[antlr-interest] Guarded Predicates

[David Ewing]

on 12/13/02 11:33 AM, Terence Parr at parrt at jguru.com wrote:

> Yeah, I have a note to look into this...are you using Java output?
> Does it look like ANTLR generates something for the guard?

David's using C++ output (your favorite ;-). I've got an example of what
looks like bogus code to me too. I'll send it to you off-line.

Dave

> 
> On Friday, December 13, 2002, at 06:42  AM, David Wigg wrote:
> 
>> Guarded predicates do not appear to do what I expected from them
>> as described in the ANTLR documentation (which seemed sensible
>> and intuitively correct).
>> 
>> It says the following;
>> 
>> ANTLR provides a guarded predicate to allow you to specify the
>> lookahead context under which a predicate should be evaluated.
>> The syntax is:
>> 
>> (lookahead-context-for-predicate)=>{predicate}?
>> 
>> A guarded predicate is useful in situations where the semantic
>> predicate should be hoisted into the prediction decision only
>> when the lookahead is consistent with some context. For example:
>> 
>> a   :   (ID)=>{isType(LT(1))}? (ID|INT)
>>      |   ID
>>      ;
>> 
>> Here, the predicate is only applicable when an ID is found on
>> the input stream. It should not be evaluated when an INT is found.
>> 
>> 
>> So, in the case illustrated above if an INT was next the guarded
>> predicate would not be evaluated and parsing would continue
>> within the first alternative to (ID|INT).
>> 
>> In fact as far as I can see the effect of the current
>> implementation is that unless the predicate is true parsing will
>> not fall through to the rest of the alternative anyway.
>> 
>> It's quite simple to put the above rule in a .g file to compile
>> it and to examine the resulting generated code.
>> 
>> I find it rather ominous that I have only been able to find any
>> examples of guarded predicates in TinyBasic.g in the java
>> examples. Does this mean that other people have found that
>> guarded predicates don't work?
>> 
>> Needless to say I have not found anything about this in the
>> FAQs. "No results found" was the response to a search for "Guarded".
>> 
>> David.
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> abstract_declarator
>>    :  (SCOPE|ID)=> {qualifiedItemIsOneOf(qiPtrMember)}?
>>       ptr_operator abstract_declarator {declaratorPointerTo();}
>>    |  LPAREN abstract_declarator RPAREN
>>       (abstract_declarator_suffix)+
>>    |  (LSQUARE (constant_expression )? RSQUARE  {declaratorArray();}
>>       )+
>>    |  /* empty */
>>    ;
>> 
>> 
>> 
>> 
>> Your use of Yahoo! Groups is subject to
>> http://docs.yahoo.com/info/terms/
>> 
>> 
> --
> Co-founder, http://www.jguru.com
> Creator, ANTLR Parser Generator: http://www.antlr.org
> Lecturer in Comp. Sci., University of San Francisco
> 
> 
> 
> 
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
> 
> 
> 


 

Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/