optional.rb

class Optional
  private_class_method :new

  def self.maybe(possible)
    possible.nil? ? absent : of(possible)
  end

  def self.absent
    None.new
  end

  def self.of(value)
    raise ArgumentError if value.nil?
    Some.new(value)
  end

  class None
    def or(default)
      default
    end

    def empty?
      true
    end

    def present?
      false
    end

    def transform(&block)
      self.class.new
    end
  end

  class Some
    def initialize(value)
      @value = value
    end

    def or(_default)
      @value
    end

    def empty?
      false
    end

    def present?
      true
    end

    def transform(&block)
      Optional.maybe(yield @value)
    end
  end
end