parser.rb

module Internal
  def self.parsers
    @parsers ||= {}
  end
end

class Parser
  def initialize(input)
    @input = input
  end
  
  class << self
    delegate :parsers, to: Internal
    
    def parse(type, *args)
      parsers[type].new(*args).value
    end
    
    def register(type, &block)
      parsers[type] = Class.new(self, &block)
    end
  end
  
  private
  
  attr_reader :input
end

class PlainTextParser < Parser
  register 'p' do
    def value
      @value ||= input.upcase
    end
  end
end