[sc34wg3] CTM draft dtd. 2007-09-09 - Templates

[Lars Marius Garshol]

* Lars Heuer
>
> In the previous example it would be impossible to create an occurrence
> of type 'homepage' *after* we have a declared a template called
> 'homepage'.
>
> IMO this is bad. The user gets two different behaviours for the same
> syntax. And if the user makes a typo within the template name, the
> parser would not claim that error, but create an occurrence silently
> (unless the argument cannot be a valid occurrence value).
>
> With the CTM syntax *before* Montreal, the parser would have
> detected typos and the user wouldn't have got two different things
> within the same syntax.
>
> I propose to make template invocation syntax and occurrence syntax
> distinct (again). Either we should go back to the syntax before
> Montréal or someone should come up with a better one.

What we have here is a group of requirements which are mutually  
incompatible, as follows:

#1: Syntax should have a uniform look
     (ie: occurrences & templates should look the same)

#2: The meaning of a single expression like "foo: bar" should not
     change throughout the file
     (ie: template definitions should not change the meaning of this
     expression)

#3: It should be possible to define templates anywhere in a file

It's possible to have any two of these, but not all three. So, it  
boils down to a question of which requirement is the least important  
to people. I guess we could do a quick poll or something.

Steve suggested an alternative solution: require template identifiers  
and topic identifiers to be distinct. In this case you'd have to  
check every declared template to see if there is a topic with that  
identifier and if so report and error, and conversely also check for  
conflicts with every new topic. This does add complexity to  
implementations.

There is the potential for trouble with %include, but given that % 
include will only work where identifiers are carefully managed across  
files anyway, I think we can ignore that problem.

In addition, we have another requirement, introduced by Lars Heuer:

#4: Mistyped template names should cause an error

This one conflicts directly with #1, so anyone who wants this  
basically ditches #1, so that they can have #2 and #3.

As far as I am able to tell, this leaves us with the following options:

  - #1 and #3: basically the post-Montreal syntax
  - #1, #2, #3 and errors on shared identifiers: current draft + new  
rule
  - #2, #3, #4: back to distinguishing templates & occurrences
  - #1, #2: means templates must be defined at top of file
    not clear if this is compatible with new %include semantics

Personally, I don't like the "errors on shared identifiers" solution  
very much, and I don't think #1 is important. (I let it pass in  
Montreal, not realizing that this was a side-effect.) So basically, I  
prefer the #2-#4 solution.

As far as I can tell, Steve favours #1-#3 plus errors.

Lars, I think, favours #2-#4, like me.

Unless I've missed it, nobody else has given any opinion so far.

--Lars M.