Question

I’m trying to create a mixin module, Parser, that will allow me to do the following :

class MyParser

 include Parser

 field :my_field, 1, 10
 field :my_other_field, 11, 15

end 

m = MyParser.new("1234567890abcde")

m.my_field # - > "1234567890"
m.my_other_field  # ->  "abcde"

I’m new to meta-programming in ruby

Here are my questions ?
I need to create a @fields array, for each class that includes Parser, how do I do that

I want a field class method that can add new fields to the @fields array, how can I access a instance variable from a class_method?

How can I get the MyParser.new method to work as described ?

Thanks

Answer 1

Here it is. if you have trouble understanding the code, let me know and I will try to clarify it for you.

module Parser
  def self.included(base)
    base.extend ClassMethods
  end

  def initialize(str)
    self.class.fields.each do |name, opts|
      instance_variable_set(:"@#{name}", str[opts[:start]..opts[:stop]])
    end
  end

  module ClassMethods
    def field(name, start, stop)
      @fields ||= {}
      @fields[name.to_sym] = {:start => start-1, :stop => stop-1}
      class_eval { attr_reader name }
    end

    def fields
      @fields
    end
  end
end