User = Struct.new(:name, :age, :profession)

class MonoidBuilder
  def initialize(name: nil, age: nil, profession: nil)
    @name = name
    @age = age
    @profession = profession
  end

  def self.empty
    new
  end

  def +(other)
    self.class.new(
      name: other.name || name,
      age: other.age || age,
      profession: other.profession || profession
    )
  end

  def set_name(name)
    self + MonoidBuilder.new(name: name)
  end

  def set_age(age)
    self + MonoidBuilder.new(age: age)
  end

  def set_profession(profession)
    self + MonoidBuilder.new(profession: profession)
  end

  def == (other)
    name == other.name &&
      age == other.age &&
      profession == other.profession
  end

  def build
    User.new(name, age, profession)
  end

  protected

  attr_reader :name, :age, :profession
end