FIX ME: Sample for metaprogramming: Dynamic and lazy definition of attributes and methods.

person.rb

class Person
  def initialize(args)
    args.each do |key, value|
      self.class.send(:attr_accessor, :"#{key}") unless instance_variables.include?(:"@#{key}")
      instance_variable_set(:"@#{key}", value)
    end
  end

  def self.undef_has_a_var_which_begins_with(var)
    method_name = "has_a_#{var}_which_begins_with?"
    send("undef_method", method_name) if method_defined? method_name
  end
  
  def self.define_has_a_var_which_begins_with(var)
    method_name = "has_a_#{var}_which_begins_with?"
    define_method(method_name) do |expectation|
      value = instance_variable_get(:"@#{var}")
      if value.is_a?(String)
        value.split(//)[0].downcase == expectation.downcase
      else
        self.class.undef_has_a_var_which_begins_with(var)
        method_missing(method_name.to_sym)
      end
    end
  end

  def method_missing(m, *args, &block)
    if m.to_s =~ /has_(a|the)_(.+)_(which|that)_begins_with\?/ &&
        instance_variables.include?(:"@#{Regexp.last_match[2]}") &&
        instance_variable_get(:"@#{Regexp.last_match[2]}").is_a?(String)
      var_name = Regexp.last_match[2]
      self.class.define_has_a_var_which_begins_with(var_name)
      send("has_a_#{var_name}_which_begins_with\?".to_sym, *args, &block)
    else
      super(m, *args, &block)
    end
  end
end

#### sample #####
if RUBY_VERSION < "1.9"
  raise "This script can be executed only on Ruby 1.9. Sorry."
end
shimada = Person.new(:name => "Shimada", :age => 18, :sex => :half, :fuga => "bababa")
p shimada.has_a_name_which_begins_with?("S") ? "Good" : "Suck"
p shimada.has_the_name_which_begins_with?("S") ? "Good" : "Suck"
shimada.name = 5
# the following raises Error because name isn't a String
p shimada.has_the_name_which_begins_with?("5") ? "Good" : "Suck"