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Sender: Mailing list for the LaTeX3 project <[log in to unmask]>
From: Will Robertson <[log in to unmask]>
Date: Tue, 25 Aug 2009 11:33:13 +0930
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Hello,

Before I start I'd like to thank Joseph for his fantastic effort  
putting template-alt together. It really is a "re-imagining" of  
template itself and raises many questions. It is much more useful to  
be able to compare two working implementations than to discuss  
template on its own in abstract terms.

* * *

I've been comparing in more detail template to Joseph's template-alt.  
The only differences I'm interested in are the different ways to  
define keys. I'm assuming that the collection/type/template/instance  
groundwork in template is pretty much the approach that we're happy  
with.

Here's a modified example from the template documentation:

     \DeclareTemplate{caption}{lesssimple}{4}
       {
        above-skip         =L           [0pt] \caption_above_skip ,
        below-skip         =L           [0pt] \caption_below_skip ,
        number-format      =f2      [#1~#2:~]  
\caption_number_format:nn     ,
        nonumber-format    =f1         [#1:~]  
\caption_nonumber_format:n    ,
        single-line-format =f1 [\hfil#1\hfil]  
\caption_single_line_format:n ,
        caption-font       =f0  [\normalfont] \caption_set_font:   ,
        caption-hj-setup   =i {hj}  [default] \caption_hj_instance ,
       }

And here's how something equivalent might look in template-alt:

     \DeclareTemplate{caption}{lesssimple}{4}
       {
        above-skip         .set:N       = \l_caption_above_skip     ,
        below-skip         .set:N       = \l_caption_below_skip     ,
        number-format      .function:N  = \caption_number_format:nn ,
        nonumber-format    .function:N  =  
\caption_nonumber_format:nn   ,
        single-line-format .function:N  =  
\caption_single_line_format:n ,
        caption-font       .function:N  = \caption_set_font:        ,
        caption-hj-setup   .instance:nN = {hj} \caption_hj_instance ,
        above-skip         .initial:n   = 0pt                       ,
        below-skip         .initial:n   = 0pt                       ,
        number-format      .initial:n   = {#1~#2:~}                 ,
        nonumber-format    .initial:n   = {#1:~}                    ,
        single-line-format .initial:n   = {\hfil #1\hfil}           ,
        caption-font       .initial:N   = \normalfont               ,
        caption-hj-setup   .initial:n   = default                   ,
       }

Discussion:

1.  Despite the large increase in verbosity (doubling the line count),  
I think I like splitting the defaults into separate keyvals. In fact,  
after splitting defaults from the definition of the keys, it's no  
longer clear to me whether defaults should be included within the  
template definition at all. For example, perhaps it would be more  
readable to write something like this:

     \DeclareTemplate{caption}{lesssimple}{4}
       {
        above-skip         .set:N       = \l_caption_above_skip     ,
        below-skip         .set:N       = \l_caption_below_skip     ,
        number-format      .function:N  = \caption_number_format:nn ,
        nonumber-format    .function:N  =  
\caption_nonumber_format:nn   ,
        single-line-format .function:N  =  
\caption_single_line_format:n ,
        caption-font       .function:N  = \caption_set_font:        ,
        caption-hj-setup   .instance:nN = {hj} \caption_hj_instance ,
       }
     \DeclareTemplateDefaults{caption}{lesssimple}
       {
        above-skip         = 0pt              ,
        below-skip         = 0pt              ,
        number-format      = {#1~#2:~}        ,
        nonumber-format    = {#1:~}           ,
        single-line-format = {\hfil #1\hfil}  ,
        caption-font       = \normalfont      ,
        caption-hj-setup   = default          ,
       }

This transfers much more cleanly when people are generating instances  
(e.g., they can just copy/paste some of the default list).  
\DelayEvaluation of template could still be used instead of .initial:V  
of template-alt.

But considering templates are designed to be instantiated, what's the  
point of default values? Well, it's nice to be able to generate  
instances without setting every key; furthermore, the instance  
parameters will cascade from these default values. If the template  
defaults can be altered, classes of instances can be generated much  
more easier, I think.

For example, if we create instances 'chapter', 'section',  
'subsection', and so on, it's much easier to change the "section font"  
by calling
     \DeclareTemplateDefaults{divisions}{plain}{font=italic}
than editing each instance or the definition of each instance.  
(Assuming that there's a processing stage where we can edit template  
defaults and instances *before* the document elements become  
instantiated and frozen.)


2.  If template-alt is going to auto-detect variables and set them  
appropriately, perhaps it could do the same for functions.

        number-format      .set:N  = \caption_number_format:nn

could look at the signature of \caption_number_format:nn and use  
\cs_set:Nn automagically, I think.


3.  The big difference between template and template-alt is the number  
of keyval types and how they're referred to.

template provides 11 different key types with nine different letters  
(two are uppercase variants).

template-alt provides seven key types (assuming we can  
drop .function), of which four are equivalent to template, one relates  
to initial/default values (which may be able to be dropped as well as  
discussed above), and two more deal with choices selection. Actually,  
I'm lying here, as there are actually 15 key types in template-alt  
with many variations such as
     .code:n
     .code:x
     .code:Nn
     .code:Nx
But where template has six letters for setting different variables,  
template-alt uses the catch-all ".set:N" with an _x variant.  
Furthermore, template-alt also deals with global variables properly.  
(As a possible downside, to do all of this it ties the package writer  
down to use the expl3 syntax, which template.sty is more lenient about.)

I believe that template-alt, as it currently stands, provides more  
useful tools for the template designer. Many 2e packages use keyval  
forms like
     size=a4paper
     style=SI
     font=bf   (not that I like this one!)
and template should cater for these uses. \MultiSelection doesn't  
really go far enough, I think.

This isn't to say that template-alt necessarily does things 100%  
correctly; I am saying that, in my opinion, template needs further  
improvement before it's ready, and it's not clear to me that we'll be  
able to add more functionality without overly complicating its current  
naming system.

* * *

I hope this comparison helps the discussion here, rather than bogging  
it down with too many details. From the sounds of things, we're much  
closer to finalising something with xparse than with template.

Will

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