Is there a \return command to end the current macro

errorspackage-writing

Is there a \return statement in Latex to end the current macro from being processed?

\if some condition
  \PackageError{mypackage}{main text}{help text}
  \return
\fi

I just think it would be more elegant than putting the main function of the macro within an \else statement. Or what is standard practice among package writers?

Best Answer

A macro is not a function call, it is textual expansion. So if you have

\def\abc{111  \return I don't want 222}

and used

  aaa \abc\ bbb

then the first thing that happens is that \abc gets expanded into the input buffer so TeX has seen aaa already but then the input is

aaa
   111  \return I don't want 222\ bbb

So if you want your \return to skip everything from \return to the end of the replacement text you are responsible for adding some marker at that point.

Related Solutions

[Tex/LaTex] How to generate “You can’t use \unskip in vertical mode”

Looking at the source tex.web, I gather the following, none of which I am sure of. From the (separate) lines

primitive("unskip",remove_item,glue_node);

any_mode(remove_item): delete_last;

I see that \unskip invokes the procedure delete_last, which has the following comment in tex.web:

Like \lastbox, this command is not allowed in vertical mode (except internal vertical mode), since the current list in vertical mode is sent to the page builder. But if we happen to be able to implement it in vertical mode, we do.

In other words, TeX tries its best to remove something, but this is sometimes impossible. In fact, the code for delete_last "apologizes for [the] inability to do the operation now" if the current mode is vmode and (I believe) there are no recent contributions, unless \unskip follows a non-glue node (in which case \unskip does nothing). Your assessment that we need to find out when things are moved out of the recent contributions is thus correct.

The main vertical list is split into two parts: from page_head to page_tail lies the current page, and from contrib_head to contrib_tail lie recent contributions. Searching for contrib_, I find the variables used in several places:

in the procedure show_activities, link(contrib_head) is queried, to know if there are any recent contributions;
the procedure build_page appears to move material from the recent contributions to the current page, so it is crucial for us;
link(contrib_head):=link(p) is used in two places to add a node to the recent contributions;
q:=new_skip_param(top_skip_code) then link(contrib_head):=q adds the topskip to the page when the first box is added;
when breaking the page at a given node, whatever follows in the current page is placed before the recent contributions, by manipulating contrib_head and contrib_tail;
the default output routine appears to place the current page back into the recent contributions (except of course what has already been split and placed into \box255), then shipout \box255;
the same happens after a user-defined output routine (save for the \box255 business).

From this, we see that build_page is probably what you are after. It is called in the following cases (labelled with a letter corresponding to your list):

(a) when a paragraph is started in vmode (by a horizontal command such as \indent, a letter, \discretionary etc), after inserting \everypar into the input stream;
(b) after insering the \everydisplay token list in the input stream and having set various parameters upon entering display mode, if the surrounding mode is vmode;
(b) after going back to horizontal mode (and saving the language etc) after a display is finished, if the surrounding mode is vmode;
(a) after ending a paragraph with \par in horizontal mode if the resulting mode is vmode;
(c) when \halign ends in vmode;
(d) after inserting a penalty in vmode;
(d) within the procedure box_end, if the finished box (or box register, or...) is non-null and is to be added to the main vertical list (rather than stored, shipped out, or used as a leader);
(d) when an \insert or \vadjust ends with an end-group character, if the resulting mode is vmode;
(e) after TeX has finished a user's output routine, checked that \box255 is empty, and placed all the current page material into recent contributions (see above);
when encountering \par in vmode;
after inserting \hbox to \hsize{}\vfill\penalty-'10000000000 if there is still some material in the main vertical list when meeting the primitive \end.

A few differences crop up compared to your list. After a default output routine, build_page does not seem to be used. The case of \end is missing from your list. More importantly, the case of \par in unrestricted vertical mode is not explicitly given in your list (although (a) would fit it, I guess). This case explains why

XXX\par
\vskip 1cm
%
\unskip

fails upon removing the %. Contrarily to expectations, \par in vertical mode is not ignored: it moves material from the recent contributions to the current page, but also resets some parameters (\looseness, \hangindent, \hangafter and \parshape). This comes to me as a surprise.

I found one last thing. The procedure build_page is not used in append_glue because the procedure append_glue "is used in at least one place where that would be a mistake". I do not know where the mistake would be. But this explains why \vskip 1cm does not make TeX move any material to the current page.

[Tex/LaTex] carriage return symbol, new command

To visualize keystrokes you could use either the keystroke or the menukeys package.

\documentclass{article}
\pagestyle{empty}% for cropping
\usepackage{keystroke}
\usepackage{menukeys} 
\begin{document}
\verb|\Return| \Return % from package keystroke

\verb|\keys{\return}| \keys{\return} % from package menukeys
\end{document}

Best Answer

Related Solutions

[Tex/LaTex] How to generate “You can’t use \unskip in vertical mode”

[Tex/LaTex] carriage return symbol, new command

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