[antlr-interest] disabling portions of a rule?

[Hill, Robert]

Thanks Loring, I've already gone with the re-throwing of the
failedPredicate Exception, its not really nescessary that I continue,
since the problem with the parsed file is a fatal error at this point
anyway. I didn't think about using a predicate for 2 rules, one being
empty - neat another tool to remember!.

I see now why we cant skip nodes(I'd used that in V2 so wondered why,
but have been too busy using it to pay attention as to the reasons why
:) )
Thanks for the explanation, it all makes sense now! 

Cheers!
Rob

Robert Hill
Information Engineer
EDS	UKIMEA DWP ACU, Hallamshire Business Park, 100 Napier St,
Sheffield. S11 8HD
email:	rhill03 at eds.com
Office:	+44 114 291 1928
Mobile:	+44 7903 185 516

 

>-----Original Message-----
>From: antlr-interest-bounces at antlr.org 
>[mailto:antlr-interest-bounces at antlr.org] On Behalf Of Loring Craymer
>Sent: 31 October 2006 02:12
>To: Robert Hill; 'ANTLR Interest'
>Subject: Re: [antlr-interest] disabling portions of a rule?
>
>ANTLR 3 does an inorder traversal of the tree being walked; 
>shortcuts are not supported.  This is different from ANTLR 2, 
>where you could match a node and then ignore its children (and 
>siblings).  The big gain for the inorder traversal is that you 
>can support
>k>1 for tree walkers.  A secondary benefit is that you
>get strong checking of tree structure, but that is 
>counterbalanced by the inability to do shortcutting. 
>[I consider this a gain, but I start with automtically 
>generated tree grammars.]
>
>The best you can do right now is to have two versions of the 
>subrule--one with actions and one without--and use the sempred 
>to pick which one to use.
>
>--Loring
>
>--- Robert Hill <rob.hill at blueyonder.co.uk> wrote:
>
>> Hi all,
>> I Have a rule like this
>> 
>> exampleRule
>> @init {
>> 	Symbol s = null;
>> }
>> 	: ^(MAP Identifier { s =
>> Symbol.Resolve($Identifier.text);
>> 				if (s==null) 
>> 					ShowError();
>> 			}
>> 		({s==null}? Subrule[s]);
>> 	)
>> 
>> So , basically if the identifier isn't in the symbol table we don't 
>> call the subrule. The predicate does the job but when a symbol isn't 
>> found I get a whole bunch of errors about mismatched tree nodes..
>> - how do I either
>> 
>> a) exit the rule early without displaying the antlr errors, or
>> b) consume the tokens that follow even though I cant call 
>the subrule 
>> to get rid of them?
>> 
>> The subrule assumes it will always be passed a valid symbol, and has 
>> quite a few alts in it, so I don't want to add a if
>> (s!=null) before all of the code
>> in the subrule.
>> I'd like to prevent antlr from showing its mismatched tree errors, 
>> whilst preferably not exiting with a null pointer exception  from 
>> within the subrule...
>> 
>> Whats the neat/preferable way of handling this?
>> Cheers!
>> 
>> Rob
>> 
>> 	
>> 
>> 
>> 
>
>
>
> 
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