[antlr-interest] Check if rule returns empty result

[Christian]

Thank you for your hint, but I do not use tree grammars mainly because
of Andy Tripp's article:
http://www.antlr.org/article/1170602723163/treewalkers.html

I walk through the tree by means of the visitor pattern instead. Main
reasons for me to be against tree grammars:

  * Code/rule duplications, especially I have to know how the AST looks
    like.
  * No syntax check, code highlight, and code assistant in actions,
    especially the programming language is mixed with the tree grammar
    syntax.


Christian

Am 04.11.2011 00:22, schrieb Jim Idle:
> Remember that your tree parser rule for the body should be:
>
> method_body
>     : ^(METHOD_BODY statement_list)
>     | METHOD_BODY  // Empty body
>     ;
>
> 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 2:37 PM
>> To: antlr-interest at antlr.org
>> Subject: Re: [antlr-interest] Check if rule returns empty result
>>
>> 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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