[antlr-interest] Check if rule returns empty result

[Jim Idle]

You don't want that anyway as your nodes will not have token start and
stop information and you might need that.

method_declaration
     : method_header method_body
     ;

method_body
	: (
		  ';'!	
     	| '{'!   statement_list?   '}'!
       )
		-> ^(METHOD_BODY statement_list?)
     ;

method_header
	:  attributes ID etc

		->^(METHOD_HEADER attributes ID )
	;


Jim

> -----Original Message-----
> From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-
> bounces at antlr.org] On Behalf Of Christian
> Sent: Thursday, November 03, 2011 1:01 PM
> To: antlr-interest at antlr.org
> Subject: [antlr-interest] Check if rule returns empty result
>
> Hi at all,
>
> I have the following rules:
>
> method_declaration
>     : method_header method_body  -> ^(METHOD_HEADER method_header)
> ^(METHOD_BODY method_body)
>     ;
>
> method_body:
>     ';'!
>     | '{'!   statement_list?   '}'!;
>     ;
>
> Of course, method_body is executed because it has no question mark.
> However, introducing rewrite rules and operators cause a problem. By
> using ^(METHOD_BODY method_body), method_body must return a non-empty
> value. However, statement_list? is optional and thus may not match at
> all. In this case, method_body returns nothing anymore and I get an
> RewriteEmpty exception.
>
> How can I solve this problem by only changing rewrite rules?
>
> Regards,
> Christian
>
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