[antlr-interest] Check if rule returns empty result

[Christian]

Thank you for your explanation. And your solution works great, too.

Christian

Am 03.11.2011 21:31, schrieb 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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