Question

The following is the Xtext grammar for my DSL.

Model:
  variableTypes=VariableTypes predicateTypes=PredicateTypes variableDeclarations=
  VariableDeclarations rules=Rules;

VariableType:
  name=ID;

VariableTypes:
   var types  (variableTypes+=VariableType)+;

PredicateTypes:
   predicate types  (predicateTypes+=PredicateType)+;

PredicateType:
  name=ID  (  (variableTypes+=[VariableType|ID])+  ) ;

VariableDeclarations:
   vars  (variableDeclarations+=VariableDeclaration)+;

VariableDeclaration:
  name=ID  :  type=[VariableType|ID];

Rules:
   rules  (rules+=Rule)+;

Rule:
  head=Head  :-  body=Body;

Head:
  predicate=Predicate;

Body:
  (predicates+=Predicate)+;

Predicate:
  predicateType=[PredicateType|ID]  (  (terms+=Term)+  ) ;

Term:
  variable=Variable;

Variable:
  variableDeclaration=[VariableDeclaration|ID];

terminal WS:
  (    |  	  |  
  |  
  |  , )+;

And, the following is a program in the above DSL.

var types
  Node

predicate types
  Edge(Node, Node)
  Path(Node, Node)

vars
  x : Node
  y : Node
  z : Node

rules
  Path(x, y) :- Edge(x, y)
  Path(x, y) :- Path(x, z) Path(z, y)

The following is my subclass of the generated Switch class that demonstrates the getPredicateType() returns null on a Predicate node.

public class ModelPrinter extends MyDSLSwitch<Object> {

    protected Object visitChildren(EObject object) {
        for (EObject eobj : object.eContents()) {
            doSwitch(eobj);
        }   
        return object;
    }

    @Override
    public Object casePredicate(Predicate object) {
        System.out.println(object.getPredicateType());
        return object;
    }

    @Override
    public Object defaultCase(EObject object) {
        return visitChildren(object);
    }

}

When I used the ModelPrinter class to traverse the EMF object model corresponding to the above program, I realized that the nodes are not linked together properly. For example, the getPredicateType() method on a Predicate node returns null. Having read the Xtext user s guide, my impression is that the Xtext default linking semantics should work for my DSL. But, for some reason, the AST nodes of my DSL don t get linked together properly. Can anyone help me in diagnosing this problem?

Answer 1

Finally, I figured out the problem. The links were not set properly because I wasn t loading the model properly. I had just used the parser to load the model. So, I didn t get the links. Therefore, I used the following code snippet from Xtext FAQ to load the model correctly. Then, I passed the returned model to my switch class.

// "workspace" is a string that contains the path to the workspace containing the DSL program.
new org.eclipse.emf.mwe.utils.StandaloneSetup().setPlatformUri(workspace);

Injector injector = new MyDslStandaloneSetup().createInjectorAndDoEMFRegistration();
XtextResourceSet resourceSet = injector.getInstance(XtextResourceSet.class);
resourceSet.addLoadOption(XtextResource.OPTION_RESOLVE_ALL, Boolean.TRUE);

// "DSLProgram" is a string that contains the path to the file of the DSL program relative to the workspace set above.
Resource resource = resourceSet.getResource(URI.createURI("platform:/resource/" + DSLProgram), true);
Model model = (Model) resource.getContents().get(0);

Answer 2

I have tried it out, but I am not familiar with the Switch, I rather used Xpand/Xtend to access predicateTypes from Predicate and generated their names.

Template.xpt:

«IMPORT myDsl»;

«DEFINE main FOR Model-»
«FILE "output.txt"-»
«FOREACH this.rules.rules.body.last().predicates AS p-»
«p.predicateType.name»
«ENDFOREACH-»
«ENDFILE-»
«ENDDEFINE»

and the output.txt:

Path
Path

I guess this is the expected behaviour.

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