[antlr-interest] Re: Antlr grammar to parse Java classfile?

[Terence Parr]

On Wednesday, December 5, 2001, at 12:12  PM, Andreas Rueckert wrote:
>
>> Anyway, concerning the "match n times" thing.  You're right...it would
>> be pretty useful.  What syntax is appropriate and how to do you say
>> 0...n vs 1..n?  Perhaps, for uniformity, we use my "element modifier
>> syntax (e.g., "INT<AST=INTNode>"):
>>
>> ids4 : ( ID )+<n=4> ;  // weird looking thing
>>
>> ids4 : ( ID )+<4> ; // a little better ("n" would be the default
>> variable)
>>
>> ids2opt : ( ID )*<2> ; // 0..2 not 1..2
>
> But in all these cases, you used constants. Many fileformats come a 
> counter,
> that defines, how long the following data block is. So you have to 
> parse the
> counter 1st, and then match the next rule <counter> times.

Oh, right.  Yeah not a problem at all except for syntax.  Should it 
assume non-int implies variable defined in C++, Sather, or Java 
(whatever your output is)?  I'm partial to keeping all non-grammatical 
stuff in {...}.  Perhaps this then:

( ID )+{n=blockCount;}

or something?

>> The implementation would be pretty simple I guess.  Just defines a
>> counter like the (...)+ does and generates an error if you don't get n;
>> counter<=n would need to go in the while loop, but should be easy.
>
> Yup! The implementation should be trivial. But since we don't have that 
> feature
> yet, I'm trying to come up with something, that works with the current 
> Antlr.
> I thought about the semantic predicate again and that might work, too. 
> Like
> =======================================
> rule
> {int counter=0}
> 	: counter=blockCount
> 	  ( {counter>0}? blockEntry {counter--;} )*
> 	;
> =======================================
> Do you think it would work? So that block entry is matched <counter> 
> times?

Let's see.  Well, it's close, but it wouldn't give you an error if you 
have less than blockCount.  You need that "did I get enough" check at 
the end of the loop.  Could do with a simple "validating" sem pred after 
loop.

Ter
--
Chief Scientist & Co-founder, http://www.jguru.com
Creator, ANTLR Parser Generator: http://www.antlr.org


 

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