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Frank Mittelbach <[log in to unmask]>
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Mailing list for the LaTeX3 project <[log in to unmask]>
Sat, 1 Mar 1997 18:56:19 +0100
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As a special subject related (or currently related at least) to the
language discussion we have the syntax and semantics of shortrefs.

a couple of thoughts are written down below and i would be interested
in hearing your comments and your own thoughts on that subject


\section{Short references}

With the development of language packages and later on within the
Babel system it became common practise to extend the markup language
of \LaTeX{} using so called ``shortrefs'' as a compact method for
inputting certain commands. Shortrefs are character sequences that do
not start with \TeX's escape character, i.e., usually `|\|', but
nevertheless act like commands. That is, they do not represent the
equivalent glyph sequence but have either additional side effects
(like the punctuation marks in french typography producing additional
space) or even denote completely different actions (e.g., |""|
for a break point without a hyphen).

In addition to the above shortrefs some \TeX{} fonts implement shortrefs
by using (or misusing) the ligature mechanism to implement input
arbitrary input syntax, e.g., |``| generating `` or |---| generating
an emdash.

\subsection{Possible application areas}

Short references can be used for different purposes:
  providing a compact input notation for commonly used textual
  commands such as characters with diacritical marks
  providing a compact and readable input notation for special
  applications, e.g., |==>| for |\Longrightarrow|
  providing typographical features not otherwise supported (e.g.,
  extra space in front of punctuation characters)
The first two items are related to input syntax and not directly
linked to the language of the current text although historically they
have been provided by language packages, e.g., |"a| as a shortref for
|\"{a}| was implemented by |german.sty| and within Babel its meaning
gets deactivated within regions marked up as belonging to other

The third item is language related, e.g., used to implement a certain
typography style without forcing the user to mark up its document


Within the framework of \TeX{} shortrefs have to be implemented by
making the first character in the sequence active (which means that
this character alone already behaves like a command). To implement
shortref sequences a clever mechanism has to be introduced that
somehow looks ahead to determine which of the following characters (if
any) still belong to the sequence and the after determining that
launching the associated action.

This approach can have undesired side effects with other \TeX{}
typesetting actions. If the implemented mechanism for determining the
shortref sequence is not fully expandable it will prevent \TeX{} from
inserting ligature or kerning information between preceding characters
and characters produced by the action finally launched, e.g., if
|"a| is a shortref to produce the single glyph ``\"a'' then a
word like |F"alle| will show no ligature between ``F'' and ``\"a''
even if the font contains such a ligature.

On the other hand the disadvantage of using a fully expandable
shortref mechanism is that shortref sequences have to be assembled
using arguments which means that spaces between shortref chars will
not be significant and shortref characters at the end of a brace group
might produce nasty \TeX{} errors.


The above means that the shortref mechanism either has to be fully
expandable or that one can't use it to produce glyphs that might play
a part in ligature or kerning tables. Since Babel 3.6 this mechanism
isn't any longer expandable which poses a serious problem for several
language packages using this mechanism.

It is questionable if any shortref mechanism should be directly linked
to language tags even if the use of shortrefs might be traditionally
linked to certain languages. In other words if a document is mainly in
German shortrefs for producing German umlauts should probably be still
available within quotations in other languages. Even if the usual
meaning of a shortref sequence differs for two languages it seems
advisable to at least make it customizable whether or not the switch
from one language to the other affects the current set of shortref

For shortrefs that implement typographical features the same applies
since for a document written in, say, French, the designer might
reasonably decide that these conventions are implemented throughout
the document even for portions written in other languages (such as