[antlr-interest] Re: syn pred / lookahaed doesn't work

[Albert Huh]

You're right about it not being ambiguous.  It's an issue with the antlr code generation.

lgcraymer actually posted a version that is better than mine.  you should use his.

But ya, you're right about it not being ambiguous.  It's the way antlr generates code.  Check out http://www.antlr.org/doc/lexer.html#Limitations_of_Nongreedy_Subrules for an example.

The problem here is that the first thing can be either a function or a statement.  Look at the generated code and you will see how antlr decides to exit the loop.




-----Original Message-----
From: Gunnar Wagenknecht [mailto:kreismeister at yahoo.de]
Sent: Wednesday, March 12, 2003 12:26 AM
To: antlr-interest at yahoogroups.com
Subject: [antlr-interest] Re: syn pred / lookahaed doesn't work



--- In antlr-interest at yahoogroups.com, "Albert Huh" <albert.huh at e...> 
wrote:

> ambiguity with k=2
> ------------------
> 1) the second token of a function can be either an IDENTIFIER 
or "function"
> 2) the second token of a variableDeclaration is an IDENTIFIER
> 3) we know that variableDeclaration is a statement
> 4) so a statement looks exactly like a function at k=2

Mhm, but I thought this is not an ambiguity because if the second 
token of a function is an IDENTIFIER than the first token of a 
function is NOT "static" or "ecit".

> i'm probably not stating anything new here to anybody.  i *think* 
this is a linear approximation issue, can somebody else confirm this?

What the hack is that?

> alternatively, you can get rid of teh warning with a combination of 
recursion and syntactic predicates.  try rewriting your statementList 
rule like this:
> 
> statementList:
>   (statement statement)=>statement statementList
>   |(statement)=>statement
>   | //empty rule, do nothing
> ;


Thank you, that was something I was trying without getting warnings 
but I couldn't finished it. Now I will try it. I'm wondering if this 
construct slows down the generated parser in a very bad way. That's 
why I threw away this solution.

Cu, Gunnar


 

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