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

[mzukowski at bco.com]

> Actually, I would argue that semantic predicates are the way 
> to go; as  
> Monty pointed out 
> 
>      ( { n > 0 }? foo { n--; } )*
> 
> will parse at most n iterations and you can add a sem pred as a 
> termination check, as Ter suggested.  Recursive definitions also work:
> 
> loop
> { int n = N; }
>      :
>        { n > 0 }?  foo { n--; } loop
>        | { n == 0}?
>      ;
> 
> will parse exactly n copies of foo or throw an exception.
> 
> Unfortunately, syntactic predicates don't behave the same in loops as 
> they do in recursive definitions--the recursive solution works as 
> ANTLR is currently implemented.  

It doesn't work quite as you have it above.  The action n-- is ignored when
guessing.  But you are right that recursive definitions will do the trick:

loop[int N] 
{ int n = N; }
     :
       { n > 0 }?  foo loop[n-1]
       | { n == 0}?
     ;

generates:

if (((LA(1)==LITERAL_hey))&&( n > 0 )) {
	foo();
	loop(n-1);
}
else if (((LA(1)==EOF||LA(1)==LITERAL_hey))&&( n == 0)) {
}
else {
	throw new NoViableAltException(LT(1), getFilename());
}

With a (valid) warning that is generated because of the possibility of
matching nothing in the loop rule.

Monty

 

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