Btukfyl

I'm reading source2e.pdf and I have problems in understanding the sense of the definition of LaTeX 2ε's @esphack:

Commands like label{...} , which themselves do not/shall not produce any visible output, might in the source/in the .tex-input-file be surrounded by spaces. If so, you don't want two space-tokens as this would double the horizontal glue.

If I got it right, @bsphack and @esphack are there for avoiding the coming into being of a space-token after tokenizing and processing the command in case there already came one into being before tokenizing and processing the command.

So let's look at the definition of @bsphack:

> @bsphack=macro:

->relax ifhmode @savsk lastskip @savsf spacefactor fi .

l.1    show@bsphack

In words:

When in horizontal mode or in restricted horizontal mode, then save the value of lastskip to @savsk and save the value of spacefactor to @savsf.

Now let's look at the definition of @esphack:

> @esphack=macro:

->relax ifhmode spacefactor @savsf ifdim @savsk >z@ ifdim lastskip =z

@ nobreak hskip z@skip fi ignorespaces fi fi .

l.2    show@esphack

In words:

When in horizontal mode or in restricted horizontal mode, then:

• restore the value of spacefactor to the value saved as savsf.


• In case @savsk is larger than zero, i.e., in case there was some horizontal glue before carrying out the command which at its start called @bsphack, then do
  ifdim lastskip =z@ nobreak hskip z@skip fi ignorespaces
  .

I understand that ignorespaces will make sense as there was already some horizontal glue before carrying out the command which at its start called @bsphack.

But I don't understand what the
ifdim lastskip =z@ nobreak hskip z@skip fi
-part is good for.

What is the sense of this?

In the situation where this is carried out, the saved value in @savsk is larger than z@, thus one can conclude that something from within the command which at its start called @bsphack did change the value of lastskip to z@.

But doing something like nobreakhskipz@skip will not revert that change/will not restore lastskip to its previous value.

Thus: What is the gist/sense/benefit of performing that hskip of zero-width?

If you wish lastskip to be restored, shouldn't it then be something like:

def@esphack{%

  relax

  ifhmode

    spacefactor@savsf

    ifdim@savsk>z@

      ifdimlastskip=z@

        nobreak

        hskip-@savsk

        nobreak

        hskip@savsk

        % the total skip is zero and lastskip is restored.

      fi

      ignorespaces

    fi

  fi

}%

Or perhaps just

def@esphack{%

  relax

  ifhmode

    spacefactor@savsf %now spacefactor is restored.

    nobreak

    hskip-@savsk

    nobreak

    hskip@savsk

    % the total skip is zero and lastskip is restored.

    ifdim@savsk>z@

      ignorespaces

    fi

  fi

}%

Is it possible that in some previous release, it was done this way and in later releases somebody erroneously "optimized" the
nobreakhskip-@savsknobreakhskip@savsk to nobreakhskipz@skip, overlooking that this won't restore lastskip any more?

If I understand correctly, you need to restore lastskip correctly as otherwise things with consecutive sequences of @bsphack..@esphack, e.g., label{foo}label{bar} won't work correctly:

If by now you do something like
A label{foo} label{bar} A

or
A label{foo}label{bar} A

, lastskip will in any case be 0 after label{foo} which affects the behavior of label{bar} because with label{bar} @savsk will not be larger than z@ any more so that ignorespaces won't get carried out although it should be carried out!

With the example below you can see a subtle difference when redefining @esphack so that it restores lastskip:

documentclass{article}



newsaveboxmybox



begin{document}



parnoindentbegin{lrbox}{mybox}

verb*|Alabel{1}B|:

end{lrbox}hbox to 4.5cm{useboxmyboxhfil} Alabel{1}B



parnoindentbegin{lrbox}{mybox}

verb*|A label{2} B|:

end{lrbox}hbox to 4.5cm{useboxmyboxhfil} A label{2} B



parnoindentbegin{lrbox}{mybox}

verb*|A label{3}label{4} B|:

end{lrbox}hbox to 4.5cm{useboxmyboxhfil} A label{3}label{4} B



parnoindentbegin{lrbox}{mybox}

verb*|  label{5} label{6} B|:

end{lrbox}hbox to 4.5cm{useboxmyboxhfil}  A label{5} label{6} B



makeatletter

def@esphack{%

  relax

  ifhmode

    spacefactor@savsf

    ifdim@savsk>z@

      ifdimlastskip=z@

        nobreak

        hskip-@savsk

        nobreak

        hskip@savsk

        % the total skip is zero and lastskip is restored.

      fi

      ignorespaces

    fi

  fi

}%

makeatother



noindentnullhrulefillnull



parnoindentbegin{lrbox}{mybox}

verb*|Alabel{a}B|:

end{lrbox}hbox to 4.5cm{useboxmyboxhfil} Alabel{a}B



parnoindentbegin{lrbox}{mybox}

verb*|A label{b} B|:

end{lrbox}hbox to 4.5cm{useboxmyboxhfil} A label{b} B



parnoindentbegin{lrbox}{mybox}

verb*|A label{c}label{d} B|:

end{lrbox}hbox to 4.5cm{useboxmyboxhfil} A label{c}label{d} B



parnoindentbegin{lrbox}{mybox}

verb*|A label{e} label{f} B|:

end{lrbox}hbox to 4.5cm{useboxmyboxhfil}  A label{e} label{f} B



end{document}

Does my modification of @esphack have drawbacks?

Am I overlooking some caveat/something relevant?

You see more change comments, and older versions in the source:

% begin{macro}{@esphack}

%   Companion to |@bsphack|.  If this command is not properly paired

%   with |@bsphack| one might end up with a low-level TeX{} error:

%   ``BAD spacefactor''. One possible cause is calling |@bsphack| in

%   vertical mode, then doing something that gets you (sometimes) into

%   horizontal mode and finally calling |@esphack|. Even if no error

%   is generated that is wrong, because |@esphack| will then use the

%   saved values for |@savsk| and |@savsf| from some earlier

%   invocation of |@bsphack| which will have nothing to do with the

%   current situation.

% changes{v1.3d}{2015/01/11}{Allow hyphenation (Donald Arseneau pr/3498) (latexrelease)}

%    begin{macrocode}

%</2ekernel>

%<latexrelease>IncludeInRelease{2018/10/10}%

%<latexrelease>                 {@esphack}{hyphenation and nobreak after space hack}%

%<*2ekernel|latexrelease>

def@esphack{%

  relax

  ifhmode

    spacefactor@savsf

    ifdim@savsk>z@

%    end{macrocode}

% changes{v1.3f}{2015/11/07}

%         {Only space if there is no space at the end of the hlist latex/4443}

%    begin{macrocode}

      ifdimlastskip=z@ 

        nobreak hskipz@skip

      fi

      ignorespaces

    fi

%    end{macrocode}

% changes{v1.3i}{2018/10/10}

%         {Don't introduce breakpoints if @nobreak is true and after sections}

%    begin{macrocode}

  else

    ifvmode

      if@nobreaknobreakelseif@noskipsecnobreakfifi

    fi

%    end{macrocode}

%

%    begin{macrocode}

  fi}%

%</2ekernel|latexrelease>

%<latexrelease>EndIncludeInRelease

%<latexrelease>IncludeInRelease{2015/10/01}%

%<latexrelease>                 {@esphack}{hyphenation and nobreak after space hack}%

%<latexrelease>def@esphack{%

%<latexrelease>  relax

%<latexrelease>  ifhmode

%<latexrelease>    spacefactor@savsf

%<latexrelease>    ifdim@savsk>z@

%<latexrelease>      ifdimlastskip=z@ 

%<latexrelease>        nobreak hskipz@skip

%<latexrelease>      fi

%<latexrelease>      ignorespaces

%<latexrelease>    fi

%<latexrelease>  fi}%

%<latexrelease>EndIncludeInRelease

%<latexrelease>IncludeInRelease{2015/01/01}%

%<latexrelease>                 {@esphack}{hyphenation and nobreak after space hack}%

%<latexrelease>def@esphack{%

%<latexrelease>  relax

%<latexrelease>  ifhmode

%<latexrelease>    spacefactor@savsf

%<latexrelease>    ifdim@savsk>z@

%<latexrelease>      nobreak hskipz@skip

%<latexrelease>      ignorespaces

%<latexrelease>    fi

%<latexrelease>  fi}%

%<latexrelease>EndIncludeInRelease

%<latexrelease>IncludeInRelease{0000/00/00}%

%<latexrelease>                 {@esphack}{hyphenation and nobreak after space hack}%

%<latexrelease>def@esphack{%

%<latexrelease>  relax

%<latexrelease>  ifhmode

%<latexrelease>    spacefactor@savsf

%<latexrelease>    ifdim@savsk>z@

%<latexrelease>      ignorespaces

%<latexrelease>    fi

%<latexrelease>  fi}%

%<latexrelease>EndIncludeInRelease

%<*2ekernel>

%    end{macrocode}

% end{macro}

Basically the bit you query, you need to add a space (or zero width) so as not to inhibit hyphenation, and then need to add the nobreak so the space doesn't introduce a breakpoint.