[antlr-interest] optional rule refs

[Johannes Luber]

Terence Parr wrote:
> Howdy,
> 
> When obtained results from optional rule refs in template generation,
> you can get a null ptr:
> 
>         ^('if' completeExpression s=statement (el=statement)?)
>         -> if(cond={$completeExpression.st}, stat={$s.st},
> elsestat={$el.st})
> 
> here, el is optional and hence null sometimes.  $el.st will cause an
> exception.  There is no way to test $el by itself as it needs a
> property.  There is no other token/node to test either.
> 
> I propose that all refs to rule properties become the equivalent of:
> 
> $rr!=null?$rr.property:null
> 
> It causes a lot of branching unnecessarily when rr is not null, but
> makes life much easier.  A number of people have run into this.  I
> suppose if it's too "inefficient" for some, they can tweak the templates
> to remove the ?: operation.
> 
> Comments?
> 
> Ter
> 

I like this. I've looked over my uses and I used either your pattern or
implemented my own. For the first your proposal simplifies things and
the other cases don't seem do be affected negatively.

Best regards,
Johannes Luber