conditionalslatex-basemacros
I understand the definitions of the \makeatletter and \makeatother commands and also \def and \newcommand somewhat. But, explained in a simple way, what is happening in the following line?
\@ifnextchar[ \@myitem{\@noitemargtrue\@myitem[\@itemlabel]}}
Of Gonzalo Medina's answer at:
A slightly longer code, which allows you to only type \footnote. The idea is to make each footnote append \footnotetext{#1} to a token list. This token list is then typeset at the right place. Some trickery is required to get nesting right, as well as hyperref support. Normally, this can be used for other (non-verbatim-ish) environments.
Comments in the code.
\documentclass{article}
\usepackage{etoolbox} % for "\pretocmd".
\makeatletter
%
% User command, see the end of the preamble for use.
% We redefine the environment by adding "\savefoot@begin" at
% the beginning, and "\savefoot@end" at the end.
%
\newcommand{\DeclareSavefootEnvironment}[1]{%
\expandafter\pretocmd\csname #1\endcsname
{\savefoot@begin}{}{\savefoot@patch@error{#1}}%
\expandafter\pretocmd\csname end#1\endcsname
{\savefoot@end}{}{\savefoot@patch@error{#1}}%
}
\newcommand{\savefoot@patch@error}[1]{%
\PackageError{savefoot}{etoolbox failed to patch the environment #1, sorry}{}}
%
% Register allocation.
%
\newcount\c@savefoot@depth
\newtoks\savefoot@toks
%
% "\savefoot@begin" starts an environment which saves footnotes.
% We only want to output footnotes outside any such environment,
% so we count how deep we are in this kind of environment using
% the count "\c@savefoot@depth", which pretends to be a LaTeX
% counter.
%
% We will store all the "\footnotetext" into the toks
% "\savefoot@toks". This is initialized _locally_ at the start
% of each "savefoot" environment.
%
\newcommand{\savefoot@begin}{%
\advance\c@savefoot@depth by 1\relax
\savefoot@toks={}}
%
% When ending a savefoot environment, we check if it was the
% outermost one. In that case, "\@secondoftwo", we output the
% "\footnotetext" that we saved, after the end of the group.
% The "\endgroup" must be explicitly there; this is the case
% because of how "\end{...}" works in LaTeX2e.
%
% Since the register "\savefoot@toks" is restored at the end
% of the group, we need to expand it inside the group, then
% send "\savefoot@append{<expanded \savefoot@toks>}" after the
% "\endgroup", hence the "\expandafter"s. Same thing when we
% need to typeset.
%
\newcommand{\savefoot@end}{%
\ifnum\c@savefoot@depth>\@ne
\expandafter\@firstoftwo
\else
\expandafter\@secondoftwo
\fi
{% Nested, don't output, save to the level above.
\expandafter\do@after@explicit@endgroup\expandafter{%
\expandafter\savefoot@append\expandafter{\the\savefoot@toks}}%
}%
{% Outside, output.
\expandafter\do@after@explicit@endgroup\expandafter{%
\the\savefoot@toks}%
}%
}
%
% Appending to a toks is standard.
%
\newcommand{\savefoot@append}[1]{%
\savefoot@toks\expandafter{\the\savefoot@toks#1}}%
%
% To put after "\endgroup", we need delimited parameters,
% hence the "\def". We first check that the command is new,
% just in case.
%
\newcommand{\do@after@explicit@endgroup}{}
\def\do@after@explicit@endgroup#1#2\endgroup{#2\endgroup#1}
%
% Now the footnote command. We save the old definition,
% used in the case where we are outside any savefoot
% environment. If we are inside a savefoot environment,
% place the "\footnotemark", and append
%
% \savefoot@setcounter{<explicit number>}\footnotetext{<text>}
%
% to our register. This will be typeset at the end. We cannot
% just do "\footnotetext[<explicit number>]{<text>}", because
% the hyperref package does not like that.
%
\let\savefoot@old@footnote\footnote
\renewcommand{\footnote}[1]{%
\ifnum\c@savefoot@depth>\z@
\expandafter\@firstoftwo
\else
\expandafter\@secondoftwo
\fi
{% Inside a leftbar, save the text in the relevant toks.
\footnotemark
\savefoot@toks=\expandafter{%
\the\expandafter\savefoot@toks
\expandafter\savefoot@setcounter\expandafter{\the\c@footnote}%
\footnotetext{#1}%
}%
}%
{% Don't save, just typeset.
\savefoot@old@footnote{#1}%
}%
}
%
% When not using hyperref, just give the counter "footnote"
% the right value (in that case, "\savefoot@hhyperref@support{#1}"
% will be defined to do nothing).
%
\newcommand{\savefoot@setcounter}[1]{%
\setcounter{footnote}{#1}%
\savefoot@hyperref@support{#1}%
}
%
% Getting "\footnotetext" and "\footnotemark" to work with hyperref
% is non-trivial, and I probably did something wrong. But at first
% sight it seems ok.
%
\newcommand{\savefoot@hyperref@support}[1]{%
\setcounter{Hfootnote}{#1}%
\global \let \Hy@saved@currentHlabel \@currentHlabel
\global \let \Hy@saved@currentHref \@currentHref
\hyper@makecurrent {Hfootnote}%
\global \let \Hy@footnote@currentHlabel \@currentHlabel
\global \let \Hy@footnote@currentHref \@currentHref
\global \let \@currentHlabel \Hy@saved@currentHlabel
\global \let \@currentHref \Hy@saved@currentHref
}
%
% If hyperref is not loaded, we disable the hyperref support.
% We check at the "\begin{document}".
%
\AtBeginDocument{%
\@ifpackageloaded{hyperref}{}{\let\savefoot@hyperref@support\@gobble}%
}
\makeatother
% ============= Example =====================
\usepackage{framed}
\usepackage{hyperref}
\DeclareSavefootEnvironment{leftbar}
\begin{document}
Some text,\footnote{With a normal footnote.} in which
footnotes are preserved.\footnote{Even when you put several
in a row.}\footnote{Like this.}
\begin{leftbar}
Also true for the leftbar environment.\footnote{The relevant
commands are placed after the end of the environment.}
\begin{leftbar}
And we check that nesting works.\footnote{That was the tricky part\ldots}
\end{leftbar}
Maybe it breaks after nesting?\footnote{Let's see.}
\end{leftbar}
More footnotes.\footnote{Like this.}\footnote{And that.}
\begin{leftbar}
And another leftbar environment to be sure.\footnote{Does it
work? I'll see soon, when I compile.}
\end{leftbar}
\end{document}
If you want that the rule goes automatically on the next line if it can't be placed in the current one, just say
\newcommand{\blank}[2][100]{\hfil\penalty#1\hfilneg\rule[-3pt]{#2}{0.4pt}}
This is modeled on the \filbreak macro in the TeXbook.
The default penalty is 100, which discourages a line break. In cases of emergency you can insert a different one as optional argument:
\blank[0]{3cm}
Best Answer
\@ifnextcharis a LaTeX conditional that peeks ahead at the following character. So,\@ifnextchar[looks ahead to see if the following character in the input stream is a[(opening left bracket). If this is true, then it executes the immediately following token, otherwise, it skips it and executes the token following that.The first token, executed upon a true evaluation of
\@ifnextchar[is\@myitem. If the evaluation is false, it executes{\@noitemargtrue\@myitem[\@itemlabel]}}- the following group or "token", which also calls\@myitemnow with an optional argument set[\@itemlabel]. Before calling\@myitem, the boolean\if@noitemargis set to true (\@noitemargtrue).The general idea behind this expression is to condition on whether an optional argument is supplied or not, and therefore avoid an error if you pass arguments to a macro where the parameter text does not match what is given. Using the above example,
\@mymacrois defined usingSince this uses
\deffor the definition, the "optional" argument is actually not optional, but required. That's why the above usage is necessary. A more conventional way of conditioning is supplied usingwhere
<arg>specifies the value of the optional argument if it is not explicitly specified.xparseprovides similar conditioning that, for the upper-level user, might be more intuitive.