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[m2t-dev] [MTL] Template invocations
Hi Arjan,

I'm working along with Jonathan to implement a first version of the MTL parser and evaluation engine.

The specifications troubles me about the particular point of 'TemplateInvocation'; quote :

The rules for determining which template to invoke and how many times are as follows:
     • The types of arguments should match the types of the corresponding parameters. An argument type matches the param-
       eter type when the latter is either the same type or a super type. If the argument is a set and the parameter is a singleton,
       then the template is invoked for each member of the set.
     • If the argument is a set and the parameter is a set, then the template is invoked once.
     • If the argument is a singleton and the parameter is a set, then the template is invoked once with the singleton set.
     • If the template has k singleton parameters and the corresponding arguments are sets, then the template is invoked for
       each member of the cross product of the k sets.
     • If a template is overridden, then the overriding template is invoked as described earlier.

Now let's consider the template "testTemplate" defined as such :
[template public testTemplate(s1:String, s2:String)]
    ...
[/template]
  1. How is the implementation expected to behave when calling "testTemplate" with a Set of Strings and a singleton String?
  2. What if I invoke this template with two Sequences of different sizes?
For a little more fun, Let's alter "testTemplate" definition for it to look like :
[template public testTemplate(s1:String, s2:String, set:Set(String))]
    ...
[/template]
   3. What will happen If I try and invoke this template with three sets of Strings?

The specification does not state this explicitly, but I think the second point is handled by the fact that the template will be called for each member of the cross product of the sets, regardless of these sets' size?

If so, should the first point be handled by invoking the template with the cross product of the set parameters with a single-element list containing only the singleton parameter?

As for the third issue, well I believe it crosses the first issue but I'd like to have your input on this.

All in all, I believe that allowing such leniency in template invocations will prove really error prone, from the development side as much as for the user side. As a matter of example, if a File block is defined in a template called in such manner, the user could end up with a considerable amount of generated files (since we call the template with a cross product) if he didn't pay enough attention to the issue. As for the "development side" ... this leniency introduces many issues for polymorphic resolutions among others : if I have two templates of the same name, the first having parameters "String, Set(String)" and the second "String, String", which should I choose when the user wants to invoke this template with two sets of Strings?

Regards,

Laurent Goubet
Obeo
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