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

[Andreas Rueckert]

Hi!

> 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:       
  
--<snip>--

Ok, I use the following construct now:
=======================================
// The constant pool.
constant_pool
{ int poolSize=0; }
	: poolSize=u2			             		// Parse the size of the constant pool + 1
	  ( {--poolSize > 0}? cp_info )* {poolSize==0}?  	// Parse <poolSize-1> cp_info structures.
	;
=======================================
and get the following warning:
=======================================
     [java] ANTLR Parser Generator   Version 2.7.1   1989-2000 jGuru.com
     [java] ../../modules/classfile/src/org/argouml/uml/reveng/classfile/classfile.g:66: warning: nondeterminism upon
     [java] ../../modules/classfile/src/org/argouml/uml/reveng/classfile/classfile.g:66:        k==1:BYTE
     [java] ../../modules/classfile/src/org/argouml/uml/reveng/classfile/classfile.g:66:        between alt 1 and exit branch of block  
=======================================
No idea for a workaround so far on my side (but I haven't really tried a lot,
since I was busy hacking the classfile specs in a grammar file).

I just learned, that the 'match n times' might have a few advantages, that have
not yet discussed IIRC.

- It seems, that I cannot assign the result of the n-th match to the n-th
element of an array. At least, I get an error, when I try to do something, like
=======================================
	: length=u2 { bytes = new byte[length]; }
	  ( {length-- > 0}? bytes[bytepos++]=u1 )* {length==0}? { name = new String(bytes); } 	;  
=======================================
Workaround is simply to use a buffer for the result.

- The trailing sematic predicate throws a semantic exception, while a
recognition exception makes more sense in my eyes.

- The predicate loop is very readable, if you don't have the generated code in
mind. Maybe some explanation in the Antlr manual would help.

Ciao,
Andreas

 

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