EXPRESS_ — The metamodel doesn't seem to support a Schema -> Interface -> Schema relationship

Issue Submitter: David Price, Eurostep

Document: EXPRESS Metamodel, Alpha Version

Issue:

The metamodel doesn't seem to support a Schema -> Interface -> Schema
relationship. I do see Schema -> InterfaceElement -> SchemaElement
relationship in the metamodel but it's as often as not the entire Schema
that gets interfaced.

If I were using the metamodel in a tool to create EXPRESS, this seems to mean
I can't establish schema interfaces beforehand (e.g. to split a domain being
modeled into subdomains) unless I have elements in every schema. This seems
like a gap in the metamodel.

What I mean by this is that I should be able to take the EXPRESS metamodel,
build an application that directly implements the classes it defines and then
I should be able to create any and all valid EXPRESS schemas.

Since:
SCHEMA A;
 USE FROM X;
END_SCHEMA;
SCHEMA X;
END_SCHEMA;

is valid EXPRESS it seems to me the metamodel should support this. It doesn't
seem to me that the distinction is purely syntactic if there are valid
schemas that cannot be represented using the metamodel. I do agree with Ed
Barkmeyers analysis of what the text in Part 11 says, it's just that the text
appears to be incomplete in that it doesn't cover the A and X situation
above.

Related Discussion Summary:

>From Ed Barkmeyer: I would say that USE FROM <schema>; is just a syntactic
shorthand. Â What is actually interfaced is a specific collection of
InterfacedElements. It is not clear that any additional semantic
relationship is established between the two schemas. Â The only relationship
between the two schemas is that some of the InterfacedElements in one Schema
refer to SchemaElements defined in the other one.

In the UML case, the relationship is package-to-package, and circular
import/merge relationships are not valid. Â But EXPRESS is very clear that the
relationship is schema-to-InterfacedElement, and circular references among
the schemas themselves are permitted. Â Also in UML, importing a package
doesn't change the local package namespace. Â In EXPRESS, interfacing elements
adds identifiers to the local schema namespace. Â So Exp:Schema and
UML:Package are only superficially similar.

I guess we never discussed using the metamodel to "create EXPRESS", and
I'm not quite sure what that means. Â But Bernd Wenzel pointed out at one point
that certain purely syntactic information should be maintained in the
metamodel to allow a reasonable approximation to the original EXPRESS
schema to be constructed.

My understanding was that EXPRESS was being treated as the legacy.
So the intent of the metamodel was to capture the knowledge contained in
a set of EXPRESS schemas, for the purpose of rendering that knowledge
into more useful forms.

It is not clear whether USE FROM X (<list of everything in X>); is a
"reasonable approximation" to USE FROM X; . Â But it is certainly true
that there is no way in the metamodel to represent USE FROM X; if X
contains no declarations.

Note also that, unlike USE FROM X;, what REFERENCE FROM X; interfaces is
selective, and there you want the metamodel to capture the
interpretation – the results of the algorithm described in clause 11 –
not just the parse. Â So I would never want to see a metamodel population
in which some schema-to-schema relationship was captured INSTEAD OF the
collection of schema-to-InterfacedElement relationships that that syntax
MEANS. Â (But the other experts may not share my view.)

Proposed Resolution: No

Add Interface to clause 8.4 and Figure 2, with two associations: interfaced-schema, interfacing-schema and an isUSE attribute