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On template types

From:

Frank Mittelbach <[log in to unmask]>

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

Date:

Thu, 7 Oct 1999 10:19:02 +0200

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 Lars, > Here are some thoughts about templates and font selection. Perhaps I > thought about it a bit too long, because the mail got a bit > long-winded, but I hope you'll endure it. long-winded or not your post does contain a lot of interesting thoughts which are worth pursuing further. but before looking at fonts let me first discuss template types a bit further and explain how i understand that concept (this might also answers some of the questions James posed in a different message). you wrote: > \DeclareTemplateType{noparams}{0} > \DeclareTemplate {noparams}{author-fonts}{0}{ > medium =n [m] \mddefault, > boldface =n [b] \bfdefault, > light =n [m] \ltdefault, > upright =n [n] \urdefault, > italic =n [it] \itdefault, > slanted =n [sl] \sldefault, > smallcaps =n [sc] \scdefault > }{\DoParameterAssignments} > > (Or perhaps author-fonts should be a type and the template should be > named default or something. I'm not sure, but it seems odd that all > templates without parameters (i.e., having type noparams in the above > setting) should always be affected by the same \UseCollection > commands.) if we would have a template type "noparams" what is it supposed to mean? As Lars said: "it seems odd that all templates without parameters should always be affected by the same \UseCollection commands". And indeed, this is not only odd it is against the philosophy of template types! The purpose of the template type is the following:  loosely speaking it is supposed to ensure that an instance of some  template type could be exchanged with another instance of the same  template type without making the code produce rubbish or doesn't work  any more.  now the above example template type called "noparams" is setting font  defaults which is something completely different than say \DeclareTemplate{noparams}{TeX}{0}{   uchyph =c [1] \uchyph,   hyphenpenalty =c [50] \hyphenpenalty,   exhyphenpenalty =c [50] \exhyphenpenalty,   lefthyphenmin =c \lefthyphenmin,   righthyphenmin =c \righthyphenmin, }  {\DoParameterAssignments} which is also a template that doesn't take mandatory parameter. So if one would claim that both such templates are of the same type we would claim that a class designer could replace an instance of the first template with an instance of the second. we don't mean this do we? so the template type is more than just the number of parameters and their meaning: it is an informal summary of the semantics of the code/purpose/action/effect it has. this includes the semantics of its mandatory parameters but it also includes its overall effect. Thus a template which is setting up the hyphenation mechanism of TeX (like the one above) should have a template type with a dedicated name (say "hyphenation") which has the following informal description: "templates of my type customize the TeX hyphenation algorithm and take no mandatory parameter". (it should probably be far more verbose, eg saying something about the scope of the customization etc etc)  And a template which sets up the font defaults should perhaps have a type called "fontdefaults" and an informal description ... This way we would not mistakenly replace an instance which sets up fonts with an instance setting up hyphenation and vice versa but only with instances that are "compatible in nature". as a side remark: my experience with templates that are doing declaration only (eg do not have mandatory arguments and no code section worth mentioning (like those above)) is that you seldom end up with more than a single template per type. as a result you may start wondering about the whole concept of separating type and template. Well the answer is that with template types that encapsulate more complex stuff (eg a list template type) you will soon find that there are several different templates possible that all fit this type. but even with such declaration thingies you sometime want to provide more than one template, for example, to offer different styles of declaration. So instead of the template TeX (of type hyphenation no noparams :-) above which is simply setting the low-level TeX parameters one could also offer the following template: \newcount\@@lefthyphenmin \DeclareTemplate{hyphenation}{std}{0}{   hyphen-disable-boolean =s {\ifnum\@@lefthyphenmin<\@M                                   \@@lefthyphenmin\lefthyphenmin                                   \lefthyphenmin\@M                                 \fi}                                {\ifnum\@@lefthyphenmin>\z@                                   \lefthyphenmin\@@lefthyphenmin                                 \fi},   hyphen-uppercase-boolean =s {\uchyph\@ne}{\uchyph\z@},   hyphen-discourage-boolean=s {\hyphenpenalty\@highpenalty                                 \exhyphenpenalty\@highpenalty}                                {\hyphenpenalty\@lowpenalty                                 \exhyphenpenalty\@lowpenalty}, }  {\DoParameterAssignments} with instances like \DeclareInstance{hyphenation}{off}{std}     {hyphen-disable-boolean = true} \DeclareInstance{hyphenation}{on}{std}     {hyphen-disable-boolean = false,      hyphen-uppercase-boolean = true,      hyphen-discourage-boolean = false} \DeclareInstance{hyphenation}{discourage}{std}     {hyphen-disable-boolean = false,      hyphen-uppercase-boolean = true,      hyphen-discourage-boolean = true} ============================= confused everybody enough? hope not :-) here is another way to look at it via some example: below is a user declaration for a description environment.  \DeclareDocumentEnvironment {description}     { }     {\UseInstance{list}{description}        \NoValue \NoValue \BooleanFalse      }     { \EndThisList } \DeclareInstance{list}{description}{vertical-std}{ ... } what do we see here? a) An instance of type "list" is called which seems to take three     mandatory arguments (all of which have preset values, ie \NoValue     \NoValue and \BooleanFalse) b) There is this strange \EndThisList c) The instance description is declared using a template "vertical-std" In fact this is an example of the list template type as described in the Vancouver talk as follows:     To format list structures such as \texttt{itemize} there might be a     template type called \texttt{list} with the following     characteristics:     \begin{itemize}     \item       Starts a list structure embedding it into the surrounding formatting     \item       Sets up a command |\ListItem| (one argument)       to start a new item' of the list     \item       Sets up a command |\EndThisList| to finish the current list       structure properly     \item       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}     \end{itemize} in other words, not only the semantics of the three arguments are part of the template type but also the fact that each such template is supposed to set up \EndThisList and \ListItem. And of course this has to be the case if i want to be able to replace the instance "description" above with an different declaration which might look like this:  \DeclareInstance{list}{description}{inline}{ ... } While the template "vertical-std" produces lists which look like current LaTeX lists (ie vertically oriented ...) the "inline" template would contain code that makes a run-in list ie one that is horizontally oriented. All this would only work if both templates take the same number of arguments (and interpret them in the same way) and in addition also behave otherwise compatible, ie they both have to set up \EndThisList etc --- thus the template type is the sum of all such characteristics. hope all this helps a little bit in understanding what we try to provide frank ps who is long-winded??? :-)`

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