[antlr-interest] Why cant += be used without an "output=" option (and other Q's)

[Gavin Lambert]

At 14:32 19/02/2008, Benjamin Shropshire wrote (off-list):
 >I'm guessing from that statement that something like this would 
be
 >done
 >
 >a returns [MyType c] :
 >        (b {if(c == null) c = new MyType(); c.accumulateB($b); 
})*
 >        (g {if(c == null) c = new MyType(); c.accumulateG($g); 
})*
 >        ;

Well, you could do that, but probably a more common use would be:

a returns [MyType c]
@init { $c = new MyType(); }
   :  (  b { $c.accumulateB($b); }
      |  g { $c.accumulateG($g); }
      )*
   ;

Note: I changed the grammar slightly to permit 'b's and 'g's to be 
intermixed.  If they're strictly ordered then things get even 
simpler than this, since they can be factored out to type-specific 
subrules.  Also this version will return an 'empty' object instead 
of a null when no items are present, but that seems reasonable to 
me.

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