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 Subject: Re: template interfaces and what they mean From: Achim Blumensath <[log in to unmask]> Reply To: Mailing list for the LaTeX3 project <[log in to unmask]> Date: Fri, 28 Jan 2000 18:08:40 CET Content-Type: text/plain Parts/Attachments: text/plain (136 lines)
Frank Mittelbach wrote:
> main point is i would like you to think about is, what are the
> invariants of that interface, ie what can be assumed to be the same
> from one template to the next (within a type) and what can differ.
>
>  1) how much freedom has a template to interpret its arguments (ie data
>     passed to it from the document)?
>
>  2) what is the scope a template instance apply itself? is it
>     well-defined?
>
> let's tackle 1) first. Achim suggests that the pshape type should get
> two arguments those meaning should depend on the template. in my opinion
> this is likely to result in chaos.
>
> the main idea behind the template types is that

[...]

> so to make this work templates of the same type need to interpret data
> coming from the document (ie via arguments) in the same way. At best I
> see that they might be allowed to ignore it.

I propose the following rule:

Templates of the same type must be exchangeable.

To be able to apply this rule each template type must have defined
semantics including a description of which parts of its behaviour are
"garanteed/essential" and which are left "undefined".

> To return to the pshape template: so I don't think it should get
> arguments that are interpreted differently by different templates.

Well, I said the "exact" meaning would depent on the template not that
the interpretation should be completely arbitrary. In terms of the
rule I presented above one could say that the semantic of a pshape
template is to set the shape of a paragraph. Obviously, different
instances can produce different shapes. Thus, exchanging instances
can result in a change of the shape. If two arguments are added
standing, say, for the horizontal and vertical deviation of the
shape from a rectangle, this doesn't matter. Exchanging instances
still results in a change of the shape.

> In fact for that particular template type I don't think it should
> get arguments at all.
>
> I see this template as something (again) to be used only as part of
> other templates to define very specialised shapes in certain layouts
> --- but shapes which are independent of of document data but exist in
> their own right.

When I wrote my suggestion I was thinking of packages like wrapfig and
picinpar. In those cases the type of the shape doesn't depend on the
document data but the exact dimension do.

> For example, I came recently across a very nicely formatted book whose
> Capter openings have all be done by having the first paragraph with a
> triangle like cutout that was part of the design for the chapter head.
> That's the kind of area where I see applications for pshape.
>
> In contrast, if one wants to provide commands that allow the user to
> directly manipulate the paragraph shape somehow (one example in a
> restricted sense being Initials) then this should not be done via
> the templates provided in xhj but by making use of the lower-level
> interface that comes with galley2.dtx

Agreed, so forget my suggestion.

> ===================================================================
>
> do you remember what the second abstract question was? No? Here is
> another rephrase:
>
>   to what elements should templates apply that do set parameters of
>   underlying algorithms (in some way or other)
>
> for example the "std" template of type justification sets the
> justification for all upcoming paragraphs of the galley (potentially
> at least) while the "single" template sets it only for the next. Achim
> suggested that not the template but the user should decide on the scope.
>
> Now I agree with him but who is the "user"? The user is the designer of
> the document class layout and if this "user" says that the first paragraph
> of a caption should have the first line centered and any following
> paragraphs should be justified without indentation then he can do this
> now by setting a
>
>  justification-setup =i
>
> key to the appropriate value. by making the (i agree) somewhat different
> feeling templates one and the same type I allowed for flexibility in
> design. In an earlier implementation I had two types but that resulted
> in either strange templates (making use of both) or in a lot of repitition,
> ie the need to have nearly identical templates differing only in one key
> since that key was of a different type.
>
> another way to look at it is to say that the "single" one is actually
> also setting all paragraphs in the galley (it sets the first to X and
> the rest to whatever was before X).
>
> yet another way is to get rid of "single" and implement what i called
> "complex" differently since what in practise will be needed are the
> instances from "std" and from "complex"

IMHO this design is, well, not optimal. I agreed that having two
different template type is ridiculus. But instead of having
separate instances for "single" and "std" it would be better to
have just one instance with an boolean argument specifying whether
to apply the template for the next or for all paragraphs.

Thus a class designer would have two parameters in his template

initial-par-justification =i \ipj
par-justification         =i \pj

and would say in the template body

\UseInstance{\pj}{\True}
\UseInstance{\ipj}{\False}

The reason I prefer this desing is that IMHO a justification template
consists of information about the formatting of a single paragraph.
To question to which paragraphs this formatting is applied is
independent. Each format does make sense either way. Thus in your
design one would have to define each instance twice. Once for
the "single" and once for the "std" case.

Achim
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//  Achim Blumensath       | \  _                        \O/ \___/\ |