variant.rb

class Variant
  attr_accessor :variant_tag

  def self.variants(hsh)
    hsh.map do |tag, args|
      variant(tag, args)
    end
  end

  def self.variant(tag, args)
    args = [args] unless args.is_a?(Array)

    @@variants ||= {}
    @@variants[tag] = args

    arg_definitions = @@variants[tag].map do |arg_name|
      "tmp.#{arg_name} = %s"
    end.join("\n        ")

    code = <<-EOF
      def #{tag.to_s}(#{args.join(', ')})
        attr_accessor *#{args.inspect}

        tag = #{tag.inspect}
        tmp = self.new

        tmp.variant_tag = tag
        #{format(arg_definitions, *args)}

        tmp
      end
    EOF

    self.class.class_eval code

    tag
  end

  def cases(hsh)
    hsh.each do |k, v|
      if variant_tag == k
        args = @@variants[k].map{|x| send(x)}

        return v.call(*args)
      end
    end
  end
end


class Order < Variant
  variants  delivery: [:address],
            digital:  [:email],
            pickup:   [:store_id]
end

def test(order)
  puts order.cases(
    delivery: ->(address)  { "delivering to #{address}" },
    digital:  ->(email)    { "emailing to #{email}" },
    pickup:   ->(store_id) {
      "picking up from store ##{store_id}"
    },
  )
end


# n constructors
test Order.delivery('123 Main St')         #=> delivering to 123 Main St
test Order.digital('customer@example.com') #=> emailing to customer@example.com
test Order.pickup(456)                     #=> picking up from store #456