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LATEX-L  March 1997

LATEX-L March 1997

Subject:

Re: Shortref mechanism

From:

Frank Mittelbach <[log in to unmask]>

Reply-To:

Mailing list for the LaTeX3 project <[log in to unmask]>

Date:

Sun, 2 Mar 1997 18:30:12 +0100

Content-Type:

text/plain

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text/plain (85 lines)

Hans Aberg writes:
 > Frank Mittelbach <[log in to unmask]> wrote:
 >
 > >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.
 >
 >   I cannot follow the details in your reasoning, but I can note that with
 > deterministic parsing, the method generally used in LaTeX, conditional
 > parsing have such limits.

well this is a fact of the way TeX behaves, just try

  f\x i

and define \x to contain a non expandable primitive (such as
\def\x{\let\foo\bar} ) and you will find that even though \x does not
produce any output it has nevertheless suppressed the ligature between
f and i. Thus when one tries to peek at the following token (and the
only way to do this is using \futurelet unless one swallows the token
using arguments) this side-effect will be the result.

the only way to trick TeX into regenerating the ligature (or the kern)
is to force a hyphenation pass of the paragraph. however this isn't
any good either for two reasons

 a) it will only work for words that TeX considers worth hyphenating
    (which is a subset of all words depending on the settings of
    \lefthyphenmin, etc)

 b) it will not work for restricted horizontal text, eg \hbox{f\x fi}

 >   But with non-deterministic parsing more general things can be done:
 >   For example, I just made a definition command that can produce commands
 > having optional arguments; in this general approach, I had to switch from
 > LaTeX style deterministic parsing to non-deterministic parsing.

i fear that you do underestimate certain restrictions (like the above)
posed TeX's highly efficient but somewhat inflexible parser.

 >   A command then picks down a look-ahead portion, like to the next space
 >     \def\foo#1 {<def text> } %% Putting back the space consumed by the argument.
 > It then examines this argument, and then has the opportunity of rejecting
 > whatever non-parsed stuff unaffected, by returning (a part of) the
 > _original_ look-ahead portion. Certainly, spaces do count in this approach.
 > If the the only problem is the next character (like a brace) after the
 > command or active character, perhaps you can combine, doing a simple
 > one-token look-ahead, before the argument pickup; otherwise, TeX requires
 > arguments to not have an extra "}", so one cannot enclose such a command in
 > braces without at least one space occurs within those braces. (So there
 > must be a look-ahead token that you can ensure to be on the same group
 > level as the command.)

again that single token lookahead is the killing factor. there is only
one solution that would avoid that kind of deadlock which is to
abandom TeX's parsing mechanism completely and writing your own parser
within TeX (eg by making braces being ordinary characters etc). that
is possible although incredibly slow even on fast machines and to do
more than just a prove of concept type implementation you get
dangerously close to proving that TeX is Turing complete. --- i have
implemented some way back an SGML interpreter within TeX that was
reading and interpreting DTD's (although only restricted) as well so i
think i do know what i'm talking about. In other words this is not
something that i do seriously consider as an adequate use of TeX the
program.

 >   Actually, what is needed, is a simple extension of the TeX \def command:
 >     \def\name<parameter text>{definition text}[exception text]
 > If a command somehow is unable to process the parameter text, then the
 > original input text (before the parsing of the parameter text began) is
 > left unaffected, and the execution passes to the exception text.
 >   Then stuff like optional commands could be implemented most easily, and
 > perhaps the stuff you are asking for, too.

well yes, if we would/could modify TeX's parser on the program level
you certainly could do better. but we are not talking here about
proposals for NTS or Omega functionality, in other words this is
unfortunately irrelevant within the TeX framework.

however, perhaps some of the Omega/NTS/e-TeX people listening in might
find this general idea useful. Nevertheless i would prefer if we stop
here and don't start a TeX extension discussions as this is not the
right list for this type of topic.

frank

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