[antlr-interest] Problems with semantic predicates

[Jim Idle]

Hari,

 

I think that you probably need to step back from the specific thing you are trying to do here and rework the grammar without backtracking. The backtracking option is a good thing for prototyping or when the thing you are going to parser doesn't particularly require speed, but if you use it without regard to the grammar construction you will usually end up in trouble. This is because it masks poor grammar construction, which, if you built up the grammar one 'rule' at a time would become more obvious.  

 

If you are getting syntax errors with conditional rewrites, then there must be something else wrong in there first I suspect. Additionally, I am having trouble seeing why you would 'need' to rewrite the same construct in two different tree shapes. Beware constructing a tree that 'looks' good, as though it isn't  bad way to think through your tree construction, it can make you forget that what you want is an efficient tree structure that has no ambiguities when parsing it. Perhaps you just always need ^(NODE xxx)?

 

You may be reluctant to restart your grammar as you feel you have put a lot of work in to it, but if you start with a blank slate and no backtracking, you will be able to borrow from your original grammar and get a cleaner, faster grammar that won't cause you maintenance headaches. Otherwise, it looks like you will be spending a lot of time on the 'last 10%' of this grammar. Once you forgo backtracking you will find that generally things like this will actually be easier, not more difficult.

 

Jim

 

From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-bounces at antlr.org] On Behalf Of Haralambi Haralambiev
Sent: Friday, April 18, 2008 5:12 AM
To: Johannes Luber
Cc: antlr-interest at antlr.org
Subject: Re: [antlr-interest] Problems with semantic predicates

 

Hello,

Is there a way to lure Ter's interest to answer whether this braces (sorry for the mistake ;-) ) functionality is in place?

I am using the latest version, 3.0.1, that is downloadable from the site, so I guess it's not implemented :-(

Hari

On 4/18/08, Johannes Luber <HYPERLINK "mailto:jaluber at gmx.de"jaluber at gmx.de> wrote:

Haralambi Haralambiev schrieb:

Hi Johannes,

I don't understand what's the benefit of the two brackets.

When I write {{$r::flag = false;}} the code that ANTLR produces is "{((r_scope)r_stack.peek()).flag = false;}", i.e. the brackets are added also, but they are still in "if (backtracking == 0)" statement, which is the check I want to skip somehow.

Hari


The two braces (brackets are "[]") should be the syntax which enables an action to be executed always, even during backtracking. But either you are using an too old version of ANTLR or Ter didn't implement this variant after all.

Johannes

 


On 4/17/08, *Johannes Luber* <HYPERLINK "mailto:jaluber at gmx.de" \njaluber at gmx.de <mailto:HYPERLINK "mailto:jaluber at gmx.de" \njaluber at gmx.de>> wrote:

   Haralambi Haralambiev schrieb:

       Hello Johannes,

       So, if I understand correctly, you suggest to have the following
       (using a scope):

       r2
       : {$r::flag}? Token1 -> ^(NODE Token1)
       | Token1
       ;

       If the above is a correct assumption, then how can I set the
       scope variable, as I could only think of the following way:

       r
       scope {
        boolean flag;
       }
       : Token1^ Token2!
          {$r::flag = true;}    r2
       | Token1^
        {$r::flag = false;}
        r2
       ;

       This solution, however, will fail, as the action for setting the
       scope variable will not be executed until the backtracking is
       over. But I need this variable set, as it will be used as a
       predicate for the parsing. Thus, we have a catch-22 situation.

       A solution I could think of is not to have two alternatives for
       r2, but rather construct the CommonTree manually with java code
       and within to use the knowledge of the boolean flag. But I
       dislike this solution and am open to suggestions.


   Try using {{$r::flag = true;}}.

   Johannes



 

 

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