .tl_set:N = \l_brusamento_rule_exceptions_tl,
 }

\NewDocumentCommand{\newrule}{ m O{} }
 {
  \subsubsection{#1}
  \group_begin: % keep the keys local
  \keys_set:nn { brusamento/rule } { #2 }
  \tl_if_blank:VF \l_brusamento_rule_addendum_tl
   {
    \paragraph{Addendum}
    \tl_use:N \l_brusamento_rule_addendum_tl
   }
  \tl_if_blank:VF \l_brusamento_rule_rationale_tl
   {
    \paragraph{Rationale}
    \tl_use:N \l_brusamento_rule_rationale_tl
   }
  \tl_if_blank:VF \l_brusamento_rule_examples_tl
   {
    \paragraph{Examples}
    \tl_use:N \l_brusamento_rule_examples_tl
    }
  \tl_if_blank:VF \l_brusamento_rule_exceptions_tl
   {
    \paragraph{Exceptions}
    \tl_use:N \l_brusamento_rule_exceptions_tl
   }
  \group_end:
}

\ExplSyntaxOff

\begin{document}

\section{Section}

\subsection{Subsection}

\newrule{Rule 1}[addendum=blah blah]
 
\newrule{Rule 2}[rationale=blah blah]
 
\newrule{Rule 3}[examples=blah blah]
 
\newrule{Rule 4}[exceptions=blah blah]

\end{document}