xtext prase of context dependant grammars - xtext

Question

I have the grammar
Model:
vars+=Vars*
funcs+=Funcs*;
Name:
name=ID;
VarName:
Name;
FuncName:
Name;
Funcs:
'func' left=FuncName (bracket?='(' ')')? '=' right=[Name]';';
Vars:
'var' VarName ';';
where the right hand size of the Func rule can be either of type VarName or FuncName depending is the brackets on the left hand size appear.
Must I modify the xtext grammar or do a type of validation/scoping?
Update 1
the scope function:
override getScope(EObject context, EReference reference) {
if (context instanceof Funcs) {
val func = context as Funcs
if (reference == MultiNameDslPackage.Literals.FUNCS__RIGHT) {
if (func.bracket) {
val rootElement = EcoreUtil2.getRootContainer(context)
val candidates = EcoreUtil2.getAllContentsOfType(rootElement, VarName)
return Scopes.scopeFor(candidates)
} else {
val rootElement = EcoreUtil2.getRootContainer(context)
val candidates = EcoreUtil2.getAllContentsOfType(rootElement, FuncName)
return Scopes.scopeFor(candidates)
}
}
return super.getScope(context, reference);
}
}
The left hand size is independent of the presence of the brackets in the editor.
Update 2
Using validation
#Check
def checkFuncContext(Funcs func) {
if (func.bracket) {
if (!(func.right instanceof VarName)) {
warning("Right hand size must be of Var type",
MultiNameDslPackage.Literals.FUNCS__RIGHT
)
}
} else {
if (!(func.right instanceof FuncName)) {
warning("Right hand size must be of Function type",
MultiNameDslPackage.Literals.FUNCS__RIGHT
)
}
}
}
The warning statements are not executed. The statement func.right instanceof FuncName) does not behave as expected.
How can I test for the correct instance?
Update 3
Using a modified grammar
VarName:
name=ID;
FuncName:
name=ID;
Funcs:
'func' left=FuncName (bracket?='(' ')')? '=' (right=[FuncName] | r1=[VarName]) ';';
does not compile: Decision can match input such as "RULE_ID" using multiple alternatives: 1, 2

Answer 1

You need to change your grammar to get the inheritance order for Name, FuncName and VarName right (Name super type of both)
Either use a parser fragment
fragment Name: name=ID;
Or use
Name:VarName|FuncName;
VarName: name=ID;
FuncName:name=ID;