From: Tom Aadland <tomhaa@...>
Date: 2008-07-14T10:00:30+09:00
Subject: Re: Treetop positive lookahead problem

Clifford Heath wrote:
> Tom Aadland wrote:
>> 
>> This should as I understand it parse the string "foobar" and return only
>> "foo". For some reason this returns nil. I tried the opposite (switched
>> '&' with '!') and that works like a charm.
> 
> Tom,
> 
> For us to know why, you should post the entire grammar, together with
> the text you're parsing, and preferably also a snippet showing how
> you're calling the parser.

Hi Clifford,

Thank you for responding and I'm sorry I didn't provide more 
information. Also as I said I think I was missing something fundamental. 
I was gonna paste this in pastie, but pastie is down :(

I thought that the 'foo' &'bar' rule meant that bar has to come togehter 
with foo as in foobar and on top of that not really consume bar so that 
the text_value returned would only be foo. However the following test 
below runs fine with that in mind :)

What I want with this rule is: match foobar or even foobarsomething, but 
return the textvalue without bar.

grammar FooGrammar

    rule test_with_bar
    'foo' (&'bar' .+) {
      def value
        text_value
      end
    }
  end

end

require 'rubygems'
require 'treetop'
require 'test/unit'

class TestFoo < Test::Unit::TestCase
  Treetop.load "foo_grammar"

  def test_parse_foo_with_bar
    res = FooGrammarParser.new.parse("foobar")

    assert_equal res.value, "foobar"
  end
end


Thanks,

Tom

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