immutable-example.rb

Person = Immutable.new(:name, :age)

p1 = Person.new(name: "Sasha", age: 30) # <Person @name="Sasha", @age=30>

p2 = p1.with_name("Tori") # <Person @name="Tori", @age=30>

p3 = p2.update(name: "Craudio", age: 10) # <Person @name="Craudio", @age=10>

Person.new(name: "Sasha").with_age(30).with_name { |name| "#{name} Grey" } # <Person @name="Sasha Grey", @age=30>

immutable.rb

class Immutable
  def self.new(*attributes)
    Class.new do
      attributes.each do |attribute|
        attr_reader attribute

        define_method(:"with_#{attribute}") do |value = nil, &block|
          update(attribute => block ? block.call(send(attribute)) : value)
        end
      end

      define_method(:initialize) do |**kwargs|
        attributes.each do |attribute|
          instance_variable_set("@#{attribute}", kwargs[attribute])
        end
      end

      define_method(:to_h) do
        attributes
          .map { |attribute| [attribute, instance_variable_get("@#{attribute}")] }
          .to_h
      end

      define_method(:update) do |values|
        self.class.new(to_h.merge(values))
      end
    end
  end
end