h_struct.rb

class HStruct
  def self.new(*members, defaults: {}, &block)
    invalid_defaults = defaults.keys - members
    fail "Invalid defaults #{invalid_defaults}" if invalid_defaults.any?

    klass = Class.new do
      @members = members
      @defaults = defaults

      class << self
        attr_reader :members, :defaults
      end

      attr_reader(*members) 

      def initialize(attrs = {})
        invalid_members = attrs.keys - self.class.members

        fail "No members #{invalid_members} in HStruct" if invalid_members.any?

        self.class.members.each do |key|
          value = attrs[key] || self.class.defaults[key]
          instance_variable_set :"@#{key}", value
        end
      end

      def ==(other)
        self.class == other.class && to_h == other.to_h
      end

      alias_method :eql?, :==

      def to_h
        self.class.members.map { |key| [key, public_send(key)] }.to_h
      end

      def each(&block)
        return to_enum(__callee__) unless block_given?

        to_h.each(&block)
      end

      def inspect
        contents = self.class.members
          .map { |key| %{#{key}="#{public_send(key).to_s.gsub('"', '\"')}"} }
          .join(' ')

        "#<hstruct #{self.class.name} #{contents}>"
      end
    end

    klass.class_eval(&block) if block_given?
    klass
  end
end

# Usage

MyStruct = HStruct.new(:foo, :bar, :baz, :bat, defaults: { baz: 3 }) do
  def batz
    4
  end
end

data_bag = MyStruct.new(foo: 1, bar: 2)
data_bag.foo #=> 1
data_bag.bar #=> 2
data_bag.baz #=> 3
data_bag.bat #=> nil
data_bag.batz #=> 4