Re: Newbie requestion on operator precedence and how to resolve an s/r conflict correctly.

[Hans Aberg]

On 20 Feb 2008, at 09:00, Arlen Cuss wrote:

> I'm writing a parser for Ruby (http://www.ruby-lang.org/). So far,  
> it can
> parse a sizeable subset of the language. The problem I'm  
> encountering is as
> follows:
>
> `obj.method' is a method call on `obj'. This is parsed correctly.
> `obj.method(args)' is a method call with arguments given. Also fine.
> `obj.method(args).another_method' is a method call to `obj' with some
> arguments, then a method call to the return value of that method.  
> This is
> parsed incorrectly.
>
> A shift/reduce conflict occurs where `obj.method(args)' is seen by my
> parser. Instead of reducing when it sees the upcoming `.', it  
> shifts, and it
> ends up with the whole `obj.method(args).another_method' on the  
> stack. Then
> it reduces, and we get `(args).another_method' and hence
> `args.another_method'. It's recognised by the parser as something like
> `obj.method(args.another_method)' instead.
>
> I'm trying to work out how to convince bison to reduce at this  
> point, by
> manipulating the precedence of ')', '.', etc., but so far to no  
> avail. Any
> ideas? I'll be happy to post the code on request.

The general method to set token precedences for resolving S/R  
conflicts is to look in the .output file for the conflicting states,  
and in them, at the tokens immediately before and after the parsing  
dot "." in those rules. The set token precedences on those.

In your case, it seems that the method "." should be parse the the  
left, as in "+" say, and not to the right as in "=". So try use a % 
left ".".

Also, there is a Yaccable C++ grammar that might have similar  
constructs:
  http://www.parashift.com/c++-faq-lite/compiler- 
dependencies.html#faq-38.11

  Hans Aberg