From: Steven Jenkins <steven.jenkins@...>
Date: 2005-04-25T08:30:02+09:00
Subject: Re: Racc

Peter Suk wrote:
> I need help using Racc.  I've been trying to go through the basic
> calculator example, but no go.  The grammar is processed without any
> shift-reduce conflicts.  It appears I am feeding the wrong kind of
> tokens to it.  I keep getting:
> 
> 
> Parsing file:
> /usr/local/lib/ruby/site_ruby/1.8/racc/parser.rb:377:in `on_error': 
> (Racc::ParseError)
> parse error on value 1 (NUMBER) from
> /usr/local/lib/ruby/site_ruby/1.8/racc/parser.rb:104:in `_racc_do_parse_c'
>         from /usr/local/lib/ruby/site_ruby/1.8/racc/parser.rb:104:in
> `__send__'
>         from /usr/local/lib/ruby/site_ruby/1.8/racc/parser.rb:104:in
> `do_parse'
>         from racc-calc-2.y:47:in `parse'
>         from ruby_calc_test.rb:20

That means there's no rule that matches the input.

> I am feeding it tokens that are two-element arrays, as stated in the
> documentation.  <http://i.loveruby.net/en/man/racc/parser.html>  But it
> is not at all clear what should be in the second element.

Racc doesn't care. As long as the lexer and your productions agree on
the type (or on the duck-typing), it's OK.

Your understanding appears to be fine. You just have a simple bug--see
below.

> [snip]
> -------- FakeLexer.rb --------
> require 'RubyCalcParser'
> 
> class FakeLexer
> 
>     def initialize(tokens)
>         @tokens = tokens
>         @index = 0
>     end
> 
>     def reset()
>         @index = 0
>     end
> 
>     def yylex()
>         rval = @tokens[@index]
>         @index += @index

          @index += 1

>         return rval
>     end
> 
> end
> -------- end --------

It's returning two NUMBERS in a row, which is a syntax error in your
grammar.

Don't you love bugs like this? If you didn't suspect your understanding
of Racc, you'd probably have found in in two minutes. HTH.

Steve