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Since this list is currently in one of its active episodes, and
questions are actually being answered, I thought maybe I should reraise
some issues regarding the evolution of xparse that I put forth in
February (according to my mailbox) but didn't get much of a response
to. If memory serves, Frank wrote he wanted 3 weeks to think about it
(the time I had taken to respond to a posting of his), but now it's
been more like 30 weeks, and Frank is obviously not the only one around...

But no need to go digging through mailboxes, just read on:

PART I: Syntax

I now have a working implementation (for LaTeX2e) of an argspec based
\NewDocumentCommand, which seems to work very well. The current code
can be found at

http://abel.math.umu.se/~lars/xdoc/xdoc2l3.dtx
http://abel.math.umu.se/~lars/xdoc/xdoc2l3.ins

and an example document demonstrating various argspecs is

http://abel.math.umu.se/~lars/xdoc/xdoc2l3test.dvi

When I started coding this, I though I needed to stick very close to
the final xparse (a somewhat difficult task, considering xparse is
still experimental) -- hence the use of \NewDocumentCommand as command
name -- but as time went on I've drifted more towards the opinion that
it will be sufficient to have a path for manual migration to xparse. In
other words, I'd like the basic features to be available (in the core
or after additional definitions), but it's no big deal if the names and
details in syntax are different.

[What could maybe be an issue for you is that I'm considering splitting
off generic parts of xdoc2 into separate packages -- like shortvrb is
from doc -- and one of these would then be the referenced xdoc2l3:
select LaTeX3(ish) features for xdoc2. If for some reason it would
become popular, then there could be a corresponding pressure for
backward compatibility, regarding for example \IfNoValueTF. ;-) ]

Anyway...
Since I wanted to stay close to the xparse syntax, I stuck with the
idea of having each argument type be denoted by a single character,
followed by zero or more arguments. Since the main new argument type I
introduced was "composition of processors", I chose the syntax

@{<processors>}

for this; @ happens to be the function composition operation (\circ) in
at least Maple (although you'll probably see that it isn't so much the
composition that is going to be characteristic of this specifier type,
so I should probably rethink this). Like functions under composition,
the processors are going to act on the argument in order right to left;
this turned out to simplify the translation of these specifiers into
actual code.

For the individual processors, I again chose the "single character
followed by zero or more arguments" syntax, with the choice of
character being influenced by xparse argument specifiers. Thus there is
for example

O{<default>}     Look ahead for a left bracket. If there is one, grab
the entire optional argument and place it within
braces. Otherwise insert <default> within braces.

o                Look ahead for a left bracket. If there is one, grab
the entire optional argument and place it within
braces. Otherwise insert \NoValue and skip all
following processors.

S{<char>}        Look ahead for a <char>. If there is one, gobble it
and insert \BooleanTrue, otherwise insert
\BooleanFalse.

g{<assignments>} Make the specified <assignments> (usually to
\catcode or other parameters relevant for scanning).

With these, I can implement several (existing or proposed) xparse
argument specifiers as mere shorthands for @{...} constructions:

m                is  @{}  (no processors, just grab an argument)
o                is  @{o}
O{<default>}     is  @{O{<default>}}
S{<symbol>}      is  @{S{<symbol>}}
s                is  @{S{*}}
g{<assignments>} is  @{g{<assignments>}}

where the last was implemented by Morten in his mail of December 13,
2007. Listed as above, this doesn't look too impressive, but the fun
begins when one can start composing processors. With only the above,
there aren't all that many combinations that make sense, but one that

@{ o g{\@sanitize} }

---first change catcodes (as in the argument of \index), then look for
and grab an optional argument while these catcodes are in effect.

To really see the power of this, it is however necessary to have some
processors which do things to mandatory arguments. Those that there are
implementations for in the code above are:

h                         convert argument to a "harmless character
sequence", i.e., any problematic character
is encoded as \PrintChar{<ASCII code>}

t                         argument is \stringed

x{<bool>}{<pre>}{<post>}  argument gets expanded; more precisely, if
<bool> is true then
<pre>{<argument>}<post>
gets expanded and otherwise
<pre><argument><post>
gets expanded

.{intrange}{<min>}{<max>} Argument must be some <number>, at least
<min> and at most <max>.

The . here is syntactic sugar for "multiletter processor name follows";
as with xparse specifiers currently, it would be possible to write
{intrange}{<min>}{<max>} instead, but having an unbraced character at
the beginning of a processor specifier makes it easier to read.

With the above, Morten's d specifier (grab and \detokenize) would be a
shorthand for

@{ x{\BooleanTrue}{\detokenize}{} }

and it would have the optional counterpart

@{ x{\BooleanTrue}{\detokenize}{} o }

or it could be combined with catcode changes as

@{ x{\BooleanTrue}{\detokenize}{} g{\catcode\%=12} }

and of course combined all three, into

@{ x{\BooleanTrue}{\detokenize}{} o g{\catcode\%=12} }

However, \detokenize isn't satisfactory for my needs; the main problem
being that detokenized material cannot be written to a file (in
particular .idx/.glo file) and then reliably read back. The "h"
processor above employs xdoc2's more robust (and powerful) alternative
of harmless character sequences. The reason I implemented this x
processor in the first place was instead that I had a legacy grabber
\XD@grab@harmless@asmacro which can be specified as

@{
x{\BooleanFalse}{\XD@unbackslash}{\@empty}
h
g{\catcode92=12\MakePrivateLetters}
}

using the above; the x part here removes a leading backslash in the
argument, and the \@empty is there in case the argument was empty.

Additional examples of grabbers in xdoc2 which can be expressed using
this syntax are

\XD@grab@harmless@oarg  as  @{ho}
\XD@grab@harmless@cs    as
@{h t g{\MakePrivateLetters\escapechar=-1}}

(see table in xdoc2l3.dtx).

Finally, I also implemented a argument specifier = which stores the
argument into a macro. Its syntax is

={<control sequence>}{<processors>}

but since the <processors> part is just as for the @ specifier, the
composition turned out to not be what was characteristic for this. I
considered renaming @ to #, but decided not to (for now) as that is
likely to lead to quoting problems in real life usage. And maybe this
list can come up with better names anyway.

It probably wouldn't be hard to convert \DeclareArgumentType from
defining an argument type specifier to defining an argument processor
specifier---concretely one that takes some fancy sort of argument and
converts it to a more regular mandatory argument. IMHO defining a
processor would be much better, at it allows for independent control of
orthogonal aspects of argument processing. I didn't implement anything
in that area though, as I don't have any immediate need for exotic
argument delimiters.

It may however be even better to construct argument specifiers as
defined by \DeclareArgumentType as a composition of two argument
processors: one that looks ahead for an optional argument and one which
actually grabs it. The reason is demonstrated by \testI in the example
document; concretely the problem is that in

\NewDocumentCommand{\test}{ @{ O{world} g{\catcode\%=12} } }{%
Hello, #1!%
}
\test % Is this a comment?
\relax

the % gets tokenized already when looking for the left bracket
delimiter of the optional argument, and it stays a token even after it
has been determined that there wasn't such an argument.

If instead Q was a processor that does everything of O short of
grabbing the argument, and b was a processor that just grabs a
bracket-delimited argument, then the above specifier could have been

\NewDocumentCommand{\test}{ @{ b g{\catcode\%=12} Q{world} } }{%
Hello, #1!%
}

instead, for which things work more as expected:

\test % This is a comment!
\test[% This is not a comment!]

Splitting the two provides higher orthogonality (I find the long
argument sequence of \DeclareArgumentType bewildering, and I don't
think it is only a matter of lack of documentation), but it could make
it harder to produce good error messages.

PART II: Technical issues

This mail discusses details of the implementation of the xdoc2l3
\NewDocumentCommand command, and issues with these.

First a bit of context: In my old code, all argument grabbers were
being expanded in the context

<grabber> <subsequent grabbers> \XD@endgrab <next argument> ...

A grabber would do whatever it had to do to grab the <next argument>,
and was then supposed to append it to \toks@; thereafter the
thing, but has an explicit \toks_use:N\l_xparse_grabbed_args_toks
instead of the \XD@endgrab macro and uses \l_xparse_grabbed_args_toks
as the token marking the beginning of arguments.

Implementing processors in this context was actually very easy, as all
they had to do was to grab the <next argument>, do something to it, and
then put the result back as a mandatory argument after \XD@endgrab,
where the next processor would expect it. I'm a bit worried that the
xparse construction, exposing the implementation detail of
\l_xparse_grabbed_args_toks, in the long run will be restrictive;
processors don't need to know the name of the token register (or even
that there is one), but they do need to know the name of the mark token.

*Example*  The x{<bool>}{<pre>}{<post>} specifier compiles into the
processor \XD@gp@expanded{<bool>}{<pre>}{<post>}, which is simply
defined as

\def\XD@gp@expanded#1#2#3#4\XD@endgrab#5{%
\protected@edef\@tempa{{%
\IfBooleanTF{#1}{#2{#5}#3}{#2#5#3}%
}}%
\expandafter\XD@gh@put \expandafter{\@tempa}{#4}%
}
\def\XD@gh@put#1#2{#2\XD@endgrab#1} % Generic helper macro

---shovelling things around a bit, but quite straightforward. The
(possibly less roundabout) approach of not throwing a processed
argument back to the other side of the \XD@endgrab marker has the
disadvantage that it forces a distinction between different types of
processors and thus increases the complexity of the system.

Two things in xparse which I didn't implement are the P and W
specifiers. In the case of W this is mostly because the LaTeX2e
\@ifnextchar gobbles spaces, and I didn't feel like rolling my own; I
have however encountered cases where I would have wanted an "o"
specifier that didn't gobble spaces, so I certainly see the need for
it. In the case of P, I was simply lazy; I don't think xdoc will need
\long arguments anyway, and they increase the complexity quite
considerably.

However, while thinking about the matter, it occurred to me that the
status for some grabber/processor of being non-\long implies that one
cannot use \par in the <default> for an O argument, even if that
argument itself is \long; the reason being that such a \par token then
appears within the <subsequent grabbers>. This issue should at least be
remarked upon in xparse.dtx.

One *new* thing I did in this implementation is that I used groups to
isolate processing of different arguments and also the argument
grabbing procedure as a whole. Concretely the latter was just the
matter of changing the defitions

\def\XD@grab@arguments#1#2{%
\toks@={#1}%
#2\XD@endgrab
}
\def\XD@endgrab{\the\toks@}

(as found in current xdoc2) to

\def\XD@grab@arguments#1#2{%
\bgroup
\toks@={#1}%
#2\XD@endgrab
}
\def\XD@endgrab{\expandafter\egroup \the\toks@}

As for the per-argument groups, if you look at xdoc2l3test.dvi you'll
see that each argument grabber begins with an explicit \bgroup, whereas
the matching \egroup is hidden in the ending \XD@ge@parameter or
affect tokenisation and be possible to combine with optional-argument
type processors requires that things are done this way. There are
however at least two problems with doing it in general:

1. If there is an error when TeX is grabbing arguments, then the
macro simply disappears, along with all text grabbed so far. Such
errors arise easily as typos, and would in this case cause the
closing \egroups to disappear, throwing off the nesting of groups.

Seeking to limit the damage caused by this, I chose to use \bgroup and
\egroup so that at least the nest end of an environment would get
things back on track, but that causes a new problem.

2. In math mode, \bgroup\egroup is far from invisible.

For xdoc this is acceptable, since the commands defined are anyway
mostly for use in vertical or horizontal mode, but xparse should be
able to define general commands and thus cannot accept this.

\begin{TeX-extension-idea}
It occurs to me that it would be useful to have groups that nest even
weaker than \bgroup-\egroup, and which do not contribute anything to
math lists.
\end{TeX-extension-idea}

The existing alternative is to use \begingroup and \endgroup, but then
one must guard against getting these mismatched. I have one idea for
this, but I haven't tried it out yet:

\def\XD@gh@insure#1{%
\let\reserved@a=#1%
\expandafter\futurelet \expandafter\@let@token
\expandafter\XD@gh@insure@
}
\def\XD@gh@insure@{%
\ifx \reserved@a\@let@token \else \egroup\egroup \fi
}

The idea is to instead of a naked macro (like \XD@gp@expanded) that
processes material after \XD@endgrab write

\XD@gh@insure{\protected@edef}\XD@gp@expanded

where \protected@edef is the first token in the expected expansion of
expansion of \XD@gp@expanded failed, and it is necessary to insert some
emergency \egroups to get the nesting back to normal.

3. Adding groups makes the command non-mouth-expandable.
In particular, it will throw TeX out of the inner loop
(disrupt kerning etc.)

Again, this is something I can live with in xdoc, but if the LaTeX3
\NewDocumentCommand is going to replace \newcommand, then there will
probably be cases where one wants to define a command that is really
just a macro. This is probably doable, if \NewDocumentCommand only
introduces the groups if they're really needed -- using a simpler
implementation for the nice cases, similar to that for the tail of m
arguments in current xparse -- but could lead to a rather complex
codebase. Still, making the distinction be between "commands with only
m' arguments" and "all other commands" wouldn't need to make the code
any more complicated than it is today. The only disruption that one
would then get by starting and ending groups as part of argument
grabbing is between an \accent and its base character, but perhaps that
is acceptable.

However, I couldn't help dreaming a bit... If one was allowed to add
one new TeX primitive, for the purpose of solving the issues above,
then what should that be? Much of our problems stem from the
restriction that TeX does not allow assignments while expanding things
(even though the mouth is Turing-complete), so my choice would be to
offer an escape from this restriction.

\begin{TeX-extension-idea}
Add an \emph{expandable} primitive that allows for some commands
(primarily assignments) to be \emph{executed} and affect the
expansion. The syntax could be
\expandedwith{<text>}{<commands>}
and the semantics could be as follows:
\begin{enumerate}
\item
A new group is begun.
\item
The <commands> are executed.
\item
The <text> is expanded (as discussed regarding
new \expanded primitive). The result of this will
be the one-step expansion of the entire \expandedwith.
\item
The new group is closed.
\end{enumerate}
With this, a non-spacebypassing \@ifnextchar could be conservatively
implemented as
\begin{verbatim}
\long\def\@ifnextchar#1#2#3{%
\expandedwith{%
\ifx\@let@token\reserved@d
\noexpand\@firstoftwo
\else
\noexpand\@secondoftwo
\fi
\the\toks@
}{%
\let\reserved@d=#1%
\toks@={{#2}{#3}}%
\futurelet\@let@token
}%
}
\end{verbatim}
(it would look as if the \@ifnextchar<char> is replaced by
\@firstoftwo or \@secondoftwo in one expansion step).

More relevant here is however that \XD@grab@arguments could be
implemented as
\begin{verbatim}
\def\XD@grab@arguments#1#2{%
\expandedwith{\the\toks@}{\iffalse}\fi
\toks@={#1}%
#2\XD@endgrab
}
\def\XD@endgrab{\iffalse{\fi}}
\end{verbatim}
---certainly a braintwister, but everything in this area tends
to be. \emoticon{:-)}

Implementation-wise, I would try making a procedure which is
like a simplified main_control and have \expandedwith work
primarily by calling this to execute the <commands>. Since most
of the complexity of main_control seems to lie in the "inner loop"
spaghetti, this might actually be straightforward. In terms of
`