This is just an idea of value objects API and dirty implementation for Ruby. A frankenstein created from merge of Struct's and OpenStruct's APIs. It supoprts mandatory and optional fields. It raises exceptions if API of value object is misused. Feel free to comment and refactor the implementation!

value_object.rb

# TODO: ValueObject should be immutable. Setter should return new instance.

class ValueObject < Struct
  def self.new(*fields)
    all_fields, optional_fields = if fields.last.is_a?(Hash)
                   optional_fields = fields.pop
                   [fields + optional_fields.keys, optional_fields]
                 else
                   [fields, {}]
                 end
 
    super(*all_fields) do
      define_method :initialize do |attributes|
        unless (missing_fields = fields - (attributes.keys & fields)).empty?
          raise ArgumentError.new("missing fields: #{missing_fields.join(", ")}")
        end
 
        optional_fields.merge(attributes).each do |field, value|
          self.public_send("#{field}=", value)
        end
      end
    end
  end
end
 
Credentials = ValueObject.new(:user, :pass, secure: false)
 
Credentials.new(user: "Artur", pass: "abc")
# => #<struct Credentials user="Artur", pass="abc", secure=false>
 
Credentials.new(user: 'Artur', pass: 'qwerty', secure: true)
# => #<struct Credentials user="Artur", pass="qwerty", secure=true>
 
Credentials.new(user: "Artur", pass: 'qwerty', foo: 1) rescue $!
# => #<NoMethodError: undefined method `foo=' for test:Credentials>
 
Credentials.new(user: "Artur") rescue $!
# => #<ArgumentError: missing fields: pass>

Credentials.new(user: 'Artur', pass: 'qwerty').class.ancestors
# => [Credentials,
#     ValueObject,
#     Struct,
#     Enumerable,
#     Object,
#     PP::ObjectMixin,
#     Kernel,
#     BasicObject]

Credentials.new(user: 'foo', pass: 'bar') == Credentials.new(user: 'foo', pass: 'bar') # => true
Credentials.new(user: 'foo', pass: 'bar') == Credentials.new(user: 'foo', pass: 'baz') # => false