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Re: Experimental template' interface code

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Thu, 7 Oct 1999 11:58:23 +0200

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 James, i already started to explain my understanding of template types in the reply to Lars and perhaps this has already answered your questions, but anyway let me elaborate a bit more on mandatory arguments viz keyword attributes etc > About lists, and especially about the what it means for the arguments of > a template type to have the same interpretation. > > Suppose the template type has a width argument. This could be used for > the width of a box to set the list in, or a template which set each item > in a cell of a table, the width argument could be used to specify the > width of those cells. Is this within what is understood by type > arguments having the same interpretation? What if the widht was taken > as the width of the the label? as i said, instances with the same template type (whether or not are derived from the same template) should be exchangeable without making the processing of a document blow up or produce rubbish. With that in mind the the above examples should most certainly not be the same type. In other words the fact that an argument takes a dimension is not the only identifying characteristic for that argument within an informal type declaration. In addition the interpretation of this dimension is crucial. Thus if one template would interpret that argument as the width of the label but another one would interpret it as the width of the whole line or the width of the text then you would get very strange results if you replace one such instance with another, wouldn't you? So suppose we define the template type list with the following characteristics (beside others not listed):   Expects three arguments with the following semantics:   \begin{itemize}   \item     String to calculate width of left indentation, or |\NoValue|   \item     Symbol/string to be used as item label, or |\NoValue|   \item     Boolean to denote whether or not numbering continues   \end{itemize} if so the first argument if not \NoValue would determine the left indentation (eg something like the available space for the label). Thus the following declaration would be an extended description environment \DeclareDocumentEnvironment{description}     { o }     { \UseInstance{list}{description} {#1} \NoValue \BooleanFalse }     { \EndThisList } where the user in a document could go like this  \begin{description}[longlabel]  \item[short] ...  \item[longlabel] ...  \item[other] ...  \end{description} in other words the user overwrites the default left indentation with a value calculated from the string "longlabel". Now such an overwrite possibility might make sense for vertical oriented lists (like current LaTeX description lists) but if the designer decides that all such lists are horizontally oriented then that argument suddenly doesn't serve any purpose any more as there is no left indentation. Now one could argue that those two templates are so much different that they can't possibly form a replacement for each other (it is perhaps a border case indeed). However I think they can and should have the same template type and for this reason we allow one derivation from that rule that arguments of templates with the same type have to have the same interpretation. The exception is that an argument value might be ignored completely (if the informal template type description says so). That is to say all templates of a given type have to parse the same number of arguments but in certain cases they are allowed to disregard some of the parsed values completely. What they are not allowed is to use the supplied value for a different purpose. ============================================== so when should be something a mandatory argument to a template (and thus part of the template type) and when is it better or more correct to put something into the keyword attributes of some template itself? there is no simple answer for this. i tried to give some reasoning on these template type arguments when i wrote the documentation for template.dtx. here is another look at it.  - clearly everything that is variable data coming from the document  source has to be passed to the template as a mandatory argument, eg  if we try to define a template type for say headings we have to pass  the heading text as a mandatory argument. i don't think there is much  discussion about that. in case of headings same would be true for  data going to the toc and data going to a potential running head  entry (in current latex those two are combined but this is not really  what you necessarily want)  - sometimes there are some logical facts about the data in the  document that you like to pass to the template in which case this has  to happen through mandatory arguments as well. for example  enumeration lists might logically continue a previous list or start  enumeration again --- this one models all kind of lists in a single  type (which might be a mistake conceptually) then an information  about whether or not the current list continues a previous  enumeration should be passed to the template as a flag of some sort  (in fact this is just variable document data if one thinks about  this, after all it is a logical fact about the document that the  current list continues a previous one)  - any layout decisions should normally go into the keyword attributes  of the individual template unless you want for some reason provide  user level overwrites. if so you need to pass either user given  formatting values or flags down to the template The really difficult one is the last item, ie which (if any) of the layout decisions of some template should be modifiable from within the document source? A person whose soul is pure would perhaps argue "none" :-) but life is short and people like a pragmatic approach towards solving of problems. So my approach is to put in the template types a number of such "layout oriented" arguments if it seems that inclusion solves a large number of modification requests in a wide variety of different cases --- what a nonsense sentence :-) but i hope you get the the essence of what i'm trying to say. problem is to find those essential overwrites (and clearly people will have different opinions on what is essential and what not) for example, the above three arguments are those that i have come up in the context of lists but there are probably others which perhaps should get added, eg an argument which receives a value like "default" "tight" "compact" and does modify the spacing characteristics of the list in question. Maybe - maybe not ... this was what i meant when i said please think about what you would consider being an appropriate template type for lists / footnotes / ... so please ... :-) =========================================================== returning to James post: > Another thought, lists with one-line items look very widely spaced, > especially when used with non-zero parskip and double spacing. A > variant form with tighter spacing, and the item text set in an hbox for > such lists might be a good idea. this might be something which should be controlled from the document or alternatively one could think of a template which is internally smarter and allows the designer to specify different spacing values depending on the number of items in the list. on the other hand, the current latex list setup is simply wrong in the sense that if you change \parskip to a positive value you have to adjust all list spacing anyway as parskip is always added to the values (something which should not be the case) so perhaps is this not really a problem in a properly designed setup frank`