Struct instantiated with a hash

hash_struct.rb

Kernel.module_eval do
  def HashStruct(*attributes)
    Class.new do
      # keeping keys array in class
      @keys = attributes.map(&:to_sym)
      class << self
        attr_reader :keys
      end
      
      # passing keys to the child class if inherited
      define_singleton_method :inherited do |child|
        child.instance_variable_set :@keys, attributes.map(&:to_sym)
      end
      
      # accesors for all attributes
      attr_accessor *attributes
      
      # constructor that takes a hash of values
      # absent keys are filled with nil values
      # extra keys raise an ArgumentError
      def initialize(hash)
        # uncommenting this line allows to pass a hash with String keys to Klass.new
        # but requires ActiveSupport::HashWithIndifferentAccess
        # hash = hash.with_indifferent_access
        
        self.class.keys.each do |attr_name|
          var_name  = "@#{attr_name}"
          var_value = hash.delete(attr_name)
          instance_variable_set(var_name, var_value)
        end
        
        raise ArgumentError, "Attributes #{hash.keys.inspect} unknown" unless hash.empty?
      end
    end
  end
end

User = HashStruct(:name, :email)
puts "User class has keys: #{User.keys}"

u1 = User.new(name: 'Vasya', email: '[email protected]')
puts "name: '#{u1.name}', email: '#{u1.email}'"

u2 = User.new(name: 'ololo')
puts "name: '#{u2.name}', email: '#{u2.email}'"

begin
  u3 = User.new(name: 'trololo', avatar: 'trollface.jpg')
rescue ArgumentError => e
  puts "ArgumentError: '#{e}'"
end

class Book < HashStruct(:name, :author)
end

b1 = Book.new(name: 'The Lord of the Rings', author: 'JRRT')
puts "name: '#{b1.name}', author: '#{b1.author}'"

b2 = Book.new(author: 'Petya')
puts "name: '#{b2.name}', author: '#{b2.author}'"