[antlr-interest] Default values in tree rewrite rules

[Emond Papegaaij]

Hello,

I've got the following parser rule in which 'superclass' is optional:
  nodeDecl
   : (options {k = 1;} : (nodeNames ':')=> bindings=nodeNames ':')?
     (declAbstract=ABSTRACT)?
     type=targetClass
     ('extends' superclass=targetClass)?
     ( CBR_OPEN body CBR_CLOSE | SEMI )
     -> ^(NODE_SPEC $type $declAbstract? $superclass? body? $bindings?)
   ;

Now I want superclass to become the tree ^(TYPE["Node"]) when it is not 
defined in the input (and it will no longer be optional in the rewrite 
rule). Is this possible? All I've come up with is:
     ('extends' superclass=targetClass)? {
       if (superclass == null) {
         superclass = new targetClass_return();
         superclass.tree = adaptor.create(TYPE, "Node");
       }
     }
Which is quite nasty.

Best regards,
Emond