module M
  class ReplyError < StandardError; end

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

    def to_s
      "#<#{self.class.name} value=#{@value}>"
    end
  end

  class Left < Either
    def map(&block)
      self
    end

    def get_or_else(value)
      value
    end

    def get!
      raise ReplyError, @value
    end

    def success?
      false
    end
  end

  class Right < Either
    def map(&block)
      Right.new(block.call(@value))
    end

    def get_or_else(_value)
      @value
    end

    def get!
      @value
    end

    def success?
      true
    end
  end
end