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On 25/08/2009, at 4:23 PM, Frank Mittelbach wrote:

> I think some of the problem here lies in the fact that you all think
> of xparse
> as a bunch of TeX macros providing a single solution. Don't.

*click*
Aha!

> This is why I would think that separating the two is key. There is
> (and can
> be) a single interface to declare a layer 0 command. And for each
> such command
> there is an interface declaration for each layer -1 (but how that
> interface
> decl is done might differ from layer -1 to layer -1).

[...]

> The big open question for me is to describe what the language at
> layer 0
> really looks like.

Now I understand you completely. (I hope.)

We want to describe every document element in terms of a template
instance. (I know you mentioned the other day that "template instance"
isn't strictly true but I don't feel that the word "instance" on its
own carries enough weight.)

\UseInstance {sectioning} {section} {#1} {#2}

Correct me if I'm wrong:

(a) The template type defines the mandatory number of arguments
required by each instance
(b) Each variation on a command (say, the different between \section
and \section*) will be handled by a separate instance

Isn't layer 0, in these terms, simply a list of template types and
instances allowed in the document?

((Late addition: Now that I read over what I've written, I suppose
that some sort of hierarchy could be imposed as well. That makes
things more complicated.))

A description for layer 0 might then look (very hypothetically) like

\begin{DefineClassTypes}{book3}
\InheritTypes{
paragraph-shapes,
cross-references,
breqn-support,
% I don't know what I'm writing here, really.
% Just trying to imply that you might want to base classes off
other classes
% or based off simply functionality that belongs only in many
classes.
}
\TemplateTypeWithInstances{sectioning}{
front-matter-division,
chapter,
appendix,
numbered-section,
unnumbered-section,
% No other instances than these are allowed!
% The instances themselves are defined in layer 1.
}
\TemplateTypeWithInstances{text-lists}{
itemize,
enumerate,
description,
% Ditto.
}
\end{DefineClassTypes}

* * *

Any layer -1 interface would then provide tools to convert whatever
input there is into instances defined as above. So, xparse2e would
contain

\DeclareDocumentCommand\section{som}{%
\ifBoolean #1 {
\UseInstance{sectioning}{numbered-section} {#2} {#3}
}{
\UseInstance{sectioning}{unnumbered-section} {#2} {#3}
}
}

I can't really see how this could be abstracted any more, but that
might be a lack of imagination on my part.

Must run,
Will