An example of "functional core, imperative shell" as far as I understand it. For more info about this concept, see https://www.destroyallsoftware.com/screencasts/catalog/functional-core-imperative-shell.

functional_core_imperative_shell.rb

# Written for this gist, but inspired by production code (I haven't run this code).

# Imperative shell
class Timer
  def initialize(record)
    @record = record
  end

  def start
    save_changes clock.start
  end

  def stop
    save_changes clock.stop
  end
  
  private

  # Only place where we know where the data is comming from and how it's persisted.
  def clock
    Clock.new(@record.total_working_time)
  end

  def save_changes(clock)
    @record.update_attribute(:total_working_time, clock.accumulated_time)
  end
end

# Functional core
class Timer
  class Clock
    def initialize(accumulated_time, started_at = nil)
      @accumulated_time = accumulated_time
      @started_at = started_at
    end
    
    attr_reader :accumulated_time
    
    def start
      self.class.new(@accumulated_time, Time.now)
    end
    
    def stop
      self.class.new(@accumulated_time + (Time.now - @started_at), nil)
    end    
  end
end

# Example usage:
record = Record.first

Timer.new(record).start
sleep 10
Timer.new(record).stop

Timer.new(record).start
sleep 3
Timer.new(record).stop

record.total_working_time # 13

That intendation went haywire

I think github is working on the gist pages today. I've gotten several flashes of unstyled content (usually happens when you don't handle assets correctly and deploy without downtime).