Have some respect, and show some class!

classy.rb

module Classy
  module Errors
    def self.included(base)
      base.extend(ClassMethods.clone)
      base.private_class_method :error
    end

    module ClassMethods
      def error(name, super_klass = StandardError)
        klass_name = name.to_s.split('_').map(&:capitalize).join
        klass = Class.new(super_klass)
        const_set "#{klass_name}Error", klass
      end
    end

    private

    def error!(name)
      klass_name = name.to_s.split('_').map(&:capitalize).join
      raise self.class.const_get("#{klass_name}Error")
    end
  end

  module Builder
    def self.included(base)
      base.extend(ClassMethods.clone)
      base.private_class_method :builder
    end

    module ClassMethods
      DEFAULT_BUILDER_OPTIONS = {
        set_instance_variables?: true
      }.freeze

      def builder(options = DEFAULT_BUILDER_OPTIONS, &block)
        @@opt_args = []
        @@rest_args = []
        @@block_args = []
        @@klass = Class.new(Object)

        block.parameters.each do |kind, arg|
          case kind
          when :opt
            @@opt_args << arg
            @@klass.send :define_method, arg.to_s do |opt|
              instance_variable_set "@#{arg}", opt
            end
          when :rest
            @@rest_args << arg
            @@klass.send :define_method, arg.to_s do |*rest|
              instance_variable_set "@#{arg}", rest
            end
          when :block
            @@block_args << arg
            @@klass.send :define_method, arg.to_s do |&bblock|
              instance_variable_set "@#{arg}", bblock
            end
          end
        end

        if options[:set_instance_variables?]
          define_method :initialize do |*new_args, &new_block|
            opts   = @@opt_args.map { |oa| [oa, new_args.shift] }
            rests  = @@rest_args.map { |ra| [ra, new_args] }
            blocks = @@block_args.map { |ba| [ba, new_block] }
            (opts + rests + blocks).each do |name, val|
              instance_variable_set "@#{name}", val
            end
            block_args = opts.map(&:last)
            rests.map(&:last).each { |rest| block_args += rest }
            instance_exec(*block_args, &block)
          end
        else
          define_method :initialize, &block
        end
      end

      def build(&block)
        builder = @@klass.new
        builder.instance_eval(&block) if block_given?
        args = @@opt_args.map do |arg|
          builder.instance_variable_get "@#{arg}"
        end
        if rest_arg = @@rest_args.first
          ra = builder.instance_variable_get("@#{rest_arg}")
          args += ra if ra
        end
        if block_arg = @@block_args.first
          ba = builder.instance_variable_get("@#{block_arg}")
          new(*args, &ba)
        else
          new(*args)
        end
      end
    end
  end
end



class Message
  include Classy::Errors
  include Classy::Builder

  error :bad_subject
  error :no_subject
  error :no_body

  attr_reader :subject, :body, :args

  builder do |subject, body, *args, &delivery|
    error! :no_subject unless @subject
    error! :bad_subject unless @subject =~ /awesome/i
    error! :no_body unless @body
  end

  def deliver
    instance_eval(&@delivery) if @delivery
  end
end

class AutoDeliveredMessage < Message
  def initialize(*as)
    super(*as)
    deliver
  end
end

require 'json'

m1 = Message.build do
  subject 'testy awesome test'
  body 'ya boi'
end

m2 = Message.build do
  subject 'aight but awesome'
  body 'ya boi'
  args 'one', 'two', 'three'
  delivery do
    puts JSON.pretty_generate(
      kind: :message,
      subject: subject,
      body: body,
      args: args
    )
  end
end

m3 = Message.new('not awesome', 'but works', 'test', 'test', 'test') do
  puts JSON.pretty_generate(
    kind: :message,
    subject: subject,
    body: body,
    args: args
  )
end

m1.deliver
m2.deliver
m3.deliver

AutoDeliveredMessage.build do
  subject 'pretty awesome'
  body 'and works'
  delivery do
    puts JSON.pretty_generate(
      kind: :message,
      subject: subject,
      body: body,
      args: args
    )
  end
end