From: Russell Fulton <r.fulton@...>
Date: 2006-07-09T06:59:25+09:00
Subject: accessors and anonymous classes

Hmmm... odd I wrote a version of this last night but it would appear 
that I failed to post it to the forum, doh! so here it goes again.  If 
it did get out and this is a duplicate then please accept my apologies 
for the waste of bandwidth.


I am writing a recursive descent parser to parse a configuration file 
that consists of nested 'Sections' which all look like this:
[<section type> <possibly optional name>] {
  <item>
  <item>
    .
    .
}

where <item> may be another section.

I wrote a clase Section to parse the basic structure. Section.initialise 
calls method specificItems to parse non section items (see below). For 
each type of section I have an anonymous class tied to a Class variable 
-- I have included one such class below.  I had to make the classes 
anonymous because I needed to include references to the classes in a 
data structure (see subSections ) which tells the parser which section 
types are valid in this section and what to do with them.

[ aside: someone asked why I wanted reference to scalar variable in my 
previous post -- this is why. I needed to have a refernce to a variable 
in that structure into which I could store the parsed class]

Now to the problem:  As you see I have defined a bunch of accessor 
methods using attr_reader and attr_writer, but these don't work ( I get 
an undefined method error ).  Adding the accessor methods explicitly 
works fine.  Odd!

I'm not sure if this is a bug, a feature or simply some lack in my 
understanding of how things should work.

Any ideas?

Also this is my first real OO Ruby program and I would appreciate 
comments -- it is too large (currently 1000 lines) to post here but if 
anyone would be willing to have a look at it I'd be happy to email it to 
them.  My email is r.fulton@auckland.ac.nz.

I have run into various problems in this project and have managed to 
solve all of them but I am far from confident that my solution was the 
best way of doing it.  The use of anonymous classes is one example.  I 
wrote a generic text parser for this project and because Ruby does not 
support multiple inheiritance I ended up passing an instance of the 
parser as a parameter to all the methods -- I figured that since the 
parser needed to maintain state that I could not make it a mixin...

Cheers, Russell


    @@hostService = Class.new( Section ) {

      attr_reader = :services, :converted, :actions, :patterns, 
:realTime,
                    :periodic, :files
      attr_writer = :converted

      def initialize( head, parser )
        @services = []
        @actions = []
        @patterns = []
        @realTime = []
        @periodic = []
        @files = []
        @converted = false
        super( head, parser )
      end

# accessor methods --- since the shorthand way did not work
      def actions
        @actions
      end

[ boring repeditive stuff snipped ]

      def converted=(value)
        @converted = value
      end

# this handles the non section items in the section

      def specificItem( firstToken, parser )

        case firstToken
        when 'service'
          if token = parser.expect(/^(\w+)/, "service name") then
            @services.push( token )
          else
            @errors = true
            parser.restofLine   # ignore the rest of the line
          end
        else
          @errors = true
          parser.error( "#{firstToken} not valid in host section" )
          parser.restofLine   # ignore the rest of the line
        end
     end

# defines which sections may be nested within this section
# Key is the section type the vailue is [ <name required>,
# class to parse section and where to store the result ]

      def subSections( kind )
        {'actions'     => [nil, @@actionList, @actions ],
         'files'       => [nil, @@commaList, @files],
         'patterns'   => [nil, @@patternList, @patterns],
         'realtime'   => [nil, @@matchList, @realTime],
         'periodic'   => [nil, @@matchList, @periodic]
        }[kind]
      end
    } # end of hostSection

-- 
Posted via http://www.ruby-forum.com/.