Writer monad in ruby

writer.rb

class Writer
  def flat_map
    self.class.new do
      value, log = get
      new_value, new_log = yield value
      [new_value, log + new_log]
    end
  end

  def map
    flat_map do |value, log|
      new_value, log_item = yield value, log
      [new_value, [log_item]]
    end
  end

  def get
    @result ||= @block.call
  end

  def initialize(&block)
    @block = block
  end
end

def Writer(value)
  Writer.new { [value, []] }
end

w = Writer(5)
w1 = w.map { |v| [v * 2, 'multiplied by 2'] }
w2 = w1.flat_map { |v| [v + 1, ['added 1', 'with finesse']] }
w3 = w1.map { |v| ["Result is #{v}", 'stringified'] }