[news.eclipse.modeling.mdt.ocl] Re: def expression?

[Ed Willink <ed@xxxxxxxxxxxxx>]

Hi Joel

The following snippets all come from org.eclipse.qvt.declarative.parser 
or org.eclipse.qvt.declarative.parser.qvtrelation with added C++-style 
class names for clarity.

/**
 * Overridden to provided a nested scope for nested variables.
 */
@Override
protected OCLExpression<EClassifier> AbstractQVTAnalyzer::letExp(
	LetExpCS letExpCS,
	Environment<EPackage, EClassifier, EOperation,
	EStructuralFeature, EEnumLiteral, EParameter, EObject,
	CallOperationAction, SendSignalAction, Constraint, EClass,
	EObject> env) {
	return super.letExp(letExpCS,
		createdNestedEnvironment(letExpCS, env));
	}

	
@Override
protected IQVTrNodeEnvironment 
AbstractQVTrAnalyzer::createdNestedEnvironment(CSTNode cstNode,
	Environment<EPackage, EClassifier, EOperation,
	EStructuralFeature, EEnumLiteral, EParameter, EObject,
	CallOperationAction, SendSignalAction, Constraint, EClass,
	EObject> env) {
		return new
			QVTrNestedEnvironment((IQVTrNodeEnvironment)env,
			cstNode);
	}


public class QVTrNestedEnvironment extends 
QVTrEnvironment<IQVTrNodeEnvironment, EModelElement, CSTNode>
{
	public QVTrNestedEnvironment(IQVTrNodeEnvironment env, CSTNode cstNode) {
		super(env, null, cstNode);
	}	
}

the construction just propagates down through 7 level of inheritance to 
an EcoreEnvironment.

The initial letExp override for AbstractOCLAnalyzer::letExp just ensures 
that the let body has a nested environment, which is instantly forgotten 
about when proceeding past the let.

	Regards

		Ed Willink


Joel Greenyer wrote:
> Ed,
> I've been trying to get familiar with nested environments, You promised 
> that they were easy to use ;), but unfortunately I can't find code 
> examples--at least not any that point in the direction that I need to go.
>
> In short, my problem is that I match a graph pattern to a model and, 
> during the matching process, I need a scope that constantly reflects the 
> bound nodes by bound variables, i.e. the scope must contain a bound 
> variable per named node that is currently bound.
>
> While I backtrack, I need to remove variables from my environment/scope 
> that will possibly be added again. So, you suggest to do this with 
> nested environments--How do I create a nested environment (I think I 
> understood that...) and how to I go back to an outer environment when I 
> backtrack?
>
> Thanks for helping!
>
> Joel
>
>
> Ed Willink wrote:
> > Hi Joel
> > > > I'm not sure about your whole problem but when you mention removing 
> > > > variables from environments I get very uncomfortable. 
> > >
> > > Why is that?
> >
> > It means that the Environment has a very clear locality. Nested 
> > environments are always valid for the context that initialised them.
> >
> > Note that OCL 2.0 does not support remove of elements from an 
> > Environment.
> > > > It is much easier
> > > > to created a nested environment for the nested scope and revert to the
> > > > outer ebvironment when you want to recover context.
> >
> > You'll find nested environments are easy too, and you cannot forget to 
> > match your adds with removes.
> > > I haven't worked with these nested environments yet, but I'll keep it 
> > > in mind. However, I'd have to create a nested environment for each 
> > > variable that I add. Adding and removing variables is easy and works 
> > > well for me so far.
> >
> > If it works for you fine. Just beware that if you start to do more 
> > interesting things, it might bite later.
> >
> >     Regards
> >
> >         Ed Willink