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Message-ID:  <BAA303C9-B664-464E-857A-EB4670725503@gmail.com>
Date:         Tue, 25 Aug 2009 18:03:00 +0930
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From: Will Robertson <wspr81@GMAIL.COM>
Subject: Re: xparse
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
In-Reply-To:  <19091.35422.30263.945137@morse.mittelbach-online.de>
Precedence: list
Status: R

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.)

Using your example, an arbitrary section header becomes converted into

     \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