Simple Abstraction for Builder objects in Ruby

builder.rb

class Builder < Struct
  def self.with(spec, &block)
    struct = new(*(spec.keys)) do
      def initialize(options={})
        super
        options.each { |opt, value| self[opt] = value }
        freeze
      end

      def with(options, &block)
        new_struct = dup
        options.each { |opt, value| new_struct[opt] = value }
        if block_given?
          new_struct.class.class_eval do
            define_method :build, &block
          end
        end
        new_struct.freeze
      end

      members.each do |opt|
        define_method "with_#{opt}" do |value, &block|
          with(opt => value, &block)
        end
      end

      define_method :build, &block
    end
    
    struct.new spec
  end
end

example.rb

person = Builder.with first: nil, last: nil, registered: false do
  "#{first} #{last} is#{registered ? '' : ' not'} registered"
end
person.build
# => "  is not registered"

mcfly = person.with_last "McFly"
mcfly.build
# => " McFly is not registered"

george = mcfly.with_first "George" do
  "I'm #{first}.  #{first} #{last}.  I'm your density.  I mean your destiny."
end
george.build
# => "I'm George.  George McFly.  I'm your density.  I mean your destiny."
george.registered
# => false