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LATEX-L  September 2014

LATEX-L September 2014

Subject:

Re: Thoughts on xtemplate

From:

"Mittelbach, Frank" <[log in to unmask]>

Reply-To:

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

Date:

Wed, 3 Sep 2014 09:39:56 +0200

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text/plain

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sorry Sean for getting back so late into this (which is actually a topic 
very dear to me). I decided to comment to several of the posts in 
chronological order even though it would probably more appropriate to 
do a summary reply ... but ...

On 16.08.2014 07:14, Sean Allred wrote:
> Hello everyone,
>
> Disclaimer: I'm a huge fan of the template idea.  It is a good system
> and I don't want it to see unnecessary change.

we I think it was a good prototype with a number of good ideas but it is 
essentially flawed and I think you put the finger into one of its wounds

> Disclaimer to the disclaimer: it's the only design management paradigm
> that I've come into contact with in regards to TeX.  :)

I beg to disagree, I think there is a second one which is the ldb (see 
my talk from India and there is some crude implementation of it too)

both template and ldb should get married and at least in parts remodeled 
to provide a coherent concept.


> I'd like to raise attention to a possible issue with xtemplate's
> design.  Currently, an 'object' can receive no more than nine arguments
> per TeX's syntax limitations: you cannot refer to a tenth argument in a
> macro definition.  That is,
>
>      \DeclareObject { foo } { 10 }
>
> will fail.  In TeX terms, this makes total sense.  You cannot have more
> than nine mandatory arguments for any single macro---that's just the way
> it is.  But speaking in terms of design, there are instances where such
> an object can have more than nine arguments.  In reality, this is the
> decision of the document designer.  There should be no such limitation
> on the design.

There is a certain history to that interface. Initially we thought that 
the template concept should not just be deployed at the boundary between 
presentation layer and designer layer but also internally (where these 
days I believe every thing should be handled by expl3 paradigms where we 
only have positional arguments). Given that and the time when that 
xtemplate concept was designed speed was a very important factor (still 
is to some extent) and if you do key/value repeatedly rather than only 
in the transition from one layer to the next that would have slowed 
things down enormously --- thus the mandatory arguments which are so 
much faster to scan.

> I'll refer you to the original post for Joseph's full answer, but his
> suggestion is, in my opinion, a very appropriate one: going for a
> completely key--value interface on the design layer (note: not the
> author layer).  It would certainly remove the limitation on the number
> of mandatory qualities an object may have.  It would also seem to be
> more befitting of the verbose clarity of the design layer to do this.

that is probably true and I do lean towards that direction too these 
days (especially as I'm now looking at template concept as something 
that forms the designer layer only and thus its parsing happens once per 
document element only.

However, regards of mandatory (positional) or named arguments there is 
an area that I feel is even more important and that hasn't been 
resolved: what exactly are the arguments that make up the signature of 
an object declared by \DeclareObjectType?

The background idea of \DeclareObjectType was that I wanted to have a 
specification for document elements and the "document data" that they 
receive that is implemented by different templates so that you could 
separate the document class specification.

Reason for having that was to enable a document written for a certain 
document class to receive different formatting simply by replacing the 
layout spec for the class with a new one.

In LaTeX2e we don't have any such separation: article.cls does define a 
document object spec implicitly but it is mixed in by defining its 
formatting. As a result if you go to a different class you can't be sure 
that it codes against that same spec (and in fact often enough it 
doesn't, eg amsart is close but not the same - and I'm not speaking of 
the fact that it internally loads amsmath for you)

So the idea was that \DeclareObjectType would formally say how many 
arguments each template for this object type would implement (checked) 
but also informally would state what kind of document data these 
arguments should receive (not checked). The templates would not need to 
actually do something with all the data received but they should be able 
to absorb them.

The problem that then showed up (and isn't resolved) is

  - what are those "required" arguments?
  - how do you handle variation without introducing multiple 
incompatible document classes

Example:

  Giving a heading (section) object type. It is fairly clear that this 
"requires" a title but beyond that it gets hazy. One could argue that 
something like (an alternative) TOC title or running heading title is 
advisable and could be made required (after all the interface from 
document level to designer layer could duplicate arguments like it does 
with current 2e and one could also argue that something like number 
suppression as implemented by 2e is also generally a good idea but 
beyond that ... is there anything else that should be forced down into 
the interface?

probably not, but then anything in addition, say a "chapter motto" would 
either mean that you end up with templates implementing different 
document types and are incompatible with each other or you would need to 
support something like defined optional data that will be parsed by the 
interfaces and then used or not used by the templates.

So my current thoughts are in the following direction:

  \DeclareObjectType

    (1) should declare a set of data arguments that are required to be 
implemented by every template of that type

    (2) it should also declare a set of data argument are optional, 
i.e., a template could choose to not use them without violating the 
statement that this template implements the particular data type

    (3) finally for special situations there could be other data 
arguments (also optional) that a template might use that are not 
declared at all.

==============

Now (1) could be kept positional as I really doubt that you find any 
type that requires more than 9 of those despite the fact that is an 
artifical restriction. However for sake of clarity on reading a designer 
spec key/values are probably the better choice.

(2) definitely has to be of type key/value

(3) also need to be key/value and offering this is really there to 
account for the fact that there might be special situations or things 
that haven't been thought of initially. If it turns out that several 
templates implement the same key meaning of type (3) then over time one 
could promote that from (3) to (2) in the object type declaration, just 
to formalize the meaning -- that would be upward compatible.

Offering an exit of (3) also means that one would need to parse any set 
of key/values store them somewhere and offer that to the template for 
use and they could decide to ignore any or all of them.

The downside is that you can't restrict yourself against a definite list 
of required and optional key names and thus for the keys of type (2) one 
can't identify typos in the name, say.

If (1) is key value then then parsing of arguments could compare the 
list of received values against the list of required values for the 
template type and at that point already complain if something is missing.

so much for my thoughts on these matters.

frank

ps some links on ideas around the architecture and ldb etc:

http://tex.stackexchange.com/questions/45838/what-can-i-do-to-help-the-latex3-project/46427#46427

http://latex-project.org/papers/LaTeX3-architecture-2011-slides.pdf

(and/or the video of the talk - link also on the website)

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