TeX won’t hyphenate a “word” if the word already contains a “discretionary”. As a consequence, compound words such as integro-differential and Rajamahendravaram--Visakhapatnam will not be hyphenated, since an empty discretionary was inserted by TeX after each explicit hyphen and dash.
I know how to trick TeX into hyphenating these compound words (there are plenty of similar questions on this site already, each with great answers). My question is about the kinds of items that should go into the horizontal list.
In TeX by Topic, Victor Eijkhout writes
\def\={\penalty10000 \hskip0pt -\penalty0 \hskip0pt\relax}
... integro\=differential equations...
and I don’t think I fully agree with this approach.
The \penalty0 is the most problematic: I think it should be \penalty\exhyphenpenalty (or it can simply be \penalty10000 , because there is already an empty discretionary in front of it?).
Secondly, the pre-hyphen sequence \penalty10000 \hskip0pt can be simplified to a single \kern0pt .
Lastly—since kern was mentioned—this approach doesn’t address implicit kerns around the hyphen and dashes. For example, the font could contain instructions that “o followed by - has positive kerning of 0.03em” and “en dash followed by V has negative kerning of 0.09em”.
In terms of horizontal list items, these are what I have come up with:
<first word>
<kern> % kerning between last letter of first word and hyphen/dashes
<hyphen/dashes>
<empty discretionary>
<penalty> % no break penalty, or explicit hyphen penalty?
<glue> % no stretch nor shrink, kerning between hyphen/dashes and first letter of second word
<second word>
Am I on the right track? Is there something that I have missed?
There seems to be some misconception that “a \nobreak following an explicit hyphen is wrong because it prohibits line break after this hyphen”. I disagree, and I hope the following plain TeX example will clear such misconception:
% plain TeX example
\hsize=24pc
\catcode`\@=11
\def\hyph{\kern\z@-\nobreak\hskip\z@skip}
% This allows both pre- and post-hyphen words to be hyphenated,
% and the \nobreak does not prohibit line break.
\noindent
Here is a rather complicated and hard-to-understand
integro\hyph differential equation. Maybe some experts
understand this integro\hyph differential equation.
And maybe some of them can solve this integro\hyph
differential equation.
\bigskip
\def\hyph{-\penalty\z@\hskip\z@skip}
% This does not allow pre-hyphen word to be hyphenated.
% Also, breaking at explicit hyphen results in zero penalty
% instead of \exhyphenpenalty being charged.
\noindent
Here is a rather complicated and hard-to-understand
integro\hyph differential equation. Maybe some experts
understand this integro\hyph differential equation.
And maybe some of them can solve this integro\hyph
differential equation.
\bye
More on \def\hyph{-\penalty\z@\hskip\z@skip}: Apart from the wrong penalty being charged (zero instead of \exhyphenpenalty), this also messes up the demerits calculation. Since the break now happens at the penalty, \doublehyphendemerits and \finalhyphendemerits will have no influence on this explicit hyphen, which is wrong. So, by this logic, even \penalty\exhyphenpenalty is incorrect, because break should not happen at a penalty! The only correct post-hyphen penalty is \nobreak.
\=' is intended to avoid the rule that says "don't consider hyphenating a word that already contains a hyphen."tugboat.clsdefines\def\hyph{-\penalty\z@\hskip\z@skip }` to use for this purpose. – barbara beeton Mar 10 '22 at 17:22\discretionary{}{}{}is automatically inserted. Knuth still calls this auto-inserted thing “a discretionary item” nonetheless. The problem with the post-explicit-hyphen code is the\penalty\z@part: zero penalty will be charged when breaking after the hyphen in\hyph, but an\exhyphenpenaltyshould be charged. – Ruixi Zhang Mar 10 '22 at 18:38\hyphwould be\def\hyph{-\penalty10000 \hskip\z@skip}or simply\def\hyph{-\nobreak\hskip\z@skip}. The\nobreakdoes NOT prohibit breaking after the hyphen—breaking can still happen at the auto-inserted empty discretionary and such a break will be charged a penalty of\exhyphenpenalty, as desired. – Ruixi Zhang Mar 10 '22 at 18:48\hyphwill only enable hyphenation in the post-hyphen word. The pre-hyphen word still won’t be hyphenated. So an even more “correct” definition would be\def\hyph{\kern\z@-\nobreak\hskip\z@skip}. And lastly, which was my third point in this post, what about the kerning around the hyphen/dashes? Do authors just writeRajamahendravaram\hyph\hskip-0.09em\relax Visakhapatnam? – Ruixi Zhang Mar 10 '22 at 19:07or\penalty10000following the hyphen is *always wrong*. (If used, it should always precede the hyphen; I'm not sure how to express this properly in Arabic and other RTL languages.) Since actual problems are almost always found when text is (nearly) final, adding\hyph` is (or should be) intentional, and it's probably not necessary to be picky. – barbara beeton Mar 10 '22 at 19:07\nobreakfollowing the explicit hyphen will be bypassed and will not prohibit line break. Line break happens at the empty discretionary between the explicit hyphen and the\nobreak, after which the\nobreakand the zero skip will disappear (being discardable). – Ruixi Zhang Mar 10 '22 at 19:13\nobreakafter the explicit hyphen, and I hope this example is finally convincing enough. – Ruixi Zhang Mar 10 '22 at 20:21